582 PART SEVEN Ecology a. b. Figure 30.2 Studying the efects of ire. Ecology: A Biological Science Fire is a natural occurrence in lodgepole pine forests, such as these Ecology began as a part of natural history, the discipline dedicated to in Yellowstone National Park. a. Recently burned forest. b. The same observing and describing organisms in their environment, but today ecology is forest, several years after the ire. also an experimental science. A central goal of ecology is to develop models (a): © Dennis MacDonald/Age fotostock/Superstock; (b): © TMI/Alamy that explain and predict the distribution and abundance of populations based on their interactions within an ecosystem. Achieving such a goal involves testing 30.1 CONNECTING THE CONCEPTS hypotheses. For example, ecologists might formulate and test hypotheses about the role fire plays in maintaining a lodgepole pine forest (Fig. 30.2). To test Ecology is the study of how organ- these hypotheses, ecologists can compare the characteristics of a community isms interact with each other and before and after a burn. Ultimately, ecologists wish to develop models about their environment. the distribution and abundance of ecosystems within the biosphere. Ecology and Environmental Science The field of environmental science applies ecological principles to practical human concerns. Ecological principles help us understand why a functioning biosphere is critical to our survival and why our population of over 7 billion people and our overconsumption of resources pose threats to the biosphere. Conservation biology is a discipline of ecology that studies all aspects of biodiversity, with the goal of conserving natural resources, including wildlife, for the benefit of this generation and future generations. Conservation biology recognizes that wildlife species are an integral part of a well-functioning bio- sphere, on which human life depends. Check Your Progress 30.1 1. List the levels of biological organization, starting with the organism. 2. Distinguish an ecosystem from a community. 3. Deine ecology.
CHAPTER 30 Ecology and Populations 583 30.2 The Human Population Learning Outcomes Upon completion of this section, you should be able to 1. Identify the relationship among the birthrate, death rate, and annual growth rate of a population. 2. Distinguish between more-developed countries (MDCs) and less- developed countries (LDCs) with regard to population growth. 3. Compare the environmental impacts of MDCs and LDCs. The human population covers the majority of the Earth. However, on both a large scale and a small scale, the human population has a clumped distribution; 59% of the world’s people live in Asia, and most live in China and India. Mongolia has a population density of only 0.25 persons per square kilometer, while Bangladesh has a density of over 1,000 persons per square kilometer. On every continent, human population densities are highest along the coasts. Present Population Growth For most of the twentieth century, the growth of the global human population was relatively slow, but as more individuals achieved reproductive age, the growth rate began to increase. The Industrial Revolution (which began some- time before 1800) brought an increase in the production of food and medicines, along with a decrease in the death rate. At this time, the growth curve for the human population began to increase steeply, so that currently the population is undergoing rapid growth (Fig. 30.3). 12 highest growth Figure 30.3 Human population growth. 11 lowest 10 growth It is predicted that the world’s population size may 9 grow to between 9 and 10 billion by 2050, depending 8 2200 on the future growth rate. Close examination of the 7 curve shows changes in growth that occurred in the 6 fourteenth century during the Black Plague, in the 5 nineteenth century with the Industrial Revolution, and 4 in the twentieth century with advances in science and Population (billions) medicine. Population (billions) 3 2 1 0 1400 1600 1800 2000 1200 2 Year Modern Science 1.5 and Medicine 1 Industrial Black Revolution 0.5 Plague 0 1400 1600 1800 1900 1200 Year
584 PART SEVEN Ecology Just as with populations in nature, the growth rate of the human popula- tion is determined by the difference between the number of people born and the number of people who die each year. For example, the current global birthrate is an estimated 18.7 per 1,000 per year, while the global death rate is approxi- mately 7.9 per 1,000 per year. From these numbers, we can calculate that the current annual growth rate of the human population is (18.7 − 7.9)/1,000 = (10.8)/1,000 = 0.0108 × 100 = 1.08% Connections: Health How do birthrates and death rates vary among countries? In 2015, the country with the highest projected birthrate was Niger (6.76 children born per woman), followed by Burundi (6.09 children per woman) and Mali (6.06 children per woman), whereas Singapore was the country with lowest birthrate (0.81 children born per woman). For death rates, Lesotho had the highest rate (14.89 deaths per 1,000 people) and Qatar had one of © ER Productions/Corbis the lowest (1.53 deaths per 1,000). With respect to overall annual growth rates, the fastest-growing countries are projected to be South Sudan (4.02%) and Malawi (3.31%). Several countries are expected to ex- perience declines, including Latvia (–1.06%) and Lithuania (–1.04%). In compari- son, in the United States, the projected birthrate is 1.87 births per woman, the death rate is 8.15 per 1,000 people, and the annual growth rate 0.78%. Future Population Growth Future growth of the human population is of great concern. It’s possible that population increases will exceed the rate at which resources can be supplied. The potential for dire consequences can be appreciated by considering the doubling time—the length of time it takes for a population to double in size. The doubling time of the human population is now estimated to be around 67 years. Will we be able to meet the extreme demands for resources by such a large population increase in such a short period of time? Already, there are areas across the globe where people have inadequate access to fresh water, food, and shelter, yet in just 67 years the world would need to double the amount of food, jobs, water, energy, and other resources to maintain the present standard of living. Rapid growth usually begins to decline when resources, such as food and space, become scarce. Then population growth declines to zero, and the popu- lation levels off at a size appropriate to the carrying capacity of its environ- ment. The carrying capacity is the number of individuals the environment can sustain for an indefinite period of time. The Earth’s carrying capacity for humans has not been determined; some authorities think the Earth is poten- tially capable of supporting far more people than currently inhabit the planet, perhaps as many as 50 billion. Others believe that we may have already ex- ceeded the number of humans the Earth can support and that the human popu- lation may undergo a catastrophic crash.
CHAPTER 30 Ecology and Populations 585 Projected Annual Growth Rate of Country Populations, 2010–2050 Growth Rate ≤–0. 5 1 % –0. 5 0 to –0. 0 1 0.00 to 0.99 1.00 to 1.99 ≥2.00 2010 population too small to reliably compute growth rate. Figure 30.4 Projected population growth rates 2010–2050. The highest population growth rates are found in Africa and the Middle East, while the growth rates throughout Europe are much lower. Source: GlobalHealthFacts.org. Based on 2012 data. More-Developed Versus Less-Developed Countries a. Complicating the issue of future growth for the human population is the fact b. that not all countries have the same growth rate (Fig. 30.4). The countries of the world today can be divided into two groups (Fig. 30.5). In the more- Figure 30.5 More-developed versus less-developed developed countries (MDCs), such as those in North America and Europe, population growth is modest and the people enjoy a fairly good standard of countries. living. In the less-developed countries (LDCs), such as those in Latin People in (a) more-developed countries have a relatively high standard America, Asia, and Africa, population growth is dramatic and the majority of of living and will contribute the least to world population growth, while the people live in poverty. people in (b) less-developed countries have a low standard of living and will contribute the most to world population growth. The MDCs (a): © Ariel Skelley/Blend Images LLC; (b): © Henk Badenhorst/Getty RF The MDCs did not always have such a low growth rate. With the development of modern medicine, improved socioeconomic conditions, and the decrease in the death rate, their populations doubled between 1850 and 1950. The decline in the death rate was followed shortly thereafter by a decline in birthrate, so populations in the MDCs have experienced only modest growth since 1950. This sequence of events (i.e., decreased death rate followed by decreased birth- rate) is termed a demographic transition. The growth rate for the MDCs as a whole is now about 0.1%, but popula- tions in several countries are not growing at all or are actually decreasing. The MDCs are expected to increase by 52 million between 2002 and 2050, but this modest amount will still keep their total population at just about 1.2 billion. In contrast to the other MDCs, population growth in the United States is not level- ing off and continues to increase. The United States currently has a growth rate of 0.78%. This higher rate is due to the fact that many people immigrate to the United States each year, and a large number of the women are still of reproduc- tive age. Therefore, it is unlikely that the United States will experience a decline in its growth rate in the near future.
586 PART SEVEN Ecology The LDCs With the introduction of modern medicine following World War II, the death rates in the LDCs began to decline sharply but the birthrates remained high. The collective growth rate of the LDCs peaked at 2.5% between 1960 and 1965, and since that time the rate has declined to 1.5%. Unfortunately, the growth rates in the majority of the LDCs have not declined. Thirty-five of these countries are in sub-Saharan Africa, where women on average bear more than five children each. By 2050, the population of the LDCs is expected to jump to at least 8 billion. Africa will not share appreciably in this increase because of the many deaths there due to AIDS. The majority of the increase is expected to occur in Asia. With 31% of the world’s arable (farmable) land, Asia is already home to 59% of the human population. Twelve of the world’s most polluted cities are in Asia. If the human population increases as expected, Asia will experience even more urban pollution, acute water scarcity, and a significant loss of wildlife over the next 50 years. Comparing Age Structures The age structure of a population is divided into three groups: prereproductive, reproductive, and postreproductive (Fig. 30.6). Many MDCs have a stable age structure, meaning that the number of individuals is about the same in all three groups. Therefore, MDC populations will grow slowly if couples have two children, and they will slowly decline if each couple has fewer than two chil- dren. A birthrate of two children per couple is often called replacement repro- duction, since if each couple produces only two children, they are replacing themselves. In reality, due to mortality, the replacement reproduction rate is often slightly greater than two children per couple. In contrast, the age-structure diagram of most LDCs has a pyramid shape, with the prereproductive group the largest. Therefore, LDC populations will continue to expand, even if replacement reproduction is attained. It might seem at first that replacement reproduction would cause an LDC population to undergo zero population growth and therefore no increase in population size. However, if there are more young women entering the reproductive years than there are older women leav- ing them, replacement reproduction still results in population growth. We will take a closer look at age-structure diagrams in Section 30.3. Figure 30.6 Age-structure Postreproductive Ages 80+Age (in years) 75–79 diagrams. Reproductive Ages 70–74 Prereproductive Ages 65–69 The diagrams predict that 60–64 (a) the LDCs will expand rapidly 55–59 due to their age distributions, 50–54 while (b) the MDCs are 45–49 approaching stabilization. 40–44 35–39 300 200 100 0 100 200 300 30–34 25–29 Millions 20–24 15–19 a. Less-developed countries (LDCs) 10–14 5–9 0–4 100 0 100 Millions b. More-developed countries (MDCs)
CHAPTER 30 Ecology and Populations 587 Population Hazardous Waste Production Consumption MDCs LDCs fossil fuels paper 22% 10% LDCs 40% LDCs 25% MDCs 60% metals MDCs 75% LDCs MDCs 78% 90% a. b. LDCs MDCs 20% 80% MDCs = more-developed countries LDCs = less-developed countries c. Population Growth and Environmental Impact Figure 30.7 Environmental impacts. Population growth is putting extreme pressure on each country’s social organi- a. The combined population of the MDCs is much smaller than that zation, the Earth’s resources, and the biosphere. Since the population of the of the LDCs. The MDCs account for 22% of the world’s population, LDCs is still growing at a significant rate, it might seem that their population and the LDCs account for 78%. b. MDCs produce most of the world’s increase is the main cause of environmental degradation. But this is not neces- hazardous wastes—90% for MDCs compared with 10% for LDCs. sarily the case, because the MDCs consume a much larger proportion of the The production of hazardous waste is tied to (c) the consumption Earth’s resources than do the LDCs. This excessive consumption leads to envi- of fossil fuels, metals, and paper, among other resources. The MDCs ronmental degradation, which is of great concern. consume 60% of fossil fuels, compared with 40% for the LDCs, 80% of metals compared with 20% for the LDCs, and 75% of paper Environmental Impact compared with 25% for the LDCs. The environmental impact of a population is measured not only in terms of 30.2 CONNECTING THE CONCEPTS population size but also in terms of resource consumption and pollution per capita. Therefore, there are two possible causes of environmental impact: pop- The human population is still grow- ulation size and resource consumption. Overpopulation is more obvious in ing, with the majority of that growth LDCs; resource consumption is more obvious in MDCs, because per capita in the less-developed countries. consumption is so much higher in those countries. For example, an average American family, in terms of per capita energy consumption, is the equivalent of 20 people in India. We need to realize that only a limited number of people can be sustained anywhere near the standard of living enjoyed in the MDCs. The comparative environmental impacts of MDCs and LDCs are shown in Figure 30.7. The MDCs account for only about one-fourth of the world population. However, compared with the LDCs, the MDCs account for 90% of the hazardous waste production, due to their high rate of consumption of such resources as fossil fuels, metals, and paper. Check Your Progress 30.2 1. Calculate the annual growth rate of a population that is experiencing a birthrate of 18.5% and a death rate of 9.8%. 2. Compare the characteristics of an MDC with those of an LDC, and give an example of a country in each group. 3. Explain what is meant by the term replacement reproduction. 4. Contrast the environmental impact of an MDC with that of an LDC.
588 PART SEVEN Ecology Young, small 30.3 Characteristics of Populations shrubs Learning Outcomes a. Mature desert shrubs Upon completion of this section, you should be able to 1. Distinguish among the types of population distribution. 2. Describe how age-structure diagrams and survivorship curves are used to predict future growth. 3. Explain how biotic potential contributes to exponential and logistic growth. 4. Compare and contrast density-dependent and density-independent factors. Medium b. Clumped Various characteristics of populations change over time. Populations are shrubs periodically subject to environmental instability. During times of environmen- tal instability, individuals are under the pressures of natural selection. Individuals better adapted to the environment will leave behind more offspring than those that are not as well adapted. Once the population becomes adapted to the new environment, an increase in size can occur. When an ecologist lists the characteristics of a population, it is a snapshot of the characteristics at a particular time and place. c. Random Distribution and Density Large Resources are the components of an environment that support its organisms. shrubs Food, water, shelter, and space are some of the important resources for popula- tions. The availability of resources influences the spatial distribution of indi- d. Uniform viduals in a given area. Three terms—clumped distribution, random distribution, and uniform distribution—are used to describe the observed Figure 30.8 Patterns of distribution within a population. patterns of distribution. In a study of desert shrubs, it was found that the distri- bution changes from clumped to random to uniform as the plants mature a. A population of mature desert shrubs. b. Young, small desert (Fig. 30.8). After sufficient study, competition for belowground resources was shrubs are clumped. c. Medium shrubs are randomly distributed. found to be the main cause for these distribution patterns. d. Large shrubs are uniformly distributed. (a): © Evelyn Jo Johnson Suppose we were to step back and consider the distribution of individu- als not within a single desert, forest, or pond but within a species’ range. A Connections: Environment range is that portion of the globe where the species can be found. On this scale, all the members of a species within the range make up the population. Members Where are the highest and lowest population of a population are clumped within the range because they are located in areas densities of humans? that contain the resources they require. Based on 2015 demo- Population density is the number of individuals per unit area (or unit graphic data, the country volume) of a particular habitat. Population density tends to be higher in areas with the highest popula- with plentiful resources than in areas with scarce resources. Other factors can tion density is the Prin- be involved, however. In general, population density declines with increasing cipality of Monaco (in body size. Consider that tree seedlings show a much higher population density Europe), with a density of than do mature trees. Therefore, more than one factor must be taken into 25,718 people per square account when explaining population densities. kilometer. Greenland tech- © Photodisk/PunchStock RF nically has the lowest population density (0.03), followed by Population Growth Mongolia (1.87) and the Western Sahara (2.27). Overall, the land masses of Earth have a population density of approximately As mentioned in our discussion on the human population (Section 30.2), the 47 people per square kilometer. growth rate is based on the number of individuals born and the number of in- dividuals who die each year within the population. Growth occurs when the number of births exceeds the number of deaths. To take another example, if the number of births is 30 per year per 1,000 individuals and the number of deaths
Postreproductive CHAPTER 30 Ecology and Populations 589 Age Structure Reproductive Figure 30.9 Age-structure diagrams. Prereproductive Typical age-structure diagrams for hypothetical a. populations that are (a) increasing, (b) decreasing, and (c) stable. Diferent numbers of individuals in each age class create these distinctive shapes. In each diagram, the left half represents males, while the right half represents females. Increasing b. Decreasing c. Stable population population population is 10 per year per 1,000 individuals, the growth rate is 2%. In cases where the Age (years) Number of Number of number of deaths exceeds the number of births, the value of the growth rate is survivors at deaths during negative—the population is shrinking. 0 –1 beginning of year year 1–2 Demographics and Population Growth 2–3 1,000 1,000–199 199 3–4 801 801–12 12 Availability of resources and certain characteristics of a population—called 4–5 789 13 demographics—influence the population growth rate. One demographic char- 5–6 776 etc. 12 acteristic of interest for all populations is the age structure of a population. 6–7 764 30 Bar diagrams (like that shown in Figure 30.6) are typically used to depict the 7–8 734 46 age structure of a population. The population is divided into members that are 8–9 688 48 prereproductive, reproductive, or postreproductive, based on their age. If the 9–10 640 69 prereproductive and reproductive individuals outnumber those that are postre- 10–11 571 productive, the birthrate exceeds the death rate (Fig. 30.9a). In contrast, if the 11–12 439 132 postreproductive individuals exceed those that are prereproductive and/or re- 12–13 252 187 productive, the number of individuals in the population will decrease over time 13–14 96 156 because the death rate exceeds the birthrate (Fig. 30.9b). If the numbers of 14–15 6 90 prereproductive, reproductive, and postreproductive individuals are approxi- 3 mately equal and the birthrate equals the death rate, then the number of indi- 0 3 viduals in the population will remain relatively stable over time (Fig. 30.9c). 3 Ecologists also study patterns of survivorship—how age at death influ- a. ences population size. For example, if members of a population die young, the number of reproductive individuals will decrease. The first step is to construct 1,000 a life table, which lists the number of individuals of each age or age range that are alive at a given time. The best way to arrive at a life table is to identify a I large number of individuals that are born at about the same time and keep re- cords on them from birth until death. Number of Survivors 100 II A life table for Dall sheep is given in Figure 30.10a. This table was con- structed by gathering a large number of skulls and counting the growth rings on hydra the horns to estimate each sheep’s age at death, given that one growth ring is produced annually. Plotting the number of survivors per 1,000 births against age 10 produces a survivorship curve (Fig. 30.10b). Each species has its own pattern, but three types of survivorship curves are common. In a type I curve, typical of Dall sheep and humans, survival is high until old age, when deaths increase due Figure 30.10 Life table and survivorship curves. III oyster a. A life table for Dall sheep. b. Three typical survivorship curves. Among Dall sheep, 50 with a type I curve, most individuals survive until old age, when they gradually die of. Percent of Life Span Among hydras, with a type II curve, there is an equal chance of death at all ages. 0 100 Among oysters, with a type III curve, most die when they are young, and few become adults that are able to survive until old age. b.
590 PART SEVEN Ecology to illness. In a type II curve, the possibility of death is equal at any age. In a type III curve, death is likely among the young, with few individuals reaching old age. a. Biotic Potential The rate at which a population increases over time depends on its biotic potential, the maximum growth rate of a population under ideal condi- tions (Fig. 30.11). Whether the population achieves biotic potential depends on the demographic characteristics of the population, such as the following: ∙ Availability of resources ∙ Number of offspring per reproductive event ∙ Chances of survival until age of reproduction ∙ How often each individual reproduces ∙ Age at which reproduction begins Patterns of Population Growth The patterns of population growth are dependent on (1) the biotic potential of the species and (2) the availability of resources. The two fundamental patterns of population growth are exponential growth and logistic growth. Exponential Growth Suppose ecologists are studying the growth of a population of insects that are capable of infesting and taking over an area. Under these circumstances, exponential growth is expected. An exponential pattern of population growth results in a J-shaped curve (Fig. 30.12). This growth pattern can be likened to compound interest at a bank: The more your money increases, the more interest you will get. If the insect population has 2,000 individuals and the per capita rate of increase is 20% per month, there will be 2,400 insects after one month, 2,880 after two months, 3,456 after three months, and so forth. Notice that a J-shaped curve has two phases: Lag phase: Growth is slow because the number of individuals in the popula- tion is small. Exponential growth phase: Growth is accelerating. Usually, exponential growth continues as long as there are sufficient resources available. When the number of individuals in the population approaches the maximum number that can be supported by available resources, competition for these resources will increase. b. exponentialNumber of Organisms growth Figure 30.11 Biotic potential. phase A population’s maximum growth rate under ideal conditions—that is, lag phase its biotic potential—is greatly inluenced by the number of ofspring produced in each reproductive event. a. Mice, which produce many Time ofspring that quickly mature to produce more ofspring, have a much higher biotic potential than (b) the rhinoceros, which produces Figure 30.12 Exponential growth. only one or two ofspring per infrequent reproductive event. (a): © Daniel Heuclin/NHPA/Photoshot; (b): © Corbis RF Exponential growth results in a J-shaped curve because the growth rate is positive.
carrying environmental CHAPTER 30 Ecology and Populations 591 capacity resistance Figure 30.13 Logistic growth. Logistic growth results in an S-shaped growth curve because environmental resistance causes the population size to level of and be in a steady state at the carrying capacity of the environment. stable equilibrium logistic growth exponential growth lag Time Number of Organisms Logistic Growth Fish caught and remaining Amount of fish caught Amount of fish remaining Usually, exponential growth cannot continue for long because of environmen- (millions of tons) tal resistance. Environmental resistance encompasses all those environmental conditions—such as limited food supply, accumulation of waste products, in- 1950 1970 1990 2010 creased competition, and predation—that prevent populations from achieving their biotic potential. The amount of environmental resistance will increase as Year the population grows larger. As resources decrease, population growth levels off and a pattern of population growth called logistic growth occurs. Logistic Figure 30.14 Sustainable population density. growth results in an S-shaped growth curve (Fig. 30.13). As the amount of ish caught increased over the 1950s and 1960s, The characteristics of logistic growth include: the remaining amount decreased. Because ishing still continues, ish population growth is maintained in its lag phase. Fishing must Lag phase: Growth is slow because the number of individuals in the popula- be reduced to allow the ish population to enter an exponential tion is small. growth phase. Then a balance between ish population growth and ish caught could be maintained regularly at a higher amount. Exponential growth phase: Growth is accelerating due to biotic potential (see Fig. 30.11). Logistic growth phase: The rate of population growth slows down due to envi- ronmental resistance. Stable equilibrium phase: Little, if any, growth takes place because births and deaths are about equal. The stable equilibrium phase is said to occur at the carrying capacity of the environment. This number is not a constant; it tends to fluctuate around a value that is determined by the current environmental resources. For example, the carrying capacity of an island may stay constant for years, but then de- crease due to a period of drought. Applications Our knowledge of logistic growth has practical applications. The model predicts that exponential growth will occur only when population size is much lower than the carrying capacity. Thus, if humans are using a fish population as a continuous food source, it is best to maintain the fish popula- tion size in the exponential phase of growth when biotic potential is having its full effect and the birthrate is the highest. If people overfish, the fish popula- tion will sink into the lag phase, and it may be years before exponential growth recurs (Fig. 30.14). On the other hand, if people are trying to limit the growth of a pest, it is best to reduce the carrying capacity, rather than the population size. Reducing the population size only encourages exponential growth to be- gin once again. Farmers can reduce the carrying capacity for a pest by alternat- ing rows of different crops instead of growing one type of crop throughout the entire field.
592 PART SEVEN Ecology Factors That Regulate Population Growth a. Low density of mice Ecologists have long recognized that the environment contains both biotic (living) and abiotic (nonliving) components that play an important role in b. High density of mice regulating population size. Figure 30.15 Density-independent efects. Density-Independent Factors The impact of a density-independent factor, such as weather or a Abiotic factors, such as weather and natural disasters, are typically density- natural disaster, is not inluenced by population density. Two independent factors. Abiotic factors can cause sudden and catastrophic reduc- populations of ield mice are in the path of a lash lood. a. In the tions in population size. However, density-independent factors cannot in and of low-density population, 3 out of the 5 mice drown, a 60% death rate. themselves regulate population size because the effect is not influenced by the b. In the high-density population, 12 out of 20 mice drown—also a number of individuals in the population. In other words, the intensity of the 60% death rate. effect does not increase with increased population size. For example, the pro- portion of a population killed in a flash flood is independent of density— floods don’t necessarily kill a larger percentage of a dense population than of a less dense population. If a mouse population has only 5 members and 3 drown in a flash flood, the death rate is 60% (Fig. 30.15a). If a mouse population has 20 mice and 12 drown, the death rate is still 60% (Fig. 30.15b). Density-Dependent Factors Biotic factors are considered density-dependent factors, because the percent- age of the population affected increases as the density of the population in- creases. Competition, predation, and parasitism are all biotic factors that increase in intensity as the density increases. We will discuss these interactions between populations again in Section 31.1 because they influence community composition and diversity. Competition Competition results when organisms attempt to use resources (such as light, food, or space) that are in limited supply. Competition may occur between different species, or between members of the same species. Competi- tion limits access to the resources necessary to ensure survival, reproduction, or some other aspect of an organism’s life cycle. Let’s consider a woodpecker population in which members have to com- pete for nesting sites. Each pair of birds requires a tree hole in order to raise offspring. If the number of tree holes is the same as or greater than the number of breeding pairs, each pair can have a hole in which to lay eggs and rear young birds (Fig. 30.16a). But if there are fewer holes than breeding pairs, each pair must compete to acquire a nesting site (Fig. 30.16b). Pairs that fail to gain access to holes will be unable to contribute new members to the population. A well-known example of competition for food affected the reindeer on St. Paul Island, Alaska. Overpopulation led to the overconsumption of re- sources, causing the population to crash—that is, it became drastically reduced. Resource partitioning among different age groups is a way to reduce competition for food. During the life cycle of butterflies, the caterpillar stage requires different food than the adult stage. Caterpillars graze on leaves, while adult butterflies feed on nectar produced by flowers. Therefore, the parents are less apt to compete with their offspring for food. Predation Predation occurs when one organism, the predator, eats another, the prey. In the broadest sense, predation includes not only animals such as li- ons that kill zebras but also filter-feeding blue whales, which strain krill from the ocean waters, and herbivorous deer, which browse on trees and bushes. The effect of predation generally increases as the prey population in- creases in density, because prey are easier to find when their population is
CHAPTER 30 Ecology and Populations 593 Figure 30.16 Density-dependent efects—competition. The impact of competition is directly proportional to the density of a population. a. When density is low, every member of the population has access to the resource. b. When density is high, members of the population must compete to gain access to the available resources, and some fail to do so. a. Low density of birds b. High density of birds larger. Consider a field inhabited by a population of mice (Fig. 30.17). Each 30.3 CONNECTING THE CONCEPTS mouse must have a hole in which to hide to avoid being eaten by a hawk. If there are 102 mice but only 100 holes, 2 mice will be left out in the open. It Population growth influences the might be hard for the hawk to find only 2 mice in the field. If neither mouse is community and ecosystem caught, the predation rate is 0/2 = 0%. However, if there are 200 mice and only structure. 100 holes, there is a greater chance that the hawk will be able to find some of the 100 mice without holes. If half of the exposed mice are caught, the preda- tion rate is 50/100 = 50%. Therefore, increasing the density of the available prey has increased the proportion of the population preyed upon. Predator-Prey Population Cycles Rather than remaining steady, some predator and prey populations experience an increase, then a decrease, in pop- ulation size. Such a cycle occurs between the snowshoe hare and the Canada lynx, a species of wild cat (Fig. 30.18). The snowshoe hare is a common her- bivore in the coniferous forests of North America, where it feeds on terminal twigs of various shrubs and small trees. Investigators first assumed that the predatory lynx brings about a decrease in the hare population, and this de- crease in prey brings about a subsequent decrease in the lynx population. Once the hare population recovers, so does the lynx population, and the result is a boom-bust cycle. But some biologists noted that the decline in snowshoe hare abundance is accompanied by low growth and reproductive rates, which could be signs of food shortage. A field experiment showed that, if the lynx predator Figure 30.17 Density-dependent efects—predation. The impact of predation on a population is directly proportional to the density of the population. a. In a low-density population, the chances of a predator inding the prey are low, resulting in little predation (a mortality rate of 0/2, or 0%). b. In a higher- density population, there is a greater likelihood of the predator locating potential prey, resulting in a greater predation rate (a mortality rate of 50/100, or 50%). a. Field with a low-density of mice. b. Field with a high-density of mice.
594 PART SEVEN Ecology Snowshoe hare and lynx cycles, boreal forest, Kluane, Yukon 100 Figure 30.18 Predator-prey cycling of a lynx and a snowshoe hare. 80 5 Research indicates that the snowshoe hare population reaches a peak snowshoe hares per ha abundance before that of the lynx by a year or more. A study conducted lynx tracks per 100 km from 1987 to 2009 shows a cycling of the lynx and snowshoe hare populations. 4 © Alan Carey/Science Source 3 60 lynx 2 40 1 20 0 1995 2000 2005 0 1987 1990 2009 Source: Krebs 2010 snowshoe hare was denied access to the hare population, the cycling of the hare population still occurred based on food availability. The results suggested that a hare-food cycle and a predator-hare cycle have combined to produce the pattern observed in Figure 30.18. Check Your Progress 30.3 1. Explain how an age-structure diagram may be used to predict the future growth of a population. 2. Describe what survivorship curves can tell you about a species’ reproductive strategies. 3. Compare and contrast an exponential growth curve and a logistic growth curve. 4. Provide examples of density-independent factors that regulate population growth. 5. Explain how competition and predation act as density-dependent factors for regulating population growth.
30.4 Life History Patterns CHAPTER 30 Ecology and Populations 595 and Extinction Opportunistic Pattern Learning Outcomes Small individuals Short life span Upon completion of this section, you should be able to Fast to mature 1. Distinguish between opportunistic and equilibrium species. Many o spring 2. Describe factors that can lead to the extinction of a species. Little or no care of o spring A life history consists of a particular mix of the characteristics we have been discussing. After studying many types of populations, from mayflies to humans, ecologists have discovered two fundamental and contrasting patterns: the opportunistic life history pattern and the equilibrium life history pattern. Opportunistic species tend to exhibit exponential growth. The members of the population are small in size, mature early, have a short life span, and provide limited parental care for a great number of offspring (Fig. 30.19a). Density-independent factors tend to regulate the population size, which is large a. enough to survive an event that threatens to annihilate it. These populations typically have a high dispersal capacity. Various types of insects and weeds are good examples of opportunistic species. Equilibrium species exhibit logistic population growth, with the pop- ulation size remaining close to or at the carrying capacity (Fig. 30.19b). Resources are relatively scarce, and the individuals best able to compete— those with phenotypes best suited to the environment—have the largest number of offspring. They allocate energy to their own growth and survival and to the growth and survival of a small number of offspring. Therefore, they are fairly large, are slow to mature, and have a fairly long life span. The population growth of equilibrium species tends to be regulated by density- dependent factors. Various birds and mammals are good examples of equilibrium species. Equilibrium Pattern Large individuals Extinction Long life span Slow to mature Extinction is the total disappearance of a species or higher Few o spring group. Which species shown in Figure 30.19, the dandelion or Much care of o spring the bears, is apt to become extinct? Because the dandelion ma- tures quickly, produces many offspring at one time, and has seeds dispersed widely by wind, it can withstand a local decimation more easily than the bears. A study of equilibrium species shows that three other factors—the size b. of the geographic range, degree of habitat tolerance, and size of local popula- Figure 30.19 Life history patterns. tions—can help determine whether an equilibrium species is in danger of extinction. Figure 30.20 compares 14 equilibrium species on the basis of a. Dandelions are an opportunistic species, whereas (b) bears are an these three factors. The mountain gorilla has a restricted geographic range, equilibrium species. narrow habitat tolerance (few preferred places to live), and small local popu- lations. This combination of characteristics makes the mountain gorilla very (a): © Elena Elisseeva/Alamy RF; (b): © Barrett Hedges/National Geographic/ Getty RF vulnerable to extinction. The possibility of extinction increases depending on whether a species is similar to the gorilla in one, two, or three ways. Such population studies can assist conservationists and others trying to preserve the biosphere’s biodiversity.
596 PART SEVEN Ecology Species Monterey Galápagos pine Most common medium ground finch Fremont × Restricted geographic range cottonwood Broad habitat tolerance California Large local population grey whale Grass fern Extensive geographic range Tiger Haleakala × Narrow habitat tolerance silversword Fish crow Large local population Welwitschia Tasmanian Extensive geographic range devil Pacific Broad habitat tolerance yew × Small local population Northern spotted owl Pritchardia × Restricted geographic range kaalae × Narrow habitat tolerance Mountain gorilla Large local population × Restricted geographic range Broad habitat tolerance × Small local population Extensive geographic range × Narrow habitat tolerance × Small local population × Restricted geographic range × Narrow habitat tolerance × Small local population Rarest Figure 30.20 Vulnerability to extinction. Vulnerability is particularly tied to range, habitat, and size of population. The number of strikes (X) in the boxes indicates the chances and causes of extinction. Check Your Progress 30.4 Connections: Environment 1. Contrast opportunistic species with equilibrium How are humans inluencing the species. rates of extinction? 2. Deine extinction. Many conservation biologists believe that 3. List ive factors that can determine whether an we are in the midst of one of the largest periods of mass extinction since the end of equilibrium population is in danger of extinction. the Cretaceous, 65 mya (see Table 16.1). The major causes of this “sixth mass extinc- 30.4 CONNECTING THE CONCEPTS tion” are human activity (population growth © Goddard Photography/iStock/Getty Knowledge of life history patterns Images Plus RF helps us predict the threat of extinction for various species. and resource use) and global climate change. While extinction is a normal aspect of life, the current extinction rates are between 100 and 1,000 times the normal levels of one to ive species per year. Some estimates suggest that as much as 38% of all species on the planet, including most primates (such as the lemur shown in photo), birds, and amphib- ians, may be in danger of extinction before the end of this century.
CHAPTER 30 Ecology and Populations 597 STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the irst adaptive textbook. SUMMARIZE Population Growth and Environmental Impact Ecology is the study of the interactions between organisms, as well as There are two causes of environmental impact: The LDCs put stress on the between organisms and their environment. Ecologists study biological biosphere due to population growth, and the MDCs put stress on the organization at a variety of levels: organism, species, population, community, biosphere due to resource consumption and waste production. ecosystem, and biosphere. Knowledge gained from ecology helps us understand all of the species on Earth. 30.3 Characteristics of Populations Ecology is the study of how organisms interact with each other and their Patterns of populations in the environment, such as clumped distribution, 30.1 environment. random distribution, and uniform distribution, along with population density are dependent on resource availability. These factors determine the 30.2 The human population is still growing, with the majority of that growth in range that a species will use. The number of individuals per unit area is the less-developed countries. higher in areas with abundant resources and lower where resources are limited. Population growth can be calculated based on the annual birthrate 30.3 Population growth influences the community and ecosystem structure. and death rate. Population growth is determined by 30.4 Knowledge of life history patterns helps us predict the threat of extinction ∙ Resource availability for various species. ∙ Demographics (age structure, survivorship, and biotic potential— 30.1 The Scope of Ecology the highest rate of increase possible) Ecology is an experimental science that studies the interactions of organisms Patterns of Population Growth with each other and with the physical environment. The general levels of biological organization studied by ecologists are The two patterns of population growth are exponential growth and logistic growth. ∙ Organism, species, population, community, ecosystem, biosphere Environmental scientists and conservation biologists both utilize the ∙ Exponential growth results in a J-shaped curve. The two phases of principles of ecology in their work. exponential growth are the lag phase (slow growth) and exponential growth phase (accelerating growth). 30.2 The Human Population ∙ Logistic growth results in an S-shaped curve. The four phases of The human population exhibits clumped distribution (on both a large scale logistic growth are the lag phase (slow growth), exponential growth and a small scale) and is undergoing rapid growth. phase (accelerating growth), logistic growth phase (slowing growth), and stable equilibrium phase (relatively no growth). Future Population Growth carrying environmental capacity resistance At the present growth rate, the doubling time for the human population is estimated to be 67 years. This rate of growth will put extreme demands stable equilibrium on resources, and growth will decline due to resource scarcity. Eventually, logistic growth the population will most likely level off at its carrying capacity. exponential ∙ More-developed countries (MDCs) have a lower growth rate but growth greater demand for resources per person. lag ∙ Less-developed countries (LDCs) have a higher growth rate and generally a lower demand for resources per person. Time Age Structure Number of OrganismsFactors That Regulate Population Growth Comparing Age Structures The two types of factors that regulate population growth are density- independent and density-dependent factors. The age structure of a population is divided into three age groups: prereproductive, reproductive, and postreproductive. The MDCs are ∙ Density-independent factors include abiotic factors, such as weather approaching stabilization, with just about equal numbers in each group. The and natural disasters. The effect of the factor is not dependent on LDCs will continue to expand, because their prereproductive group is the population density. largest. Ideally, populations would exhibit replacement reproduction and each couple would produce only two children. ∙ Density-dependent factors include biotic factors, such as competition and predation. The effect of the factor is dependent on population Postreproductive density. Reproductive b. Decreasing c. Stable population population Prereproductive a. Increasing population
598 PART SEVEN Ecology 30.3 Characteristics of Populations 30.4 Life History Patterns and Extinction 6. A population’s maximum growth rate is also called its The two fundamental life history patterns are exhibited by opportunistic a. carrying capacity. c. growth curve. species and equilibrium species. b. biotic potential. d. replacement rate. ∙ Opportunistic species are characterized by individuals that have short life spans, mature quickly, produce many offspring, have strong 7. If members of a population live only along a lake’s shoreline, this dispersal ability, provide little or no care to their offspring, and exhibit population exhibits which type of spatial distribution? exponential popuation growth. a. variable c. random ∙ Equilibrium species are characterized by individuals that have long life spans, mature slowly, produce few offspring, provide much care to the b. clumped d. uniform offspring, and exhibit logistic population growth. 8. Label each of the following age-structure diagrams to indicate whether Extinction the population is stable, increasing, or decreasing. Extinction is the total disappearance of a species or higher group. Opportunistic species are less likely than equilibrium species to become extinct. Three factors, in particular, influence the vulnerability of equilibrium species to extinction: size of geographic range, degree of habitat tolerance, and size of local populations. ASSESS a. b. c. Testing Yourself 9. An S-shaped growth curve indicates a. logistic growth. Choose the best answer for each question. b. logarithmic growth. c. exponential growth. 30.1 The Scope of Ecology d. additive growth. 1. Which of these levels of ecological study involves both abiotic and For questions 10–13, choose the term from the key that matches each scenario. biotic components? a. organisms Key: b. populations c. communities a. density-independent factor d. ecosystem b. competition e. All of these are correct. c. predation d. predator-prey cycle 2. The biological level of organization that includes all members of a 10. A severe drought destroys the entire food supply of a herd of gazelle. species in a given region is 11. A population of feral cats increases in size as the mouse population a. organism. increases and then crashes regularly after the mouse population enters b. population. periods of decline. c. community. 12. Swift coyotes are able to catch rabbits and live to reproduce. Other d. ecosystem. coyotes lack a good food source and are not strong enough to reproduce. e. biosphere. 13. Deer prefer to feed on a dense thicket of oak saplings rather than more widely spaced young oak trees. 30.2 The Human Population 30.4 Life History Patterns and Extinction 3. Decreased death rates followed by decreased birthrates has occurred in 14. Which of the following features is present in organisms that exhibit an a. MDCs. equilibrium life history pattern? a. long life span b. LDCs. b. small body size c. many offspring c. MDCs and LDCs. d. fast to mature e. no parental care d. neither MDCs nor LDCs. 15. Which of the following is not an adaptive feature of an opportunistic life 4. Calculate the growth rate of a population of 500 individuals in which history pattern? a. many offspring the birthrate is 10 per year and the death rate is 5 per year. b. little or no care of offspring c. long life span a. 5% c. 1% d. small individuals e. individuals that mature quickly b. –5% d. –1% 5. If the human birthrate were reduced to 15 per 1,000 per year and the death rate remained the same (9 per 1,000), what would be the growth rate? a. 9% d. 0.6% b. 6% e. 15% c. 10%
ENGAGE CHAPTER 30 Ecology and Populations 599 Thinking Critically to allow them to shoot red-tailed hawks, one of the main predators of rabbits and pheasants. Explain why shooting hawks will not ultimately 1. In Sri Lanka, the death rate is 6 per 1,000, while the birthrate is 17 per solve the problem of decreased rabbit and pheasant populations. 1,000. Calculate the current population growth rate. The formula for 3. What type of data would you collect to show that a population is the doubling time of a population is undergoing stabilizing selection, as in Figure 15.2? 4. Species that are more vulnerable to certain risk factors are more likely t = 0.69/r, where t is the doubling time and r is the growth rate than others to become extinct. For example, species with a unique lineage, such as the giant panda, are likely to be at severe risk of Determine the doubling time (the number of years it will take to double extinction. the population size) in Sri Lanka. Be sure to convert your growth rate a. Should our limited resources for species protection be focused from a percentage (e.g., 2.1%) to a decimal value (0.021). If the birthrate drops to 10 per 1,000, what is the doubling time? on species that are at the highest risk of extinction? 2. Upland game hunters in Illinois have been noticing a decrease in the b. Do you support the idea that high-risk species may be less rabbit and pheasant populations over the past 5 years. They have lobbied the Department of Natural Resources and the state legislature successful products of evolution and should not receive extraordinary protection?
31 Communities –5 –4 –3 –2 –1 0 1 2 3 4 5 5 and Ecosystems Degrees Celsius OUTLINE –5 –4 –3 –2 –1 0 1 2 3 4 31.1 Ecology of Communities 601 Degrees Celsius 31.2 Ecology of Ecosystems 610 31.3 Ecology of Major Ecosystems 620 (map): Source: NOAA National Centers for Environmental Information, State of the Climate: Global Analysis for March 2016, published online April 2016, retrieved on June 7, 2016 from BEFORE YOU BEGIN http://www.ncdc.noaa.gov/sotc/global/201603; (dry lake): © Vladislav T. Jirousek/Shutterstock RF; Before beginning this chapter, take a few moments to (hurricane): © Purestock/SuperStock RF review the following discussions. Section 1.1 What are the roles of producers, The Consequences of Climate Change consumers, and decomposers in an ecosystem? Section 6.1 How do photosynthetic organisms convert Almost every month, announcements from climate scientists present data CO₂ and water to sugar? supporting the observation that our planet is warming. Not only was 2015 the Section 30.2 What is the environmental impact of hottest year on record, but almost every month in 2016 broke previous heat human population growth? records, and usually by signiicant amounts. After decades of studies and anal- yses, the scientiic community has concluded that global warming, and the resulting climate changes, are not a result of natural cycles, but rather due to the emission of greenhouse gases. For most of us, it is diicult to see how global changes impact us directly. However, in many cases, the evidence is already around us. Droughts are more severe, there are reductions in mountain snow packs, and precipitation events are more unpredictable. These are all indications that the climate is changing. On a more personal level, climate change has the potential to increase our exposure to diseases that are not normally a part of our geographical area. For example, the spread of malaria, dengue fever, and even the Zika virus, is due to expansion of the range of mosquitoes that act as vectors for each disease. In the near future, heat warnings will limit outdoor activities, and water and air quality will be degraded. All of these events are tied to an imbalance in how our planet functions on a global scale. In this chapter, we will explore how ecosystems function and how varia- tions in these natural cycles, and human inluences, afect the basic structure of an ecosystem. As you read through this chapter, think about the following questions: 1. How does climate inluence a community and an ecosystem? 2. What is the relationship between greenhouse gases and the carbon cycle? 600
CHAPTER 31 Communities and Ecosystems 601 31.1 Ecology of Communities Learning Outcomes Upon completion of this section, you should be able to 1. Define community and coevolution, and provide an example of each. 2. Distinguish between species richness and species diversity. 3. Compare and contrast primary and secondary succession. 4. Describe and provide examples of how a species’ niche influences the types of interactions within a community. A community is an assemblage of populations of multiple species, interacting with one another within a single environment. For example, the various species living in and on a fallen log, such as plants, fungi, worms, and insects, interact with one another and form a community. The fungi break down the log and provide food for the earthworms and insects living in and on the log. Those insects may feed on one another, too. If birds flying throughout the forest feed on the insects and worms living in and on the log, then the insects and worms are also part of the larger forest community. Communities come in various sizes, and it is sometimes difficult to de- cide where one community ends and another one begins. The relationships and interactions between species in a community form as the products of coevolution, by which an evolutionary change in one species results in an evo- lutionary change in another. Figure 31.1 gives examples of how flowering plants and their pollinators have coevolved. The Australian orchid, Chiloglottis trapeziformis, resembles the body of a wasp, and its odor mimics the phero- mones of a female wasp. Therefore, male wasps are attracted to the flower, and when they attempt to mate with it, they become covered with pollen, which they transfer to the next orchid c. Figure 31.1 Coevolution. Flowers and pollinators have evolved to be suited to one another. a. Hummingbird-pollinated flowers are usually red, a. a color these birds can see, and the petals are recurved to allow the stamens to dust the birds’ heads. b. The reward ofered by the lower is not always food. This orchid looks and smells like the female of this wasp’s species. The male tries to copulate with lower after lower and in the process transfers pollen. c. Bats are b. nocturnal, and the lowers they pollinate are white or light-colored, making them visible in moonlight. The lowers smell like bats and are large and sturdy, enabling them to withstand insertion of the bat’s head as it uses its long, bristly tongue to lap up nectar and pollen. (a): © Ondrej Prosicky/Shutterstock RF; (b): © Perennou Nuridsany/Science Source; (c): © Dr. Merlin D. Tuttle/Bat Conservation International/Science Source
602 PART SEVEN Ecology flower (Fig. 31.1b). The orchid is dependent on wasps for pollination, because neither wind nor other insects pollinate these flowers. Figure 31.2 Community species composition. The species in each community have adaptations that are suited to the Communities difer in their species composition, as environmental conditions. An ecosystem consists of these species interacting exemplified by the predominant plants and animals in (a) with each other and with the physical environment. If the physical environment a coniferous forest and (b) a tropical rain forest. changes, the species that make up the community and the relationships among (a): © Yi Jiang Photography/Flickr Open/Getty RF; (b): © Nejron these species will also change. Extinction occurs when environmental change Photo/Shutterstock RF is too rapid for suitable adaptations to evolve. We will explore the structure of ecosystems in more detail in Section 31.2. b. Rapid environmental changes are detrimental to humans, despite our use a. of technology to adapt. Sometimes the economy of an area is dependent on the species composition of an aquatic or terrestrial ecosystem. Therefore, human activities that negatively impact the biodiversity of the area can also negatively affect the economy of the area. Knowledge of community and ecosystem ecol- ogy will help you better understand how our human activities are detrimental to our own well-being. Community Composition and Diversity Ecologists have a variety of methods of analyzing a community. Frequently, these focus on determining what species are present in the community, their distribution, and their interactions. In this chapter, we will focus on comparing communities based on their species composition and diversity. Species Composition and Richness A comparison of the species composition of a coniferous forest and a tropical rain forest (Fig. 31.2) reveals some very distinctive differences. Narrow-leaved evergreen tree species are prominent in the coniferous forest, whereas broad- leaved evergreen tree species are numerous in the tropical rain forest. There are also differences in the types and number of plant and animal species in these two communities. Ecologists often use the term species richness, which is a listing of the various species found in a community. Ecologists have con- cluded that not only are the spe- cies compositions of these two communities different but the tropical rain forest has a higher species richness, meaning that it has a greater number of species. Diversity The diversity of a community includes both species richness and species distribution. The di- versity of a community goes be- yond species richness to include distribution and relative abundance, the number of individuals of each species in a given area. For example, a decidu- ous forest in West Virginia may contain 76 poplar trees but only 1 American elm. If you were simply walking through this forest, you could miss the lone American elm. If, instead, the forest had 36 poplar trees and 41 American elm trees, the forest would seem more diverse to you and indeed would have a higher diversity value, although both forests would have the same species rich- ness. The greater the species richness and the more even the distribution of the species in the community, the greater the diversity.
Ecological Succession CHAPTER 31 Communities and Ecosystems 603 The species composition and diversity of communities change over time. It a. often takes decades for noticeable changes to occur. Natural forces, such as glaciers, volcanic eruptions, lightning-ignited forest fires, hurricanes, torna- b. does, and floods are considered disturbances that can bring about community changes. Communities also change because of human activities, such as log- Figure 31.3 Climax communities. ging, road building, sedimentation, and farming. A more or less orderly process of community change is known as ecological succession. Does succession in a particular area always lead to the same climax community? For example, (a) temperate rain forests occur only Ecologists have developed models to explain why succession occurs where there is adequate rainfall and (b) deserts occur where rainfall and to predict patterns of succession following a disturbance. The climax- is minimal. Even so, exactly the same mix of plants and animals may pattern model says that the climate of an area will always lead to the same not always arise, because the assemblage of organisms depends on assemblage of bacterial, fungal, plant, and animal species (Fig. 31.3) known which organisms, by chance, migrate to the area. as a climax community. For example, a coniferous forest community is ex- (a): © Corbis RF; (b): © Anton Foltin/Shutterstock RF pected in northern latitudes, a deciduous forest in temperate zones, and a tropical forest in areas with a tropical climate. Scientists know that distur- bances influence community composition and diversity and that, despite the climate, the composition of a climax community in a given climate is not always the same. Two Types of Succession Ecologists define two types of ecological succession: primary and secondary (Fig. 31.4). Primary succession starts where soil has not yet formed. For example, hardened lava flows and the scraped bedrock that remains following a glacial retreat are subject to primary succession. Secondary succession begins, for example, in a cultivated field that is no longer farmed, where soil is already present. With both primary and secondary succession, a progression of species occurs over time. The spores of fungi and vascular seedless plants are usually the first to grow and then seeds of nonvascular plants, followed by seeds of gymnosperms and/or angiosperms, are carried into the area by wind, water, or animals from the surrounding regions (Fig. 31.5; see also Fig. 31.4). Figure 31.4 Primary and secondary succession. Primary succession begins on areas of bare rock. Secondary succession begins in areas where soil remains following natural or human-caused disturbance. rock lichen/moss grass low shrub high shrub shrub tree low tree high tree primary succession secondary succession
604 PART SEVEN Ecology lichen Dryas plant a. Glacier/lichen stage b. Shrub stage c. Low tree stage d. High tree stage Figure 31.5 Secondary succession. The first species to appear in an area undergoing either primary or sec- ondary succession are called opportunistic pioneer species. These tend to be Secondary successional changes in a western Pennsylvania ield: small in stature, short-lived, and quick to mature, and they produce numerous (a) irst year, (b) second year, (c) ifth year, and (d) after 20 years. offspring per reproductive event. The first pioneer species to arrive are photo- synthetic organisms, such as lichens and mosses. A lichen consists of two or- (a): (glacier): © Kevin Smith/Design Pics; (a): (lichen): © Background Abstracts/ ganisms, fungus and alga, living together as one. The fungal component of a Getty RF; (b): (shrub stage): © Don Paulson/Alamy RF; (b): (Dryas plant): © Martin lichen plays a critical role in breaking down rock or lava into usable mineral Fowler/Shutterstock RF; (c): © Bruce Heinemann/Getty RF; (d): © Don Paulson/ nutrition, not only for its algal partner but also for pioneer plants. The mycor- Alamy RF rhizal fungal partners of plants (see Section 18.3) pass minerals directly to them, so that they can grow successfully in poor soil. Pioneer plant species that Table 31.1 Species Interactions become established in an area are often accompanied by pioneer herbivore spe- cies (e.g., insects) and then carnivore species (e.g., small mammals). As the Interaction Expected Outcome community continues to change, equilibrium species become established in the area. Equilibrium species, such as deer, wolves, and bears, are larger in size, Competition (− −) Abundance of both species long-lived, and slow to mature, and they produce only a few offspring per decreases. reproductive event. Predation (+ –) Abundance of predator increases, Interactions in Communities and abundance of prey decreases. Species interactions, especially competition for resources, fashion a commu- Parasitism (+ –) Abundance of parasite increases, nity into a dynamic system of interspecies relationships. In Table 31.1, the and abundance of host decreases. plus and minus signs show how the relationship affects the abundance of the two interacting species. Competition between two species for limited Commensalism (+ 0) Abundance of one species increases, and the other is not afected. Mutualism (+ +) Abundance of both species increases.
resources has a negative effect on the abundance of both species. In preda- CHAPTER 31 Communities and Ecosystems 605 tion, one animal, the predator, feeds on another, the prey (see Fig. 30.18); in parasitism, one species obtains nutrients from another species, called the Figure 31.6 Niche of a backswimmer. host, but usually does not kill the host. Commensalism is a relationship in which one species benefits, while the second species is neither harmed nor Backswimmers require warm, clear pond water containing insects benefited. Commensalism often occurs when one species provides a home or that they can eat and vegetation where they can hide from transportation for another. In mutualism, two species interact in such a way predators. that both benefit. © Steve Austin/Papilio/Corbis Ecological Niche Each species occupies a particular position in the community, in both a spatial and a functional sense. Spatially, species live in a particular area of the com- munity, or habitat, such as underground, in the trees, or in shallow water. Functionally, each species plays a role, such as photosynthesizer, predator, prey, parasite, or decomposer. The ecological niche of a species incorporates the role the species plays in its community, its habitat, and its interactions with other species. The niche includes the living and nonliving resources that indi- viduals in the population need to meet their energy, nutrient, and survival de- mands. For example, the habitat of an insect called a backswimmer is a pond or lake, where it eats other insects (Fig. 31.6). The pond or lake must contain vegetation where the backswimmer can hide from predatory fish and birds. The pond water must be clear enough for the backswimmer to see its prey and warm enough for it to maintain a good metabolic rate. It is often difficult to describe and measure the whole ecological niche of a species in a community, so ecologists often focus on a certain aspect of a species’ niche, as with the birds featured in Figure 31.7. Competition Competition for resources, such as light, space, or nutrients, contributes to the niche of each species and helps form the community structure. Laboratory Oystercatchers pry Plovers dart around open bivalve shells with on beaches and their knifelike bills and grasslands, hunting probe sand for worms for insects and small and crabs. invertebrates. Dabbling ducks feed Avocets feed on insects, Figure 31.7 Feeding niches for wading birds. by tipping, tail up, to small marine invertebrates, reach aquatic plants, and seeds by sweeping Flamingos feed in deeper water by ilter feeding; dabbling seeds, snails, and their bills from side to side ducks feed in shallower areas by upending; avocets feed by insects. in shallow water. sifting. Oystercatchers and plovers have adaptations, such as shorter legs, for feeding in shallows and on land. Flamingos feed on small molluscs, crustaceans, and vegetable matter strained from mud pumped through their bills by their power- ful tongues.
606 PART SEVEN Ecology Population experiments helped ecologists formulate the competitive exclusion principle, Densities which states that no two species can occupy the same niche at the same time. In the 1930s, G. F. Gause grew two species of Paramecium in one test tube P. aurelia grown separately P. caudatum grown separately containing a fixed amount of bacterial food. Although populations of each Time (days) Time (days) species survived when grown in separate test tubes, only one species, Parame- cium aurelia, survived when the two species were grown together (Fig. 31.8). Population P. aurelia acquired more of the food and had a higher population growth rate Densities than did P. caudatum. Eventually, as the P. aurelia population grew and obtained an increasingly greater proportion of the food resource, the number of Both species grown together P. caudatum individuals decreased and the population died out. Time (days) Niche Specialization Competition for resources does not always lead to Figure 31.8 Competitive exclusion principle demonstrated localized extinction of a species. Multiple species coexist in communities by partitioning, or sharing, resources. In another laboratory experiment using by Paramecium. other species of Paramecium, Gause found that the two species could survive in the same test tube if one species consumed bacteria at the bottom of the tube The competitive exclusion principle states that no two species and the other ate bacteria suspended in solution in the middle of the tube. This occupy exactly the same niche. When grown separately, resource partitioning decreased competition between the two species, leading Paramecium caudatum and Paramecium aurelia exhibit logistic to increased niche specialization. One niche was split into two due to differ- growth. When grown together, P. aurelia excludes P. caudatum. ences in feeding behavior. Data from G. F. Gause, The Struggle for Existence, 1934, Williams & Wilkins When three species of ground finches of the Galápagos Islands live on Company, Baltimore, MD, p. 557. the same island, their beak sizes differ, and each feeds on different-sized seeds (Fig. 31.9). When the finches live on separate islands, their beaks tend to be the G. fuliginosa G. fortis G. magnirostris same intermediate size, enabling each to feed on a wider range of seeds. Such character displacement often is viewed as evidence that competition and Percent of Sample 50 resource partitioning have taken place. 30 10 The niche specialization that permits the coexistence of multiple species can be very subtle. Species of warblers that live in North American forests are Abingdon, Bindloe, James, Jervis Islands all about the same size, and all feed on budworms, a type of caterpillar found on spruce trees. Robert MacArthur recorded the length of time each warbler 40 G. fortis species spent in different regions of spruce canopies to determine where each 20 species did most of its feeding. He discovered that each species primarily used different parts of the tree canopy and, in that way, had a more specialized niche. 0 As another example, consider that three types of birds—swallows, swifts, and Daphne Island martins—all eat flying insects and parachuting spiders. These birds even fre- quently fly in mixed flocks. But each type of bird has a different nesting site 40 G. fuliginosa and migrates at a slightly different time of year. 20 Mutualism 0 Crossman Island Mutualism, a symbiotic relationship in which both members benefit, is at least Beak Depth as important as competition in shaping community structure. The relationship between plants and their pollinators mentioned previously is a good example of Figure 31.9 Character displacement in inches on the mutualism. Perhaps the relationship began when herbivores feasted on pollen. The plant’s provision of nectar may have spared the pollen and, at the same Galápagos Islands. time, allowed the animal to become an agent of pollination. Over time, pollina- tor mouthparts have become adapted to gathering the nectar of a particular When Geospiza fuliginosa, G. fortis, and G. magnirostris coexist on plant species. This species has become dependent on the pollinator for dispers- the same island, their beak sizes are appropriate for eating small, ing its pollen. As also mentioned previously, lichens can grow on rocks be- medium-size, and large seeds, respectively. When G. fortis and G. cause the fungal member extracts minerals from the rocks, which are provided fuliginosa are on separate islands, their beaks have the same to the algal partner. The algal partner, in turn, photosynthesizes and provides intermediate size, which allows them to eat seeds of various sizes. organic food for both members of the relationship. Character displacement is evidence that resource partitioning has occurred. In tropical America, ants form mutualistic relationships with certain plants. The bullhorn acacia tree is adapted to provide a home for ants of the
species Pseudomyrmex ferruginea (Fig. 31.10). Unlike other acacias, this spe- a. cies has swollen thorns with a hollow interior, where ant larvae can grow and develop. In addition to housing the ants, acacias provide them with food. The ants feed from nectaries at the base of the leaves and eat fat- and protein- containing nodules, called Beltian bodies, at the tips of the leaves. The ants constantly protect the tree from herbivores that would like to feed on it. The ants are so critical to the trees’ survival that, when the ants on experimental trees were poisoned, the trees died. The outcome of mutualism is an intricate web of species interdepen- dencies critical to the community. For example, in areas of the western United States, the branches and cones of whitebark pine are turned up- ward, meaning that the seeds do not fall to the ground when the cones open. Birds called Clark’s nutcrackers eat the seeds of whitebark pine trees and store them in the ground (Fig. 31.11). Grizzly bears find the stored seeds and consume them. Thus, Clark’s nutcrackers and grizzly bears are critical seed dispersers for the trees. White- bark pine seeds do not germinate unless their seed coats are exposed to fire. When natural forest fires in the area are sup- pressed, whitebark pine trees decline in number, and so do Clark’s nutcrackers and grizzly bears. When lightning-ignited fires are allowed to burn, or prescribed burning is used in the area, the white- b. bark pine populations increase, as do the populations of Clark’s nutcrackers and grizzly bears. Beltian bodies c. Figure 31.10 Mutualism. The bullhorn acacia tree is adapted to provide nourishment for a mutualistic ant species. a. The thorns are hollow, and the ants live inside. b. The bases of the leaves have nectaries (openings) where ants can feed. c. The tips of the leaves of the bullhorn acacia have Beltian bodies, which ants harvest for larval food. (a): © WILDLIFE GmBh/Alamy; (b): © Carol Farneti–Foster/Oxford Scientiic/Getty Images; (c): © Bazzano Photography/Alamy Figure 31.11 Interdependence of species. a. The Clark’s nutcracker feeds on the seeds of the whitebark pine. The nutcracker also stores seeds in the ground, and these seeds may be found and eaten by (b) the grizzly bear. Clark’s nutcrackers and grizzly bears disperse the seeds of this species of pine. (a): © Peter Chadwick/Alamy; (b): © ArCaLu/ Shutterstock RF 607 a. b.
608 PART SEVEN Ecology Connections: Scientiic Inquiry Do humans form mutualistic relationships with other organisms? One of the best examples of mutualism be- tween humans and another species involves the bacteria Escherichia coli that live in our large intestine. While some strains of E. coli have a bad reputation for causing intestinal problems, the majority of the E. coli in our intestine are beneicial. In exchange for a warm environment, and plenty of food, the © Science Photo Library RF/Getty RF intestinal E. coli produce vitamin K and assist in the breakdown of iber into glucose. Community Stability As demonstrated by the phenomenon of succession, community stability is fragile. However, some communities have one species that stabilizes the com- munity, helps maintain its characteristics, and plays a critical role in holding the web of interactions together. Such a species is known as a keystone species, a species on which the majority of the community depends. The term keystone comes from the name for the center stone of an arch that holds the other stones in place, so that the arch can keep its shape. Keystone species are not necessarily the most numerous in the commu- nity. However, the loss of a keystone species can lead to the extinction of other species and a loss of diversity. For example, bats are designated as keystone species in tropical forests. Bats are pollinators, and they disperse the seeds of certain tropical trees. When bats are killed off or their roosts are destroyed, many species of trees will fail to reproduce. The grizzly bear is a keystone species in the northwestern United States and Canada; they disperse as many as 7,000 berry seeds in one dung pile. Grizzly bears also kill the young of many herbivorous mammals, such as deer, thereby keeping their populations under control. The sea otter is a keystone species of a kelp forest ecosystem. Kelp forests, created by large, brown seaweeds, provide a home for a vast assortment of organisms. The kelp forests occur just off the coast and protect coastline ecosystems from damaging wave action. Among other species, sea otters eat sea urchins, keeping their population size in check. Otherwise, sea urchins feed on the kelp, causing the kelp forest and its associated species to severely de- cline. Fishermen don’t like sea otters because they also prey on abalone, a mol- lusc prized for its commercial value. Many do not realize that, without the otters, abalone and many other species would not be around, because the kelp forest would no longer exist. Native Versus Exotic Species Native species are species indigenous to an area because they have evolved to fit into the particular community. For example, you naturally find maple trees in Vermont and many other states of the eastern United States. The introduc- tion of exotic species (also sometimes called alien or nonnative species), into a community greatly disrupts normal interactions, and therefore changes a community’s web of species. Populations of exotic species may grow exponen- tially, because they outcompete the native species or because their population
CHAPTER 31 Communities and Ecosystems 609 a. b. Figure 31.12 Exotic species. Human introduction of exotic species, such as (a) the Asian carp and (b) the emerald ash borer, have disrupted communities across the entire midwestern United States. (a): © Jason Lindsey/Alamy; (b-1): © J. D. Pooley/AP Images; (b-2): Stephen Ausmus/USDA size is not controlled by a natural predator or disease. The unique assemblage 31.1 CONNECTING THE CONCEPTS of native species on an island often cannot compete well against an exotic spe- cies. For example, myrtle trees, introduced to the Hawaiian Islands from the Community interactions are Canary Islands, are mutualistic with a type of bacterium capable of fixing complex and dynamic. atmospheric nitrogen. The bacterium provides the tree with a form of nitrogen it can use. This feature allows the trees to become established on nitrogen-poor volcanic soil, a distinct advantage in Hawaii. Once established, myrtle trees prevent the normal succession of native plants on volcanic soil. Exotic species disrupt communities in continental areas as well. Asian carp were accidentally introduced into the Mississippi River system and have spread throughout the majority of the Midwest. They have significantly de- creased the populations of many native species of fish (Fig. 31.12a). The em- erald ash borer was accidentally introduced to the United States from Asia back in 2002 (Fig. 31.12b). Since then, it has spread throughout 19 states, wiping out large numbers of ash trees in the process. On the Galápagos Islands, black rats accidentally carried to the islands by ships have reduced populations of the giant tortoise. Goats and feral pigs have changed the vegetation on the islands from highland forest to pampaslike grasslands and destroyed stands of cac- tuses. In the United States, gypsy moths, zebra mussels, the Chestnut blight fungus, fire ants, and African bees are well-known exotic species that have killed native species. At least two species, fire ants and African bees, have attacked humans, with serious consequences. Check Your Progress 31.1 1. Describe an example of coevolution. 2. Contrast species richness with species diversity. 3. Contrast primary succession with secondary succession. 4. List the ive major types of species interactions in a community that determine an organism’s ecological niche.
610 PART SEVEN Ecology 31.2 Ecology of Ecosystems Figure 31.13 Producers. Learning Outcomes Green plants and algae are photoautotrophs. Upon completion of this section, you should be able to (tree): © Authors Image/PunchStock RF; (diatoms): © Ed Reschke 1. Describe the role of autotrophs and heterotrophs in an ecosystem. 2. Compare how energy and chemicals interact with an ecosystem. 3. Describe the diferent trophic levels and the formation of ecological pyramids. 4. Summarize the biogeochemical cycles, and state how human activity is inluencing each cycle. 5. Understand the consequences of global climate change and its relationship to the carbon cycle. An ecosystem is broader than a community, because community ecology con- siders only how species interact with one another. When studying ecosystem ecology, interactions with the physical environment are also considered. For example, one important aspect of an ecological niche is how an organism ac- quires food. It is obvious that autotrophs interact with the physical environ- ment, but so do heterotrophs. a. Herbivores Autotrophs Autotrophs take in only inorganic nutrients (e.g., CO2 and minerals) and rely on an outside energy source to produce organic nutrients for their own use and for all the other members of a community. They are called producers because they produce food. Photoautotrophs are photosynthetic organisms that pro- duce most of the organic nutrients for the biosphere (Fig. 31.13). Algae of all types possess chlorophyll and carry on photosynthesis in fresh water and marine habitats. Algae make up the phytoplankton, which are photosynthesiz- ing organisms suspended in water. Green plants are the dominant photosyn- thesizers on land. All photosynthesizing organisms release O2 into the atmosphere. Some bacteria are chemoautotrophs. They obtain energy by oxidizing inorganic compounds, such as ammonia, nitrites, and sulfides, and they use this energy to synthesize organic compounds. Chemoautotrophs have been found to support communities in some caves and at hydrothermal vents along deep-sea oceanic ridges. b. Carnivores Heterotrophs Figure 31.14 Consumers. Heterotrophs consume preformed organic nutrients and release CO2 into the atmosphere. They are called consumers because they consume food. Herbi- a. Caterpillars and girafes are herbivores. b. A praying mantis and a vores are animals that graze directly on algae or plants (Fig. 31.14a). In lion are carnivores. aquatic habitats, zooplankton act as herbivores; in terrestrial habitats, many (a): (caterpillar): © Corbis RF; (girafes): © George W. Cox; (b): (mantis): © Kristina species of insects play that role. Carnivores eat other animals; for example, a Postnikova/Shutterstock RF; (lion): © Sue Green/Shutterstock RF praying mantis catches and eats caterpillars, and a lion hunts for game (Fig. 31.14b). These examples illustrate that there are primary consumers (e.g., caterpillars), secondary consumers (e.g., praying mantis), and tertiary consumers (e.g., lion). Sometimes tertiary consumers are called top predators. Omnivores are animals that eat both plants and animals. As you likely know, humans are omnivores.
CHAPTER 31 Communities and Ecosystems 611 Connections: Environment How do chemoautotrophs on the ocean loor produce food? The chemoautotrophs on the ocean loor use chemical energy instead of sunlight to make food. Volcanoes on the ocean loor release hydrogen sulide gas through cracks called hydrothermal vents. (Hydrogen sulide is the nasty-smelling gas we associate with the smell of rotten eggs.) Some chemoautotrophs split hydrogen sulide to obtain the energy © B. Murton/Southampton Oceanography Centre/Science Source needed to link carbon atoms together to form glucose. The glucose contained in these chemoautotrophs sustains a variety of bizarre organisms, such as giant tube worms, anglerish, and giant clams. The decomposers are heterotrophic bacteria and fungi, such as molds 10,000 × and mushrooms, that break down dead organic matter (Fig. 31.15). Decomposers perform a very valuable service, because they release inorganic nutrients (CO2 Figure 31.15 Decomposers. and minerals), which are then taken up by plants once more. Otherwise, plants would rely on minerals to be slowly released from rocks. Detritus is composed Fungi and bacteria are decomposers. of the remains of dead organisms plus the bacteria and fungi that aid in decay. (mushrooms): © BadZTuA/Getty RF; (bacteria): © Science Photo Library/Alamy RF Fanworms feed on detritus floating in marine waters, while clams take detritus from the sea bottom. Earthworms and some beetles, termites, and maggots are soil detritus feeders. Energy Flow and Chemical Cycling heat heat The living components of ecosystems process energy and chemicals. Energy flow through an ecosystem begins when producers absorb solar en- ergy. Chemical cycling then begins when producers take in inorganic nutrients from the physical environment (Fig. 31.16). Thereafter, via solar photosynthesis, producers convert the solar energy and inorganic nu- energy trients into chemical energy in the form of organic nutrients, such as carbohydrates. Producers synthesize organic nutrients directly for producers consumers themselves and indirectly for the heterotrophs within the ecosystem. Energy flows through an ecosystem because, as organic nutrients Key: pass from one component of the ecosystem to another, as when an herbivore eats a plant or a carnivore eats an herbivore, a portion is energy used as an energy source. Eventually, the energy dissipates into the nutrients environment as heat. Therefore, the vast majority of ecosystems can- inorganic decomposers nutrient pool not exist without a continual supply of solar energy. Only a portion of the organic nutrients made by producers are passed on to consumers. Plants use some of the organic molecules to fuel their own cel- lular respiration. Similarly, only a small percentage of nutrients consumed by lower-level consumers, such as herbivores, are available to higher-level con- sumers, or carnivores. As Figure 31.17 demonstrates, a certain amount of the heat nutrients eaten by an herbivore is eliminated as feces. Metabolic wastes are excreted as urine. Of the assimilated energy, a large portion is used during Figure 31.16 Chemical cycling and energy low. cellular respiration, which produces ATP, as well as heat. Only the remaining Chemicals cycle within, but energy lows through, an ecosystem. As energy, which is converted into increased body weight or additional offspring, energy is repeatedly passed from one component to another, all the becomes available to carnivores. chemical energy derived from solar energy dissipates as heat.
612 PART SEVEN Ecology Heat to environment cellular respiration growth and Energy to reproduction carnivores death excretion defeca tion Energy to detritus Figure 31.17 Energy balances. feeders Only about 10% of the nutrients and energy taken in by an herbivore is passed on to carnivores. A large portion goes to detritus feeders. The elimination of feces and urine by a heterotroph, and indeed the death Another large portion is used for cellular respiration. of any organism, does not mean that organic nutrients are lost to an ecosystem. © Brand X Pictures/PunchStock RF Instead, the organic nutrients are made available to decomposers. Decomposers convert the organic nutrients back into inorganic chemicals and release them to the soil or atmosphere. Chemicals complete their cycle in an ecosystem when producers absorb inorganic chemicals from the atmosphere or soil. Energy Flow Applying the principles discussed so far to a temperate deciduous forest, ecol- ogists can draw a food web to represent the interconnecting paths of energy flow between the components of the ecosystem. In Figure 31.18, the green arrows are part of a grazing food web, because the energy flow begins with plants, such as the oak trees depicted. A detrital food web (orange arrows) begins with bacteria and fungi. In the grazing food web, caterpillars and other herbivorous insects feed on the leaves of the trees, while other herbivores, in- cluding mice, rabbits, and deer, feed on leaves at or near the ground. Various birds, chipmunks, and mice feed on fruits and nuts of the trees, but, in fact, they are omnivores because they also feed on caterpillars and other insects. These herbivores and omnivores all provide food for a number of different carnivores. In the detrital food web, detritus, which includes smaller decomposers (such as bacteria and fungi), is food for larger organisms. Because some of these organ- isms, such as shrews and salamanders, become food for aboveground animals, the detrital and grazing food webs are joined. We tend to think that the aboveground parts of trees are the largest stor- age form of organic matter and energy, but this is not necessarily the case. In temperate deciduous forests, the organic matter lying on the forest floor and mixed into the soil, along with the underground roots of the trees, contains over twice the energy of the leaves, branches, and trunks of living trees combined. Therefore, more energy and matter in a forest may be stored in or funneled through the detrital food web than the grazing food web.
CHAPTER 31 Communities and Ecosystems 613 Key: birds hawks grazing food web detrital food web fruits and nuts chipmunks mice owls leaf-eating snakes insects fishers leaves rabbits old leaves, skunks dead twigs deer shrews foxes salamanders bacteria and fungi carnivorous invertebrates invertebrates Trophic Levels and Ecological Pyramids The arrangement of component Figure 31.18 Food webs. species in Figure 31.19 suggests that organisms are linked to one another in a straight line according to feeding relationships, or who eats whom. Diagrams The grazing and detrital food webs of an ecosystem are linked. that show a single path of energy flow in an ecosystem are called food chains (Fig. 31.19). A trophic level is a level of nourishment within a food chain or web. In the grazing food web (see Fig. 31.18), from left to right, the trees are
614 PART SEVEN Ecology producers primary secondary tertiary consumers consumers consumers Figure 31.19 Food chain. A food chain diagrams a single path of energy low in an ecosystem. Most food chains have three or four links. 10% 10% 10% carnivores photosynthesizers herbivores top carnivores 10% producers (the first trophic level), the first series of animals are herbivores (the 1 10% second trophic level), and many of the animals in the next series are carnivores 10% (the third and possibly fourth trophic levels). carnivores 10 Food chains are short, because energy is lost between trophic levels. In general, only about 10% of the energy of one trophic level is available to the next herbivores trophic level. Therefore, if an herbivore population consumes 1,000 kg of plant 100 material, only about 100 kg is converted to herbivore tissue, 10 kg to first-level carnivores, and 1 kg to second-level carnivores. This 10% rule explains why few producers carnivores can be supported in a food web. The transfer of energy between succes- 1,000 sive trophic levels is sometimes depicted as an ecological pyramid (Fig. 31.20). Figure 31.20 Ecological pyramid. A pyramid based on the number of organisms can run into problems be- cause, for example, one tree can support many herbivores. Pyramids based on An ecological pyramid depicts the transfer of nutrients and energy biomass, which is the number of organisms multiplied by their weight, eliminate from one trophic level to the next. size as a factor. Even then, apparent inconsistencies can arise. In aquatic ecosys- tems, such as lakes and open seas where algae are the only producers, the herbi- vores at some point in time may have a greater biomass than the producers. Why? Even though the algae reproduce rapidly, they are also consumed at a high rate. Chemical Cycling The pathways by which chemicals cycle within ecosystems involve both living components (producers, consumers, decomposers) and nonliving components (rock, inorganic nutrients, atmosphere) and therefore are known as biogeo- chemical cycles. Biogeochemical cycles can be sedimentary or gaseous. In a sedimentary cycle, such as the phosphorus cycle, the element (chemical) is absorbed from the sediment by plant roots, passed through the food chain, and eventually returned to the soil by decomposers, usually in the same general area. In a gaseous cycle, such as the nitrogen and carbon cycles, the element returns to and is withdrawn from the atmosphere as a gas. Chemical cycling of an element may involve reservoirs and exchange pools as well as the biotic community (Fig. 31.21). A reservoir is a source nor- mally unavailable to organisms. For example, much carbon is found in calcium carbonate shells in sediments on the ocean floors. An exchange pool is a source from which organisms can obtain elements. For example, photosynthesizers can use carbon dioxide in the atmosphere for their carbon needs. The biotic com- munity consists of the autotrophic and heterotrophic species of an ecosystem that feed on one another. Human activities, such as mining or burning fossil fuels, increase the amounts of chemical elements removed from reservoirs and cycling within ecosystems. As a result, the physical environment of the
CHAPTER 31 Communities and Ecosystems 615 Reservoir producers fossil fuels mineral in rock Exchange pool sediment in oceans atmosphere soil water Biotic consumers community Key: human activities decomposers natural events Figure 31.21 Model for chemical cycling. Chemical nutrients cycle between these components of ecosystems: Reservoirs, such as fossil fuels, minerals in rocks, and sediments in oceans, are normally relatively unavailable sources, but exchange pools, such as those in the atmosphere, soil, and water, are available sources of chemicals for the biotic community. When human activities (purple arrows) remove chemicals from reservoirs and make them available to the biotic community, pollution can result. ecosystem contains excess chemicals, which in turn may alter the species com- Key: position and diversity of the biotic community. human activities Phosphorus Cycle mineable weathering natural events rock On land, the very slow weathering of rocks fostered by fungi adds phosphates (PO42– and HPO42–) to the soil, some of which phosphate sewage become available for uptake by terrestrial plants (Fig. 31.22). mining treatment plants Phosphates made available by weathering also run off into aquatic ecosystems, where algae absorb the phosphates geological fertilizer runo from the water before they become trapped in sediments. uplift plants Phosphates in sediments become available again only when a geological upheaval exposes sedimentary rocks phosphates phosphates to weathering once more. in soil in solution Producers use phosphates in a variety of mole- dead organisms Biotic cules, including phospholipids, ATP, and the nucleotides and animal wastes community Figure 31.22 The phosphorus cycle. decomposers biota detritus The phosphorus cycle is a sedimentary biogeochemical cycle. sedimentation Globally, phosphates low into large bodies of water and become a part of sedimentary rocks. Thousands or millions of years later, the sealoor can rise; the phosphates are then exposed to weathering and become available. Locally, phosphates cycle within a community when plants on land and algae in the water take them up. Animals gain phosphates when they feed on plants or algae. Decomposers return phosphates to plants or algae, and the cycle begins again.
616 PART SEVEN Ecology Figure 31.23 Lemmings. that become a part of DNA and RNA. Animals consume producers and incor- porate some of the phosphates into teeth, bones, and shells. Decomposition of The population size of lemmings inluences the mineral cycles of the dead plant and animal material and animal wastes does, however, make phos- Arctic ecosystem. phates available to producers faster than does weathering. Because most of the © Tom McHugh/Science Source available phosphates are used in food chains, phosphate is usually a limiting inorganic nutrient for ecosystems. In other words, the finite supply of phos- phates limits plant growth, and therefore primary productivity. The importance of phosphates and calcium to population growth is dem- onstrated by considering the fate of lemmings every 4 years (Fig. 31.23). You may have heard that lemmings dash mindlessly over cliffs into the sea; ecolo- gists tell us that these lemmings are actually migrating to find food. What hap- pened? Every 4 years or so, grasses and sedges of the tundra (see Fig. 31.30) become rich in minerals, and the lemming population starts to explode. Once the lemmings number in the millions, the grasses and sedges of the tundra suf- fer a decline caused by a lack of minerals. Then the lemming population suffers a crash, but it takes about 4 years before the animals decompose in this cold region and minerals return to the producers. Then the cycle begins again. Human Activities A transfer rate is the amount of a nutrient that moves from one component of the environment to another within a specified period of time. Figure 31.24 The nitrogen cycle. Human activities affect the dynamics of a community by altering transfer rates. The nitrogen cycle is a gaseous biogeochemical cycle normally For example, humans mine phosphate ores and use them to make fertilizers, ani- maintained by the work of several populations of soil bacteria. (bacteria inset): © Perennou Nuridsany/Science Source mal feed supplements, and detergents. Phosphate ores are slightly radioactive; N2 in atmosphere therefore, mining phosphate poses a health threat to all organisms, including the proteins miners. Animal wastes from livestock feedlots, fertilizers from lawns and crop- in animals and plants land, and untreated and treated sewage discharged from cities all add excess phos- phates to nearby waters. The result is eutrophication, or overenrichment of a Key: body of water, which causes a rapid algal population growth called an human activities algal bloom. When the algae die and natural events decay, oxygen is consumed, causing fish kills. In the mid-1970s, Lake Erie was dying because of eutrophication. Control of nutrient phosphates, particularly in sewage human effluent and household detergents, activities reversed the situation. Nitrogen Cycle plants Nitrogen, in the form of nitrogen gas nitrogen-fixing denitrifying (N2), constitutes about 78% of the at- bacteria in nodules bacteria mosphere by volume. But plants can- not make use of nitrogen gas. Instead, and soil plants rely on various types of bacte- ria to make nitrogen available to them. dead organisms Biotic Therefore, nitrogen, like phosphorus, and animal waste community is a limiting inorganic nutrient of pro- nitrates (NO3–) ducers in ecosystems. decomposers Plants can take up both ammonium (NH4+) and nitrate (NO3–) from the soil and nitrifying incorporate the nitrogen into amino acids ammonium bacteria and nucleic acids. Two processes, nitrogen fixation (NH4+) and nitrification, convert nitrogen gas, N2, into NH4+ and NO3–, respectively (Fig. 31.24). Nitrogen fixation occurs
CHAPTER 31 Communities and Ecosystems 617 when nitrogen gas is converted to ammonium. Some cyanobacteria in aquatic Figure 31.25 Root nodules. ecosystems and some free-living, nitrogen-fixing bacteria in soil are able to fix nitrogen in this way. Other nitrogen-fixing bacteria live in nodules on the roots Bacteria that live in nodules on the roots of plants in the legume of plants such as peas, beans, and alfalfa. They make organic compounds con- family, such as pea plants, convert nitrogen in the air to a form that taining nitrogen available to the host plants. plants can use to make proteins. © Biophoto Associates/Science Source Nitrification is the production of nitrates. Ammonium in the soil is con- verted to nitrate by certain nitrifying soil bacteria in a two-step process. First, nitrite-producing bacteria convert ammonium to nitrite (NO2–), and then nitrate-producing bacteria convert nitrite to nitrate. In Figure 31.24, notice that the biotic community subcycle in the nitrogen cycle does not depend on the presence of nitrogen gas. Denitrification is the conversion of nitrate to nitrogen gas, which enters the atmosphere. Denitrifying bacteria are chemoautotrophs that live in the an- aerobic mud of lakes, bogs, and estuaries; they carry out this process as part of their own metabolism. In the nitrogen cycle, denitrification counterbalanced nitrogen fixation until humans started making fertilizer. Human Activities When humans produce fertilizers from N2, they are alter- Key: ing the transfer rates in the nitrogen cycle. In fact, humans nearly double the human activities fixation rate. The nitrate in fertilizers, just like phosphate, can leach out of agri- natural events cultural soils into waterways, leading to eutrophication. Deforestation by humans also causes a loss of nitrogen to groundwater and makes regrowth of the forest CO2 in atmosphere difficult. The underground water supplies in farming areas today are more apt to contain excess nitrate. A high concentration of nitrates interferes with blood oxy- gen levels, and infants below the age of six months who drink water containing excessive amounts of nitrate can become seriously ill and, if untreated, may die. To cut back on fertilizer use, it might be possible to genetically engineer soil bacteria with increased nitrogen fixation rates. Also, farmers could grow various plants that increase the nitrogen content of the soil (Fig. 31.25). In one study, the rotation of legumes and winter wheat produced a better yield than fertilizers after several years. Carbon Cycle combustion In the carbon cycle, organisms in both terrestrial and aquatic photosynthesis di usion ecosystems exchange carbon dioxide with the atmosphere respiration (Fig. 31.26). On land, plants take up carbon dioxide from bicarbonate (HCO3–) the air. Then through photosynthesis, they incorporate plants runo ocean carbon into organic nutrients, which are used by both decay autotrophs and heterotrophs. When aerobic organisms CaCO3 respire, a portion of this carbon is returned to the atmo- dead organisms sedimentation sphere as carbon dioxide, a waste product of cellular and animal wastes respiration. In aquatic ecosystems, the exchange of carbon di- oxide with the atmosphere is indirect. Carbon dioxide from the air combines with water to produce bicarbonate ion (HCO3–), a source of carbon for algae that produce food Figure 31.26 The carbon cycle. The carbon cycle is a gaseous biogeochemical cycle. Producers take fossil fuels in carbon dioxide from the atmosphere and convert it to the organic molecules that feed all organisms. Fossil fuels arise when organisms die but do not decompose. The burning of fossil fuels releases carbon dioxide and causes environmental pollution.
618 PART SEVEN Ecology for themselves and for heterotrophs. Similarly, when aquatic organisms respire, the carbon dioxide they give off becomes bicarbonate ion. The amount of bicar- Connections: Environment bonate in the water is in equilibrium with the amount of carbon dioxide in the air. What causes ocean acidiication? Living and dead organisms contain organic carbon and serve as one of the reservoirs for the carbon cycle. The world’s biotic components, particularly The release of excessive CO2 into the atmosphere has led to a trees, contain billions of tons of organic carbon, and additional tons are esti- change in the basic chemistry of the ocean water. The ocean mated to be held in the remains of plants and animals in the soil. If dead plant absorbs about 25% of the CO2 in the atmosphere. As the CO2 and animal remains fail to decompose, they are subjected to extremely slow levels in the atmosphere increase, so do the levels in the physical processes that transform them into coal, oil, and natural gas, the fossil ocean, leading to ocean acidiication. As this occurs, many fuels. Most of the fossil fuels were formed during the Carboniferous period, marine organisms do not have the minerals they need to pro- 286–360 MYA, when an exceptionally large amount of organic matter was bur- duce and maintain their shells. The pH level of the ocean will ied before decomposing. Another reservoir is the calcium carbonate (CaCO3) shift to a more acidic environment, leading to stress and the that accumulates in limestone and shells. Many marine organisms have cal- possible extinction of many species of shellish and corals. cium carbonate shells that remain in bottom sediments long after the organisms Ocean acidiication is a global issue due to the potential im- have died. Over time, geological forces change these sediments into limestone. pact this will have on the entire marine food chain. Human Activities The transfer rates of carbon dioxide due to photosynthe- Mean Global Temperature Change (°C) 5.5 maximum likely increase sis and cellular respiration are just about even. However, more carbon dioxide 5.0 most probable temperature is being deposited in the atmosphere than is being removed. In 1850, 4.5 2100 atmospheric CO2 was at about 280 parts per million (ppm); today, it is over 4.0 increase for 2 × CO2 400 ppm. This increase is largely due to the burning of fossil fuels and the de- 3.5 struction of forests to make way for farmland and pasture. Today, the amount 3.0 minimum likely increase of carbon dioxide released into the atmosphere is about twice the amount that 2.5 remains in the atmosphere. CO2 concentrations in the atmosphere would rise 2.0 1900 1940 1980 2020 2060 higher, except for the fact that the oceans take up CO2, and so far they have 1.5 Year been taking up more CO2 than they vent. Despite a number of international 1.0 agreements to address carbon dioxide emissions, it is believed that atmospheric 0.5 concentrations will continue to rise until around 2100. 0.0 –0.5 The increased amount of carbon dioxide (and other gases) in the atmo- sphere is causing climate change to occur. These gases allow the sun’s rays to 1860 pass through, but they absorb and reradiate heat back to Earth, a phenomenon called the greenhouse effect. Figure 31.27 Global warming. We are already beginning to see the consequences of climate change. If Mean global temperature is expected to rise due to the addition of the Earth’s temperature continues to rise, more water will evaporate, forming greenhouse gases to the atmosphere. more clouds. This sets up a positive feedback effect that could increase global warming still more. The global climate has already warmed about 0.6°C (1.1°F) since the Industrial Revolution. Enhancements in computer science are allowing scientists to explore the majority of the variables that influence the global climate. Most climate scientists agree that the Earth’s temperature may rise 1.5–4.5°C (2.0–8.1°F) by 2100 if greenhouse emissions continue at the current rates (Fig. 31.27). Figure 31.28 shows how the average temperature in the United States has steadily increased over the past two centuries. As climate change contin- ues, other effects will possibly occur. It is predicted that, as the oceans warm, temperatures in the polar regions will rise to a greater degree than in other re- gions. As a result, sea levels will rise, because glaciers will melt, and water expands as it warms. Water evaporation will increase, and most likely there will be increased rainfall along the coasts and dryer conditions inland. The oc- currence of droughts will reduce agricultural yields and cause trees to die off. Furthermore, weather pattern changes might cause the American Midwest to become a dust bowl. Expansion of forests into Arctic areas might not offset the loss of forests in the temperate zones. Coastal agricultural lands, such as the deltas of Bangladesh and China, will be inundated with water. Billions of
CHAPTER 31 Communities and Ecosystems 619 dollars will have to be spent to keep coastal cities such Temperature change (°F per century): as New Orleans, New York, Boston, Miami, and Galveston from disappearing into the sea. -4 -3 -2 -1 01 234 As we explored in the chapter opener, the Gray interval: -0.1 to 0.1° changing climate will also have an impact on our personal lives. Over the past few decades, the Figure 31.28 Climate change in the United States. mosquito species Anopheles and Aedes have in- creased their range into the United States. These The average temperature in the United States has steadily increased over the are tropical species of mosquitoes, but increase past two centuries, leading to more severe droughts and more erratic periods of in global temperatures have reduced the severity precipitation that are altering the composition of many communities. of winters. These mosquitoes are vectors for diseases such as malaria, dengue fever, 31.2 CONNECTING THE CONCEPTS and Zika virus. As the climate warms, we will see more instances of these Ecosystems are deined by the diseases. cycling of energy and nutrients. Global disruptions in nutrient There are efforts to reduce cycles are causing problems with carbon dioxide emissions. In- ecosystem function. creasingly, you hear about alter- native sources of energy (see Section 32.4), and increased energy efficiency of cars and appliances. Glob- ally, international meetings, such as the re- cent Paris Climate Change Conference, are working to reduce carbon dioxide emissions to keep global temperatures from rising more than 2°C over pre- industrial levels. Connections: Ecology What are some other greenhouse gases? In addition to carbon dioxide, the following gases also play a role in the green- house efect: • Methane (CH₄ ). A single molecule of methane has 21 times the warming potential of a molecule of carbon dioxide, making it a powerful green- house gas. Methane is a natural by-product of the decay of organic material, but it is also released by landills and the production of coal, natural gas, and oil. • Nitrous oxide (N₂O). Nitrous oxide is released from the combustion of fossil fuels and as gaseous waste from many industrial activities. • Hydrofluorocarbons. These were initially produced to reduce the levels of ozone-depleting compounds in the upper atmosphere. Unfortunately, although present in very small quantities, they are potent greenhouse gases. Check Your Progress 31.2 1. Compare how autotrophs and heterotrophs acquire energy. 2. Describe the diferences between energy low and chemical cycling in an ecosystem. 3. Compare an ecological pyramid with a food chain. 4. Explain how human activity is inluencing each of the biogeochemical cycles.
620 PART SEVEN Ecology 31.3 Ecology of Major Ecosystems a. Learning Outcomes Upon completion of this section, you should be able to 1. List the two major types of ecosystems that make up the biosphere. 2. List the major types of terrestrial ecosystems. 3. Explain primary productivity, and relate this concept to the diferent types of aquatic and terrestrial ecosystems. The biosphere, which encompasses all the ecosystems on Earth, is the final level of biological organization. Aquatic ecosystems are divided into those associated with fresh water and those associated with salt water (marine eco- systems; Fig. 31.29). The ocean, a marine ecosystem, covers 70% of the Earth’s surface. Two types of freshwater ecosystems are those with standing water, such as lakes and ponds, and those with running water, such as rivers and streams. The richest marine ecosystems lie near the coasts. Coral reefs are located offshore, while estuaries occur where rivers meet the sea. b. c. d. Figure 31.29 The major aquatic ecosystems. Aquatic ecosystems are divided into those associated with salt water, such as (a) the ocean, and (b) those with fresh water, such as a river. Saltwater, or marine, ecosystems also include (c) coral reefs and (d) estuaries. (a): © Spondylolithesis/Getty RF; (b): © Creatas RF/PunchStock RF; (c): © Olga Khoroshunova/Shutterstovck RF; (d): © Corbis RF
Scientists recognize several distinctive major types of terrestrial eco- CHAPTER 31 Communities and Ecosystems 621 systems, also called biomes (Fig. 31.30). Temperature and rainfall define the biomes, which contain communities adapted to the regional climate. The Connections: Environment northernmost biome is the tundra. A permafrost persists year-round and prevents large plants from becoming established. The taiga is a very cold What is an estuary? northern coniferous forest, and the tundra, which borders the North Pole, is also very cold, with long winters and a short growing season. Temperate An estuary is a region where fresh grasslands receive less rainfall than temperate deciduous forests (in which water, usually from a river, meets the trees lose their leaves during the winter) and more water than deserts, which ocean. The water in an estuary zone lack trees. The savanna is a tropical grassland with a high temperature and is often called “brackish,” because it alternating wet and dry seasons. The tropical rain forests, which occur near is a mixture of salt and fresh water. the equator, have a high average temperature and the greatest amount of Estuaries are considered to be some rainfall of all the biomes. They are dominated by large, evergreen, broad- of the most productive ecosystems leaved trees. in the world. Not only do they pro- vide food sources, such as shrimp, Primary Productivity clams, and other seafood, but they One way to compare ecosystems is to look at primary productivity, the rate © Roger de la Harpe/Gallo at which producers capture and store energy as organic nutrients over a certain length of time. Temperature and moisture, and secondarily the nature of the also play an important role in the Images/Getty Images soil, influence the primary productivity, which determines the assemblage of chemical cycling of nitrogen and phosphorus. Estuaries are species in an ecosystem. In terrestrial ecosystems, primary productivity is gen- fragile ecosystems; pollution, oil spills (such as the one that erally lowest in high-latitude tundras and deserts, and it is highest near the threatened Louisiana’s Bartaria Bay in 2010), and human activ- ity can easily reduce the productivity of an estuary. 30°N Equator 30°S Tropical rain forest Cold desert Tundra Tropical deciduous forest Mountain ranges Temperate grassland (prairie) Tropical grassland (savanna) Temperate deciduous forest Polar ice cap Hot desert Taiga and temperate rain forest Figure 31.30 The major terrestrial ecosystems. The tundra is the northernmost terrestrial ecosystem and has the lowest average temperature of all the terrestrial ecosystems, with minimal to moderate rainfall. The taiga, a coniferous forest that encircles the globe, also has a low average temperature, but moderate rainfall. Temperate forests have moderate temperatures and occur where rainfall is moderate yet suicient to support trees. Tropical rain forests, which generally occur near the equator, have a high average temperature and the greatest amount of rainfall of all the terrestrial ecosystems. (taiga): © John E. Marriott/All Canada Photos/Getty Images; (temperate deciduous forest): © David Sucsy/E+/Getty RF; (tundra): © Stockbyte/Getty RF; (tropical rain forest): © Digital Vision/ Getty RF
622 PART SEVEN Ecology Figure 31.31 Primary productivity. estuaries, swamps, and marshes Ecologists can compare ecosystems based on tropical primary productivity, the rate at which rain forests producers convert and store solar energy as chemical energy. coral reefs temperate deciduous forests temperate grasslands (prairies) lakes and streams rocky beaches and sandy beaches tundras open oceans deserts 800 1,600 2,400 3,200 4,000 4,800 5,600 6,400 7,200 8,000 8,800 9,600 Average Net Primary Productivity (kcal/m2/yr) equator, where tropical forests occur (Fig. 31.31). The high productivity of tropical rain forests provides varied niches and lots of food for consumers. The number and diversity of species in tropical rain forests are the highest of all the terrestrial ecosystems. Therefore, conservation biologists are interested in preserving as much of this biome as possible. The primary productivity of aquatic communities is largely dependent on the availability of inorganic nutrients. Estuaries, swamps, and marshes are rich in organic nutrients and in decomposers that convert those organic nutri- ents into their inorganic chemical components. Estuaries, swamps, and marshes also contain a large variety of species, particularly in the early stages of their development before they venture forth into the sea. Therefore, all of these coastal regions are in great need of preservation. The open ocean has a produc- tivity between that of a desert and that of the tundra, because it lacks a concen- trated supply of inorganic nutrients. Coral reefs exist near the coasts in warm tropical waters, where currents and waves bring nutrients and sunlight pene- trates to the ocean floor. Coral reefs are areas of remarkable biological abun- dance, equivalent to that of tropical rain forests. 31.3 CONNECTING THE CONCEPTS Aquatic and terrestrial ecosystems are connected. Check Your Progress 31.3 1. Describe the two major types of ecosystems that make up the biosphere. 2. List the major terrestrial ecosystems. 3. Explain why swamps have higher levels of primary productivity than do open oceans.
CHAPTER 31 Communities and Ecosystems 623 STUDY TOOLS http://connect.mheducation.com Maximize your study time with McGraw-Hill SmartBook®, the irst adaptive textbook. SUMMARIZE Autotrophs The autotrophs are the producers. They require only inorganic nutrients All of the species in a given area make up a community. Energy flow and (e.g., CO2 and minerals) and an outside energy source to produce organic chemical cycling are essential for the stability of both aquatic and terrestrial nutrients for their own use and for the use of other members of the ecosystems. community. Examples of autotrophs are algae, cyanobacteria, and plants. 31.1 Community interactions are complex and dynamic. Heterotrophs Ecosystems are defined by the cycling of energy and nutrients. Global The heterotrophs are consumers. They require a preformed source of organic nutrients and give off CO2. Examples of heterotrophs are herbivores 31.2 disruptions in nutrient cycles are causing problems with ecosystem (feed on plants), carnivores (feed on animals), and omnivores (feed on both function. plants and animals). Other heterotrophs are the decomposers (the bacteria and fungi that aid in decay by breaking down detritus). 31.3 Aquatic and terrestrial ecosystems are connected. Energy Flow and Chemical Cycling 31.1 Ecology of Communities Energy flows through an ecosystem, while chemicals cycle within an ecosystem. Knowledge of community and ecosystem ecology is important for understanding the impacts of human activities on the environment. heat heat ∙ A community is an assemblage of the populations of different species solar consumers interacting with each other in a given area. Coevolution has shaped the energy interactions of many species. Communities are often characterized by their diversity and species richness. producers ∙ An ecosystem consists of species interacting with one another and with Key: inorganic decomposers the physical environment. nutrient pool energy Ecological Succession nutrients bare rock → lichens/mosses → grasses → shrubs → trees The two types of ecological succession are primary succession (begins on heat bare rock) and secondary succession (following a disturbance; begins where soil is present). Ecological succession leads to a stable climax community. ∙ Energy flows within an ecosystem through food chains and detrital food webs and grazing food webs. Interactions in Communities Species in communities interact with one another in the following ways: ∙ A trophic level is a level of nourishment in a food chain or web. ∙ An ecological pyramid diagrams the energy losses that occur between ∙ Competition. Species vie with one another for resources, such as light, space, and nutrients. Aspects of competition are the competitive exclusion trophic levels. Only about 10% of the energy of one trophic level is principle, resource partitioning, and character displacement. available to the next trophic level. Top carnivores occupy the topmost and smallest trophic level. The biomass of a trophic level is the total ∙ Predation. One species (predator) eats another species (prey). weight of the living organisms at that level. ∙ Parasitism. One species (parasite) obtains nutrients from another Biogeochemical Cycle species (host) but does not kill the host species. Chemicals cycle within ecosystems through various biogeochemical cycles, ∙ Commensalism. One species benefits from the relationship, while the such as the phosphorus cycle, the nitrogen cycle, and the carbon cycle. The transfer rate is the amount of a nutrient that moves from one component of other species is neither harmed nor benefited. the environment to another within a specified time. ∙ Mutualism. Two species interact in a way that benefits both. Human activities significantly alter the transfer rates in the Ecological Niche biogeochemical cycles. The ecological niche of a species is defined by the role it plays in its community, its habitat, and its interactions with other species. ∙ In the phosphorus and nitrogen cycles, excess nutrients being released into the environment cause eutrophication. Keystone Species The interactions of a keystone species in the community hold the community and its species together. Removal of a keystone species can lead to extinction of other species and loss of diversity. An example of a keystone species is the grizzly bear. Native Versus Exotic Species Native species are indigenous to a given area and thrive without assistance. Exotic species are introduced into an area and may disrupt the balance and interactions among native species in that area’s community. 31.2 Ecology of Ecosystems In a food chain in an ecosystem, some populations are autotrophs and some are heterotrophs.
624 PART SEVEN Ecology 31.2 Ecology of Ecosystems ∙ In the carbon cycle, the burning of fossil fuels has increased the level of 8. In the following diagram, fill in the components of chemical cycling and greenhouse gases in the atmosphere, resulting in global warming and nutrient flow. climate change. c. d. 31.3 Ecology of Major Ecosystems solar e. f. The biosphere encompasses all the ecosystems on Earth. energy h. g. Aquatic Ecosystems Key: The aquatic ecosystems are classified as freshwater ecosystems (rivers, streams, a. lakes, ponds) and marine ecosystems (oceans, coral reefs, saltwater marshes). b. Terrestrial Ecosystems The terrestrial ecosystems are called biomes. The major biomes are the tundra, taiga, temperate deciduous forest, tropical grassland (savanna), temperate grassland (prairie), desert, and tropical rain forest. Primary Productivity Primary productivity is the rate at which producers capture and store energy and convert it to organic nutrients over a certain length of time. The number of species in an ecosystem is positively related to its primary productivity. ASSESS i. Testing Yourself 9. An ecological pyramid depicts the amount of _______ in various trophic levels. Choose the best answer for each question. a. food b. organisms 31.1 Ecology of Communities c. energy d. waste 1. As diversity increases, a. species richness increases and the distribution of species becomes 10. Which of the following represents a grazing food chain? more even. a. leaves → detritus feeders → deer → owls b. species richness decreases and the distribution of species becomes b. birds → mice → snakes more even. c. nuts → leaf-eating insects → chipmunks → hawks c. species richness increases and the distribution of species becomes d. leaves → leaf-eating insects → mice → snakes less even. d. species richness decreases and the distribution of species becomes 11. Identify the components of the ecological pyramid in the following less even. diagram. For statements 2–6, indicate the type of interaction described in each scenario. a. Key: b. a. competition d. commensalism c. b. predation e. mutualism c. parasitism d. 2. An alfalfa plant gains fixed nitrogen from the bacterial species 12. Ecosystems include which of the following components that Rhizobium living on its root system, while Rhizobium gains communities do not? carbohydrates from the plant. a. energy transfer between members b. interaction with the physical environment 3. Both foxes and coyotes in an area feed primarily on a limited supply of c. intricate food webs rabbits. d. herbivores and carnivores 4. Roundworms live and reproduce within a cat’s digestive tract. 5. A fungus captures nematodes as a food source. 6. An orchid plant lives in the treetops, gaining access to sunlight and pollinators but not harming the trees. 7. According to the competitive exclusion principle, a. one species is always more competitive than another for a particular food source. b. competition excludes multiple species from using the same food source. c. no two species can occupy the same niche at the same time. d. competition limits the reproductive capacity of species.
31.3 Ecology of Major Ecosystems CHAPTER 31 Communities and Ecosystems 625 13. Which biome is characterized by a coniferous forest with low average 2. Sea otters play an important role in maintaining the kelp forests of the temperature and moderate rainfall? Pacific coast. Otters maintain the sea urchin population, preventing the sea urchins from overgrazing the kelp beds. If the kelp beds are a. taiga d. temperate deciduous forest overgrazed, a multitude of other species will decline as a result. Identify the chain of events that could occur if the sea otter population b. savanna e. tropical rain forest were reduced along the Pacific coast. c. tundra 3. Many exotic species, such as zebra mussels and sea lampreys, are so obviously troublesome that most people do not object to programs 14. Which biome has the lowest primary productivity? aimed at controlling their populations. However, some eradication programs directed toward exotic species meet with more resistance. For a. tundra d. prairie example, the mute swan, one of the world’s largest flying birds, is beautiful and graceful, and it has an impressive presence. However, it is b. lake e. temperate deciduous forest very aggressive and territorial. The mute swan was introduced to the United States from Asia and Europe in the nineteenth century as an c. sandy beach ornamental bird but has since established a large wild population. The birds consume large amounts of aquatic vegetation and ENGAGE displace native birds from feeding and nesting areas. The U.S. Fish and Wildlife Service has programs for reducing mute swans in Maryland in BioNOW order to protect native bird populations. Attempts to limit the size of the mute swan populations in Maryland and other states have been met Want to know how this science is relevant to your life? Check out the with opposition by citizens who find the birds beautiful. Do you feel BioNow video below. that native populations need not be protected as long as the exotic species serves a suitable human purpose? Or do you feel that native ∙ Biodiversity species should be protected regardless? What was the effect of the exotic (alien) species on the biodiversity? Why do you think this was the case? Thinking Critically 1. One of the most striking examples of coevolution is between insects and flowers. The earliest angiosperms produced large amounts of pollen on flowers that were wind-pollinated. The ovules were partially exposed and exuded tiny droplets of sugary sap to catch passing pollen. Outline a course of events that could have resulted in the coevolution we observe today between a flower and its pollinator.
32 Human Impact © Brett Carlsen/Stringer/Getty Images News on the Biosphere Flint Water Crisis OUTLINE 32.1 Conservation Biology 627 In 2014, the city of Flint, Michigan, in an attempt to save money due to a inan- 32.2 Biodiversity 628 cial crisis, switched the source of its drinking water from Lake Huron to the Flint 32.3 Resources and Environmental River. Almost immediately, the residents of Flint began to notice a change in their water quality. Foul odors, sediment, and bacteria were now present in the Impact 632 water, and residents were complaining of feeling sick after using the water. 32.4 Sustainable Societies 643 But the real problem was lead. Portions of Flint’s water system date back BEFORE YOU BEGIN to the early decades of the twentieth century, when lead was used in the con- nections between pipes. Unfortunately, the Flint River water contains high lev- Before beginning this chapter, take a few moments to els of chloride, which is a corrosive material. The chloride caused the lead (and review the following discussions. other heavy metals) in the pipes to leach out, exposing residents to high levels Section 12.3 What is a transgenic organism? of lead in their drinking water. Section 30.2 How do the environmental impacts of a more-developed country (MDC) compare with those of The presence of lead in drinking water is a major health hazard. The a less-developed country (LDC)? amount of lead in water is limited by government regulations to less than Section 31.2 How much energy is transferred from one 15 parts per billion (ppb), but even low levels of lead exposure can be danger- level of an ecological pyramid to the next level? ous. Lead exposure in adults can lead to kidney and liver problems, and an increased rate of illness. The efects of lead on children are more dangerous. 626 Lead afects the developing nervous system and can cause permanent devel- opmental problems. An estimated 7,000–12,000 children in Flint may have been exposed to unsafe levels of lead. In this chapter, we will explore how humans are interacting with their ecosystems, and how those interactions may sometimes have negative consequences. As you read through this chapter, think about the following questions: 1. In addition to chemical pollution, what other threats to the water cycle are associated with human activity? 2. Does the lead in the Flint water crisis represent a point or nonpoint source of pollution?
CHAPTER 32 Human Impact on the Biosphere 627 32.1 Conservation Biology Learning Outcomes Upon completion of this section, you should be able to 1. Identify the role of conservation biology with regard to maintaining biodiversity. 2. Recognize the subields of biology that support conservation biology. In order to understand the diversity of life on Earth, we need to know more about species other than their total numbers. Conservation biology is a field of biology that focuses on conserving natural resources for this and future genera- tions. Conservation biology is concerned with developing new scientific con- cepts and applying them to our lives, along with sustainably managing the Earth’s biodiversity for human use. Multiple subfields of biology blend together to form the concepts of conservation biology. Basic Biology systematics behavior ecology physiology field biology genetics Conservation Biology evolutionary biology biopark management wildlife management forestry agronomy veterinary science range management Applied Biology fisheries biology Conservation biology is unique among the life sciences, because it sup- ports a variety of ethical principles: (1) Biodiversity is good for the Earth and therefore good for humans; (2) extinctions are undesirable; (3) the interac- tions within ecosystems support biodiversity and are beneficial to humans; and (4) biodiversity is the result of evolutionary change and has great value to humans. Conservation biology is often referred to as a crisis discipline. In the next 20–50 years, approximately 10–20% of the biodiversity on Earth will disap- pear due to the extinction of species. It is extremely important that everyone realize the importance and value of biodiversity, as well as how human actions contribute to the extinction crisis before us. Check Your Progress 32.1 32.1 CONNECTING THE CONCEPTS 1. Describe how conservation biology is supported by a variety of Conservation biology seeks to disciplines. balance the needs of humans with the needs of the rest of the species 2. List the principles supported by conservation biology. on Earth.
% Species A ected628 PART SEVEN Ecology 32.2 Biodiversity by Cause 100 Learning Outcomes Habitat90 loss80 Upon completion of this section, you should be able to 70 Exotic60 1. Deine biodiversity, and briely list some of the threats to it. species50 2. List the direct values of biodiversity, and explain how they are beneicial Pollution40 30 to the human population. Over-20 3. List the indirect values of biodiversity, and briely describe their exploitation10 economic beneits. Disease0 Biodiversity can be defined as the diversity of life on Earth, described in terms a. of the number of different species. We are presently in a biodiversity crisis— the number of extinctions (loss of species) expected to occur in the near future b. will, for the first time, be due to human activities. According to the U.S. Fish and Wildlife Service (FWS), as of 2016, there are over 694 animal species and Figure 32.1 Habitat loss. 898 plant species in the United States that are threatened or in danger of extinc- tion. The majority of these species (85%) are threatened by habitat loss a. In a study examining records of imperiled U.S. plants and animals, (Fig. 32.1a), usually associated with the sprawl of urban areas. Other factors habitat loss emerged as the greatest threat to wildlife. b. The contributing to the biodiversity crisis are the introduction of exotic species Canada lynx is threatened by the loss of habitat and the (50%), water and air pollution (24%), and the overexploitation of natural re- overexploitation of forests for wood. sources (17%). In many cases, endangered species are threatened by multiple © Don Johnston/All Canada Photos/Getty Images factors. For example, the Canada lynx (Lynx canadensis) (Fig. 32.1b) lives in the northern forests of the United States. It is a relatively rare species that pre- fers dense forests. It is currently listed as a threatened species due to habitat loss caused by the creation of roads for snowmobiling and skiing, as well as increased harvesting of timber. Biodiversity is not evenly distributed over the Earth. It is highest at the tropics and declines toward the poles, whether we are considering terrestrial, freshwater, or marine species. The regions of the world that contain the great- est concentration of species are known as biodiversity hotspots (Fig. 32.2). The hotspots contain over 50% of all known plant species and 42% of all ter- restrial vertebrate species but cover only about 2.4% of the Earth’s land area. Therefore, when trying to save the greatest number of species, hotspots should be prioritized. North Europe Asia America Africa South America Australia Figure 32.2 Biodiversity hotspots. Antarctica biodiversity hotspot Hotspots cover only 2.4% of the Earth, yet they contain over 50% of the world’s plant species and 42% of terrestrial vertebrates.
Conservation biology strives to reverse the trend toward the CHAPTER 32 Human Impact on the Biosphere 629 extinction of thousands of plants and animals. To bring this about, it is necessary to make all people aware that biodiversity is a resource a. Wild species, like the rosy with immense value—both direct and indirect. periwinkle, are sources of many medicines. Direct Values of Biodiversity b. Wild species, like the nine-banded armadillo, play a role in medical research. The direct values of biodiversity include medicines, foods, and other products that benefit humans. Figure 32.3 Medicinal value of biodiversity. Medicinal Value Many wildlife species, such as the (a) rosy periwinkle and (b) nine-banded armadillo, are sources of medical beneits to Most of the prescription drugs used in the United States were humans. originally derived from organisms. The rosy periwinkle from Mada- (a): © Steven P. Lynch; (b): © Steve Bower/Shutterstock RF gascar is an excellent example of a tropical plant that has provided useful medicines (Fig. 32.3a). Potent chemicals from this plant are used to treat two forms of cancer: leukemia and Hodgkin’s disease. Due to these drugs, the sur- vival rate for childhood leukemia has gone from 10% to 90%, and Hodgkin’s disease is now usually curable. Although the value of saving a life cannot be calculated, it is still sometimes easier for us to appreciate the worth of a resource if it is explained in monetary terms. Thus, researchers tell us that, judging from the success rate in the past, hundreds of additional types of drugs are yet to be found in tropical rain forests, and the value of this resource to society is probably in excess of several hundred billion dollars. The popular antibiotic penicillin is derived from a fungus, and certain species of bacteria produce the antibiotics tetracycline and streptomycin. These drugs have proven to be indispensable in the treatment of diseases, including certain sexually transmitted diseases. Leprosy is among the diseases for which there is, as yet, no cure. The bacterium that causes leprosy will not grow in the laboratory, but scientists discovered that it grows naturally in the nine-banded armadillo (Fig. 32.3b). Having a source for the bacterium may make it possible to find a potential cure for leprosy. The blood of horseshoe crabs contains a substance called limulus amoebocyte lysate, which is used to ensure that medical devices, such as pace- makers, surgical implants, and prostheses, are free of bacteria. Blood is taken from 250,000 horseshoe crabs a year, and then they are returned to the sea unharmed. Agricultural Value Crops such as wheat, corn, and rice were originally derived from wild plants that were modified to be high producers. The same high-yield, genetically similar strains tend to be grown worldwide. When cultivated rice crops in Africa were being devastated by a virus, researchers grew wild rice plants from thousands of seed samples until they found one that contained a gene for resis- tance to the virus. These wild plants were then used in a breeding program to transfer the gene into high-yield rice plants. If this variety of wild rice had become extinct before its resistance could be discovered, rice cultivation in Africa might have collapsed. Biological pest controls—specifically, natural predators and parasites— are often preferable to chemical pesticides (Fig. 32.4). When a rice pest called the brown planthopper became resistant to pesticides, farmers began to use natural enemies of the brown planthopper instead. The economic savings were calculated at well over $1 billion. Similarly, cotton growers in Cañete Valley, Peru, found that pesticides were no longer working against the cotton aphid
630 PART SEVEN Ecology Wild species, like ladybugs, due to resistance. Research identified natural predators, which cotton farmers play a role in biological are now using to an even greater degree. control of agricultural pests. Most flowering plants are pollinated by animals, such as bees, wasps, butterflies, beetles, birds, and bats. The honeybee has been domesticated, and it pollinates almost $10 billion worth of food crops annually in the United States. The danger of this dependency on a single species is exemplified by mites that have wiped out more than 20% of the commercial honeybee population in the United States. Where can we get resistant bees? From the wild, of course; the value of wild pollinators to the U.S. agricultural economy has been calcu- lated at $15 billion a year. Wild species, like the long-nosed Consumptive Use Value bat, are pollinators of agricultural and other plants. We have had much success in cultivating crops, keeping domesticated animals, growing trees on plantations, and so on. However, the environ- ment provides all sorts of other products that are sold in marketplaces worldwide, including wild fruits and vegetables, skins, fibers, beeswax, and seaweed. Also, some people obtain their meat di- rectly from the environment. In one study, researchers calculated that the economic value of wild pig in the diet of native hunters in Sarawak, East Malaysia, was about $40 million per year. Similarly, many trees are still felled in the natural environment for their wood. Researchers have calculated that a species-rich forest in the Peruvian Amazon is worth far more if the forest is used for fruit and rubber produc- tion than for timber production. Fruit and the latex needed to produce rubber can be brought to market for an unlim- ited number of years, whereas once the trees are gone, no more forest products can be harvested. Indirect Values of Biodiversity All wild species play important roles in the ecosystems to which they belong. If we want to preserve them, it is more beneficial to save the entire ecosystem than the individual species. Ecosystems perform many indirect services that cannot always be measured economically. These services are said to be indirect values because they are wide- ranging and not easily perceptible (Fig. 32.5). Maintaining Biogeochemical Cycles Wild species, like many marine species, We observed in Section 31.2 that ecosystems are charac- provide us with food. terized by energy flow and chemical cycling. The biodi- versity within ecosystems contributes to the functioning of the water, carbon, nitrogen, phosphorus, and other biogeochemical cycles. We are dependent on these cycles for fresh water, the removal of carbon dioxide from the atmosphere, Wild species, like rubber trees, can Figure 32.4 Agriculture and the consumptive value of wildlife. provide a product indefinitely if the forest is not destroyed. Many wildlife species help account for our bountiful harvests and are sources of food for the world. (ladybug): © Anthony Mercieca/Science Source; (bat): © Dr. Merlin D. Tuttle/Bat Conservation International/Science Source; (ishing boat): © Herve Donnezan/Science Source; (rubber tree): © Bryn Campbell/Stone/Getty
the uptake of excess soil nitrogen, and the provision of phosphate. When CHAPTER 32 Human Impact on the Biosphere 631 human activities upset one aspect of a biogeochemical cycle, other parts within the cycle are also affected. Technology has been unable to artifi- Figure 32.5 Indirect values of ecosystems. cially contribute to or replicate any of the biogeochemical cycles. Forests and coral reefs perform many of the functions listed as the Waste Disposal indirect values of ecosystems. (deciduous forest): © BananaStock/PunchStock RF; (coral reef): © Vlad61/ Decomposers break down dead organic matter and other types of wastes Shutterstock RF to inorganic nutrients, which are used by the producers within ecosys- tems. This function aids humans immensely, because we dump millions of tons of waste material into natural ecosystems each year. If not for decomposition, waste would soon cover the entire surface of our planet. We can build sewage treatment plants, but they are expensive, and few of them break down solid wastes completely to inorganic nutrients. It is less expensive and more efficient to water plants and trees with partially treated wastewater and let soil bacteria cleanse it completely. Biological communities are also capable of breaking down and immobilizing pollutants, such as the heavy metals and pesticides that humans release into the environment. A review of wetland functions in Canada assigned a value of $50,000 per hectare (2.471 acres, or 10,000 square meters) per year to the ability of natural areas to purify water and take up pollutants. Provision of Fresh Water Few terrestrial organisms are adapted to living in a salty environment—they need fresh water. The water cycle continually supplies fresh water to terrestrial ecosystems. Humans use fresh water in innumerable ways, including drinking it and irrigating their crops with it. Freshwater ecosystems, such as rivers and lakes, also supply fish and other types of organisms for our consumption. Unlike other commodities, there is no substitute for fresh water. We can remove salt from seawater to obtain fresh water, but the cost of desalination is about four to eight times the average cost of fresh water acquired via the water cycle. Forests and other natural ecosystems exert a “sponge effect.” They soak up water and then release it at a controlled rate. When rain falls in a natural area, plant foliage and dead leaves lessen its impact, and the soil slowly absorbs it, especially if the soil has been aerated by organisms. The water-holding ca- pacity of forests reduces the possibility and degree of flooding. The value of a marshland outside Boston, Massachusetts, has been estimated at $72,000 per hectare per year solely on its ability to reduce floods. Forests release water slowly for days or weeks after rains have ceased. Comparing rivers in West African coffee plantations, those flowing through forests release twice as much water halfway through the dry season and three to five times as much at the end of the dry season. These data show the water-retaining ability of forests. Prevention of Soil Erosion Intact terrestrial ecosystems naturally retain soil and prevent soil erosion. The importance of this ecosystem attribute is especially noticeable following defor- estation. In Pakistan, the world’s largest dam, the Tarbela Dam, is losing 12 billion cubic meters of storage capacity sooner than expected because of the silt that is building up behind the dam due to deforestation of areas upriver. At one time, the Philippines were exporting $100 million worth of oysters, mus- sels, clams, and cockles each year. Now, silt carried down rivers following deforestation is smothering the mangrove ecosystem that serves as a nursery
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