_{Part VII} Ecology

Ecology of Populations

Chapter

30

Outline

30.1 The Human Population

• Humans have a clumped distribution pattern. Population densities are highest along the coasts of all the continents.534

• The human population is undergoing rapid growth, and most of this growth takes place in the less-developed countries.535–36

• Comparison of age structures indicates future growth trends.536

• Most resource consumption and pollution occur in more-developed countries.537

30.2 Characteristics of Populations

• Populations have a particular distribution and density.538

• Population growth is dependent on the birthrate versus the death rate.538

• Population demographics such as age and survivorship influence growth.539

• Population growth is exponential when biotic potential expresses itself or logistic when resources are limited.540–41

• Population growth is regulated by density-independent (abiotic) and density-dependent (biotic) factors.542

30.3 Life History Patterns and Extinction

• Opportunistic populations have characteristics that allow them to survive unfavorable conditions and disperse easily.544

• Equilibrium populations have characteristics that make it difficult for them to survive unfavorable conditions.544

• Extinction is more likely in equilibrium populations with limited range.545

30.4 The Scope of Ecology

• Ecology is the study of the interactions of organisms with each other and with the physical environment.546

• Ecologists study these levels of biological organization: organism, population, community, ecosystem, and biosphere.546

The world’s human population is growing at an alarming rate. Some estimates predict it will exceed 9 billion people within 50 years. If this rate of growth continues, the planet will eventually be unable to support the human population. Different countries have found ways to encourage their citizens to limit population growth. One of the most scrutinized strategies is China’s one-child policy. This policy allows each Chinese couple to have only one child. Punishments for those who do not follow the policy include imposing fines of as much as eight times the annual household income, destroying a family’s home, or even forcing a pregnant female to terminate her pregnancy. While some officials believe that such a drastic policy is necessary to decrease the growth of the Chinese population, others feel the policy is too harsh and no more effective than the much less restrictive policies, such as family planning and birth control education, that are being practiced in other overpopulated countries. The situation in China poses a major ethical dilemma. Is it more important to control population growth or to respect individuals’ reproductive rights?

In this chapter, you will learn about ecology and population dynamics in all sorts of populations, including humans. One important lesson to be learned is that there are no easy solutions when it comes to ecological problems.

_{30.1 The Human Population}

The human population practically covers the face of the Earth. However, on both a large scale and a small scale, the human population has a clumped distribution. Over half of the world’s people live in Asia, and most Asian populations live in China and India. Mongolia has a population density of only 1.5 persons per square kilometer, while Bangladesh has a density of nearly 1,000 persons per square kilometer. On every continent, human population densities are highest along the coasts.

Present Population Growth

For quite a while, the growth of the global human population was relatively slow, but as more reproducing individuals were added to the population, growth began to increase. At the time of the Industrial Revolution (1800s), the growth curve for the human population began to slope steeply upward, so that by now the population is undergoing rapid growth (Fig. 30.1).

Just as with populations in nature, the growth rate of the human population 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 22 per 1,000 per year, while the global death rate is approximately 9 per 1,000 per year. From these numbers we can calculate that the current annual growth rate of the human population is:

(2229)/1,000 5 (13)/1,000 5 1.3%

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 23 years. Will we be able to meet the extreme demands for resources by such a large population increase within 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 23 years, the world would need to double the amount of food, jobs, water, energy, and other resources in order 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 population levels off at a size appropriate to the carrying capacity of its environment. The carrying capacity reflects 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 potentially capable of supporting far more people than currently inhabit the planet, perhaps as many as 50–100 billion people. Others believe that we may have already exceeded the number of humans the Earth can support, and that the human population may undergo a catastrophic crash to a much smaller size.

More-Developed Versus Less-Developed Countries

Complicating the issue of future human population growth is the fact that not all countries have the same growth rate (Fig. 30.2). The countries of the world today can be divided into two groups (Fig. 30.3). In the more-developed countries (MDCs), such as those in North America and Europe, population growth is modest, and the people enjoy a fairly good standard of living. In the less-developed countries (LDCs), such as those in Latin America, Asia, and Africa, population growth is dramatic, and the majority of the people live in poverty.

The MDCs

The MDCs did not always have a low growth rate. Between 1850 and 1950, they doubled their populations, largely because of a decline in the death rate due to the development of modern medicine and improved socioeconomic conditions. The decline in the death rate was followed shortly thereafter by a decline in birthrate, so that populations in the MDCs have experienced only modest growth since 1950. This sequence of events (i.e., decreased death rate followed by decreased birthrate) is termed a demographic transition.

The growth rate for the MDCs as a whole is now about 0.1%, but populations in several countries are not growing at all, or are actually decreasing in size. 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 leveling off and continues to increase. The U.S. currently has a growth rate of 0.6%. This higher rate is due to the fact that many people immigrate to the U.S. each year, and a large number of the women are still of reproductive age. Therefore, it is unlikely that the U.S. will experience a decline in its growth rate in the near future.

The LDCs

Death rates began to decline sharply in the LDCs following World War II with the introduction of modern medicine, but the birthrate 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 1.6%. Unfortunately, the growth rates in 46 of the less-developed countries have not declined. Thirty-five of these countries are in sub-Saharan Africa, where women on average are presently bearing more than five children each.

Between 2002 and 2050, the population of the LDCs is expected to jump from 5 billion 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. Asia, with 31% of the world’s arable (farmable) land, is already home to 56% of the human population. Twelve of the world’s most polluted cities are located 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

An age structure diagram is divided into three groups: dependency, reproductive, and postreproductive (Fig. 30.4). Many MDCs have a stable age structure, meaning that the number of individuals in each group is just about the same. Therefore, the MDC populations are expected to remain just about the same size if couples have two children, and to decline if each couple has fewer than two children. In contrast, the age structure diagram of most LDCs has a pyramid shape, with the dependency group the largest. Therefore, the LDC populations will continue to expand, even after replacement reproduction is attained. However, the more quickly replacement reproduction is achieved, the sooner zero population growth will result.

Replacement reproduction occurs when each couple in a population produces only two children. It might seem at first that replacement reproduction would cause a 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 leaving them, replacement reproduction would still result in population growth.

Population Growth and Environmental
Impact

Population growth is putting extreme pressure on each country’s social organization, the Earth’s resources, and the biosphere. Since the population of the LDCs is still growing at a significant rate, it might seem that their population increase will be the main cause of future environmental degradation. But this is not necessarily the case because the MDCs consume a much larger proportion of the Earth’s resources than do the LDCs. This consumption leads to environmental degradation, which is also of the utmost concern.

Environmental Impact

The environmental impact (E.I.) of a population is measured not only in terms of population size, but also in terms of resource consumption per capita and the pollution that results because of this consumption. In other words:

E.I.  population size 3 resource consumption per capita
 pollution per unit of resource used

Therefore, there are two possible causes of overpopulation: population size and resource consumption. Overpopulation is more obvious in LDCs; resource consumption is more obvious in MDCs, because the per capita consumption is so much higher in the MDCs. For example, an average American family, in terms of per capita resource consumption and water requirements, is the equivalent of 30 people in India. We need to realize, therefore, 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 30.5. The MDCs account for only about one-fourth of the world population. However, compared to the LDCs, MDCs account for 90% of the hazardous waste production due their high rate of consumption of such resources as fossil fuel, metal, and paper.

_{30.2 Characteristics of Populations}

The characteristics of a population can change. For example, a population is usually subject to evolution by natural selection. During times of environmental stability, most individuals of a population are well adapted to their environment, and the population may increase in size. During times of environmental change, 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 will occur again. So when an ecologist lists the characteristics of a population, this is a snapshot of the characteristics at a particular time and place.

Distribution and Density

Resources are the nonliving and living components of an environment that support its organisms. Light, water, space, mates, and food are some important resources for wildlife populations. The availability of resources influences the spatial distribution of individuals in a population. Three terms—clumped, random, and uniform—are used to describe the observed patterns of distribution. In a study of desert shrubs, it was found that the distribution changes from clumped to random to uniform as the plants mature (Fig. 30.6). After sufficient study, competition for belowground resources was found to cause the distribution pattern to become uniform.

Suppose we were to step back and consider the distribution of individuals, not within a single desert or forest or pond, but within a species’ range. A 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 of a population are clumped within the range because they are located in areas suitable for their growth. Desert shrubs, for example, are found in deserts.

Population density is the number of individuals per unit area or volume. Population density tends to be higher in areas with plentiful resources than in areas with scarce resources. Other factors can be involved, however. In general, population density declines with increasing body size. Consider that tree seedlings live at a much higher population density than do mature trees. Therefore, more than one factor must be taken into account when explaining population densities.

Population Growth

As mentioned on page 534, the growth rate is dependent on the number of individuals born each year and the number of individuals that die each year. Populations grow when the number of births exceeds the number of deaths. To take another example, if the number of births is 30 per year, and the number of deaths is 10 per year per 1,000 individuals, the growth rate is 2%.

In cases where the number of deaths exceeds the number of births, the value of the growth rate is negative—the population is shrinking.

Demographics and Population Growth

Availability of resources and also certain characteristics of a population—called demographics—influence its growth rate.

One demographic characteristic of interest for all populations is the age structure of a population. As shown in Figure 30.4, it is customary to use bar diagrams to depict the age structure of a population. The population is divided into members that are prereproductive, reproductive, or postreproductive based on their age. If the prereproductive and reproductive individuals outnumber those that are postreproductive, the birthrate will exceed the death rate (Fig. 30.7a). In contrast, if the postreproductive individuals exceed those that are prereproductive and/or reproductive, the number of individuals in the population will decrease over time because the death rate will exceed the birthrate (Fig. 30.7b). If the numbers of prereproductive, reproductive, and postreproductive individuals are approximately equal, the birthrate and death rate will be equal, and the number of individuals in the population will remain stable over time (Fig. 30.7c).

Ecologists also study patterns of survivorship—how the age at death influences population size. For example, if members of a population die young, the number of reproductive individuals is lower than otherwise. First, an ecologist constructs a life table, which lists the number of individuals that die or survive at each age. The best way to arrive at a life table is to identify a large number of individuals that are born at about the same time and keep records on them from birth until death.

A life table for Dall sheep is given in Figure 30.8a. This table was constructed by gathering a large number of skulls and counting the growth rings on the horns to estimate each sheep’s age at death. Plotting the number of survivors per 1,000 births against age produces a survivorship curve (Fig. 30.8b). While each species has its own particular pattern, 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 to illness. In a type II curve, the possibility of death is spread over all age groups. In a type III curve, death is likely among the young, and few individuals reach old age.

Biotic PotentialHow quickly a population increases over time depends upon its biotic potential, the highest possible rate of increase for a population when resources are unlimited (Fig. 30.9). Whether the biotic potential is high or low depends on demographic characteristics of the population, such as the following:

1. Usual number of offspring per reproduction

2. Chances of survival until age of reproduction

3. How often each individual reproduces

4. Age at which reproduction begins

Patterns of Population Growth

The patterns of population growth are dependent on (1) the biotic potential of the species combined with other demographics, 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. 30.10). This growth pattern can be likened to compound interest at the 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 these phases:

Lag phaseGrowth is slow because the number of individuals in the population is small.

Exponential growth phaseGrowth is accelerating.

Usually, exponential growth can only continue as long as resources in the environment are unlimited. When the number of individuals in the population approaches the number that can be supported by available resources, competition among individuals for resources increases.

Logistic Growth

As resources decrease, population growth levels off, and a pattern of population growth called logistic growth is expected. Logistic growth results in an S-shaped growth curve (Fig. 30.11).

Notice that an S-shaped curve has these phases:

Lag phaseGrowth is slow because the number of individuals in the population is small.

Exponential growth phaseGrowth is accelerating due to biotic potential
(see Fig. 30.9a).

Deceleration phaseThe rate of population growth slows because of increased competition among individuals for available resources.

Stable equilibrium phaseLittle 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. Recall from Section 30.1 that the carrying capacity is the total number of individuals the resources can support. This number is not a constant and varies with the circumstances. For example, a large island can support a larger population of penguins than a small island, because the smaller island has a limited amount of space for nesting sites.

ApplicationsOur 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. So, if humans are using a fish population as a continuous food source, it would be best to maintain that population 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 population will sink into the lag phase, and it may be years before exponential growth recurs. 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 to reduce the population size. Reducing the population size only encourages exponential growth to begin once again. Farmers can reduce the carrying capacity for a pest by alternating rows of different crops instead of growing one type of crop throughout the entire field.

Factors That Regulate Population Growth

Ecologists have long recognized that the environment contains both biotic (living) and abiotic (nonliving) components, and that these two components play an important role in regulating population size in nature.

Density-Independent Factors

Abiotic factors, such as weather or natural disasters, are typically density-independent factors. Abiotic factors can cause sudden and catastrophic reductions in population size. However, density-independent factors cannot in and of themselves regulate population size because the effect is not influenced by the number of individuals in the population. In other words, the intensity of the effect does not increase with increased population size. For example, the proportion 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 (Fig. 30.13). Of course, the larger the population, the greater the number of individuals killed, but the percentage killed does not depend on the density.

Density-Dependent Factors

Biotic factors are considered density--dependent factors because the percentage of the population affected does increase as the density of the population increases. 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 Chapter 31 because they influence community composition and diversity.

CompetitionCompetition can occur when members of the same species attempt to utilize needed resources (such as light, food, or space) that are in limited supply. As a result, not all members of the population can have access to the resource to the degree necessary to ensure survival or reproduction or some other aspect of their life history.

As an example, let’s consider a woodpecker population in which members have to compete for nesting sites. Each pair of birds requires a tree hole in order to raise offspring. If there are more holes than breeding pairs, each pair can have a hole in which to lay eggs and rear young birds (Fig. 30.14a). But if there are fewer holes than breeding pairs, each pair must compete to acquire a nesting site (Fig. 30.14b). 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 concerns the reindeer on St. Paul Island. Overpopulation led to an insufficiency of food to sustain all members, 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.

PredationPredation occurs when one organism, the predator, eats another, the prey. In the broadest sense, predation can include not only animals such as lions that kill zebras, but also filter-feeding blue whales that strain krill from the ocean waters, parasitic ticks that suck blood from their hosts, and even herbivorous deer that browse on trees and bushes.

The effect of predation generally increases as the prey population grows more dense, because prey are easier to find when hiding places are limited. Consider a field inhabited by a population of mice (Fig. 30.15). Each 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, two mice will be left out in the open. It might be hard for the hawk to find only two mice in the field. If neither mouse is caught, the predation rate is 0/2 = 0%. However, if there are 200 mice and only 100 holes, there is a greater chance that the hawk will be able to find some of these 100 mice without holes. If half of the exposed mice are caught, the predation 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 CyclesInstead of remaining steady, some predator and prey populations experience an increase, and then a decrease, in population size. A famous example of such a cycle occurs between the snowshoe hare and the Canada lynx, a type of small cat (Fig. 30.16). The snowshoe hare is a common herbivore 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 decrease 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 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 30.16.

_{30.3 Life History Patterns
and Extinction}

A life history consists of a particular mix of the characteristics we have been discussing. After studying many different 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 populations 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. 30.17a). Density-independent effects tend to regulate the size of the population, which is large enough to survive an event that threatens to annihilate it. The population has a high dispersal capacity. Various types of insects and weeds provide the best examples of opportunistic species.

Equilibrium populations exhibit logistic population growth, and the size of the population remains close to or at the carrying capacity (Fig. 30.17b). Resources are relatively scarce, and members best able to compete—those with phenotypes best suited to the environment—have the largest number of offspring. Members allocate energy to their own growth and survival, and to the growth and survival of their few offspring. Therefore, they are fairly large, slow to mature, and have a fairly long life span. The growth of equilibrium populations tends to be regulated by -density-dependent effects. Various birds and mammals provide the best examples of equilibrium species.

Extinction

Extinction is the total disappearance of a species or higher group. Which species shown in Figure 30.17, the dandelion or the mountain gorilla, is apt to become extinct? Because the dandelion matures quickly, produces many offspring at one time, and has seeds that are dispersed widely by wind, it can more easily withstand a local decimation than can the mountain gorilla.

A study of equilibrium species shows that three other factors—namely, size of geographic range, degree of habitat tolerance, and size of local populations—can help determine whether an equilibrium species is in danger of extinction. Figure 30.18 compares several equilibrium species on the basis of these three factors. The mountain gorilla has a restricted geographic range, narrow habitat tolerance (few preferred places to live), and small local population. This combination of characteristics makes the mountain gorilla very 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 who are trying to preserve the biodiversity of the biosphere.

_{30.4 The Scope of Ecology}

Ecology is the scientific study of the interactions of organisms with each other and with the physical environment. Ecology is one of the two biological sciences of most interest to the public today, the other being genetics. Genetics captures our attention because of its amazing advances, but ecology is of urgent concern because of a newly gained recognition: Unless we learn to live with the environment and not against it, our society cannot endure. Understanding ecology will help you make informed decisions, ranging from what kind of car to drive and whether to use pesticides on your lawn, to how to support the preservation of a forested area in your town. Your ecological decisions will affect not only your life, but also the lives of generations to come.

Ecology involves the study of several levels of biological organization (Fig. 30.19). Some ecologists study organisms, focusing on their adaptations to a particular environment. For example, organismal ecologists might investigate what attributes of a parrotfish in a coral reef allow it to live in warm tropical waters.

In this chapter, we learned that organisms are usually part of a population, a group of individuals of the same species in a given location. At the population level of study, ecologists describe changes in population size over time. For example, a population ecologist might compare the number of parrotfishes living in a given location today to data she obtained from that same location 20 years ago.

A community consists of all the various populations at a particular locale. A coral reef contains numerous populations of fishes, crustaceans, corals, algae, and so forth. At the community level, ecologists study how the interactions between populations affect the populations’ well-being. For example, a community ecologist might study how a decrease in algal populations affects the population sizes of crustaceans and fishes living on the coral reef.

An ecosystem consists of a community of living organisms as well as their physical environment. As an example of how the physical environment affects a community, consider that the presence of suspended particles in the water decreases the amount of sunlight reaching algae living on the coral reef. Without solar energy, algae cannot produce the organic nutrients they and the other populations require.

Finally, the biosphere is the portion of the Earth’s surface—air, water, land—where living things exist. Having information about the many levels of organization within a coral reef allows ecologists to understand how a coral reef contributes to the biodiversity and dynamics of the biosphere.

Ecology: A Biological Science

Ecology began as a part of natural history, the discipline of observing and describing organisms in their environment, but today ecology is also an experimental science. A central goal of ecology is to develop models that explain and predict the distribution and abundance of populations based on their interactions within an ecosystem. Achieving such a goal involves testing hypotheses. For example, ecologists might formulate and test hypotheses about the role fire plays in maintaining a lodgepole pine forest (Fig. 30.20). To test these hypotheses, ecologists could compare the characteristics of a community before and after a prescribed burn of the area by professionals under controlled conditions. Ultimately, ecologists wish to develop models about 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. Comprehension of ecological principles helps us understand why a functioning biosphere is critical to our survival and why our population of 6 billion people and our overconsumption of resources pose threats to the biosphere.

Conservation biology is a new discipline 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 biosphere on which human life depends.

The Chapter in Review

Summary

30.1 The Human Population

The human population exhibits clumped distribution (both on a large scale and a small scale) and is undergoing rapid growth.

Future Population Growth

At the present growth rate, the doubling time for the human population is estimated to be 23 years. This rate of growth will put extreme demands on resources, and growth will decline due to resource scarcity. Eventually the population will most likely level off at its carrying capacity.

More-Developed Countries (MDCs)The growth rate for MDCs is about 0.1%. The total MDC population between 2002 and 2050 will remain at around 1.2 billion.

Less-Developed Countries (LDCs)The growth rate for LDCs is about 1.6%. The LDC population between 2002 and 2050 is expected to increase from 5 billion to 8 billion.

Comparing Age Structures

The age structure of populations is divided into three age groups: dependency, reproductive, and postreproductive. The MDCs are approaching stabilization with just about equal numbers in each group. The LDCs will continue to expand because their prereproductive group is the largest.

Population Growth and Environmental Impact

Two types of environmental impact can occur: The LDCs put stress on the biosphere due to population growth, and the MDCs put stress on the bio-sphere due to resource consumption and waste production.

30.2 Characteristics of Populations

Distribution patterns and population density are both dependent on resource availability.

Distribution PatternsClumped, random, uniform.

Population DensityThe number of individuals per unit area is higher in areas with abundant resources, and lower when resources are limited.

Demographics and Population Growth

Population growth can be calculated based on the annual birthrate and death rate. Population growth is determined by:

• Resource availability

• Demographics (age structures, survivorship, and biotic potential—the highest rate of increase possible)

Patterns of Population Growth

The two patterns of population growth are exponential growth and logistic growth.

Exponential GrowthExponential growth results in a J-shaped curve. The two phases of exponential growth are lag phase (slow growth) and exponential growth phase (accelerating growth).

Logistic GrowthLogistic growth results in an S-shaped curve. The four phases of logistic growth are: lag phase (slow growth), exponential growth phase (accelerating growth), deceleration phase (slowing growth), and stable equilibrium phase (little growth).

Factors That Regulate Population Growth

The two factors that regulate population growth are density-independent and density-dependent factors.

Density-independent factors include abiotic factors, such as weather and natural disasters. The effect of the factor is not dependent on density.

Density-dependent factors include biotic factors, such as competition and predation. The effect of the factor is dependent on density.

30.3 Life History Patterns and Extinction

The two fundamental life history patterns are exhibited by opportunistic populations and equilibrium populations.

Opportunistic populations are characterized by small individuals with short life spans, who mature quickly, produce many offspring, have strong dispersal ability, provide little or no care of offspring, and exhibit exponential growth.

Equilibrium populations are characterized by large individuals with long life spans, who mature slowly, produce few offspring, provide much care of offspring, and exhibit logistic growth.

Extinction

Extinction is the total disappearance of a species or higher group. Opportunistic populations are less likely than equilibrium populations to become extinct. Three factors in particular influence vulnerability of equilibrium populations to extinction: size of geographic range, degree of habitat tolerance, and size of local populations. -

30.4 The Scope of Ecology

Ecology is an experimental science that studies the interactions of organisms with each other and with the physical environment. The levels of biological organization studied by ecologists are:

• Organisms

• Population

• Community

• Ecosystem

• Biosphere

Thinking Scientifically

1. In Sri Lanka, the death rate is 6 per 1,000, while the birthrate is 19 per 1,000. Calculate the current population growth rate.

The for-mula for the doubling time of a population is:

t  0.69/r, where t is the doubling time and r is the growth rate.

Determine the doubling rate (the number of years it will take to double the population size) in Sri Lanka. Be sure to convert your growth rate from a percentage (e.g., 2.1%) to a fraction (0.021). If the birthrate drops to 10 per 1,000, what will be the doubling rate?

2. Assume that a population of dandelions grows exponentially so that it doubles in size every week. The population (beginning with one plant) expands to fill a field in 20 weeks. How many weeks will it take to fill one-quarter of the field? How long will it take to fill half the field?

Testing Yourself

Choose the best answer for each question.

1. Decreased death rate followed by decreased birthrate has occurred in

a. MDCs.

b. LDCs.

c. MDCs and LDCs.

d. neither MDCs nor LDCs.

2. Human societies at present are characterized by

a. ever-increasing population growth.

b. unsustainable practices.

c. overreliance on fossil fuels.

d. All of these are correct.

3. A population’s maximum growth rate is also called its

a. carrying capacity.

b. biotic potential.

c. growth curve.

d. replacement rate.

4. Which of these levels of ecological study involves both abiotic and biotic components?

a. organisms

b. populations

c. communities

d. ecosystem

e. All of these are correct.

5. The level of biological organization subject to evolution by natural selection is called

a. organism.

b. population.

c. community.

d. ecosystem.

e. biosphere.

6. Within a range, members of a population exhibit this type of spatial distribution.

a. variable c. random

b. clumped d. uniform

7. Which are likely to have the highest population density?

a. zebras on the African savanna during the dry season

b. mice in a city park

c. moose in a Canadian forest

d. earthworms in an organically rich soil

8. Calculate the growth rate of a population of 500 individuals in which the birthrate is 10 per year and the death rate is 5 per year.

a. 5%

b. 5%

c. 1%

d. 1%

9. If the human birthrate were reduced to 15 per 1,000 per year and the death rate remained the same (9 per thousand), what would be the growth rate?

a. 9%

b. 6%

c. 10%

d. 0.6%

e. 15%

10. Replacement reproduction in a population with a pyramid-shaped age structure diagram results in

a. no population growth.

b. population growth.

c. a decline in the population.

11. Label the following age structure diagrams to indicate whether the population is stable, increasing, or decreasing.

12. If the number of prereproductive and reproductive members of a population exceeds the number of postreproductive members, the population will

a. grow.

b. remain stable.

c. decline.

13. A J-shaped growth curve indicates

a. logistic growth.

b. logarithmic growth.

c. exponential growth.

d. additive growth.

14. Exponential growth occurs

a. when population size is increasing to an ever higher amount.

b. at the carrying capacity of the environment.

c. when people are poor and don’t have enough to eat.

For statements 15–18, indicate the type of factor in the key exemplified by the scenario.

Key:

a. density-independent factor

b. competition

c. predation

d. predator-prey cycle

15. A severe drought destroys the entire food supply of a herd of gazelle.

16. A population of feral cats increases in size as the mouse population increases and then crashes regularly after the mouse population enters periods of decline.

17. Only the swiftest coyotes are able to catch the limited supply of rabbits available as a food source. The remaining animals are not strong enough to reproduce.

18. Deer in a forest damage a dense thicket of oak saplings more severely than a few young oak trees.

19. Which of the following is not a feature of an opportunistic life history pattern?

a. many offspring

b. little or no care of offspring

c. long life span

d. small individuals

e. fast to mature

20. The distribution of the human population is

a. variable.

b. clumped.

c. random.

d. uniform.

21. When the carrying capacity of the environment is exceeded, the population typically

a. increases, but at a slower rate.

b. stabilizes at the highest level reached.

c. decreases.

d. dies off entirely.

22. A pyramid-shaped age distribution means that

a. the prereproductive group is the largest group.

b. the population will grow for some time in the future.

c. the country is more likely an LDC than an MDC.

d. fewer women are leaving the reproductive years than entering them.

e. All of these are correct.

23. Which of these is a density-independent factor?

a. competition

b. predation

c. weather

d. resource availability

Go to www.mhhe.com/maderessentials for more quiz questions.

Bioethical Issue

Species that are more prone to certain risk factors are more likely to become extinct than others. For example, species with a unique lineage, such as the giant panda, are at severe risk for extinction. Should our limited resources for species protection be focused on species that are at the highest risk for extinction? Some argue that high-risk species are less successful products of evolution and should not receive extraordinary protection. Consequently, all species at risk for extinction should be equally protected. Which camp are you in? Support your position.

Understanding the Terms

age structure539

biosphere546

biotic potential540

carrying capacity535

clumped538

community546

competition542

demographics539

density-dependent factor542

density-independent factor542

ecology546

ecosystem546

equilibrium population544

exponential growth540

extinction545

less-developed country
(LDC)535

life history544

logistic growth541

more-developed country
(MDC)535

natural history547

opportunistic population544

organism546

population546

population density538

predation543

random538

range538

resource538

survivorship539

uniform538

Match the terms to these definitions:

a. _______________ All the populations at a particular locale.

b. _______________ Nonliving and living components of the environment that support organisms.

c. _______________ Portion of the globe in which a species is found.

d. _______________ Number of individuals per unit area or volume.

e. _______________ The highest possible rate of increase for a population when resources are unlimited.

f. _______________ Total number of individuals that available resources can support.

g. _______________ Members of the same species attempting to use the same limited resource.

h. _______________ Life history pattern in which population growth is logistic.

i. _______________ The total disappearance of a group.

j. _______________ Country where population growth is modest and people enjoy a good standard of living.

To control population growth, China has imposed the one-child-per-couple policy on its citizens.

The world population increases by 85 million persons each year.

Over half the world’s population lives in Asia.

Figure 30.1Human population growth.

It is predicted that the world’s population size may level off at 9 billion or increase to 12 billion by 2200, depending on the future growth rate. Close examination of the curve shows changes in growth that occurred in the 1300s during the black plague, in the 1800s with the Industrial Revolution, and in the 1900s with advances in science and medicine.

Figure 30.2Global population growth rates.

The highest population growth rates are found in Africa and the Middle East, while the growth rates throughout Europe are much lower.

Source: United Nations Population Division, 1993. Note: Data refer to 1990–95 (World Resource Institute).

Check Your Progress

1. Usually, why does rapid population growth begin to decline?

2. Contrast MDCs with LDCs.

Answers:1. Historically, when resources such as food and space become scarce, population size levels off at the carrying capacity of the environment.2. MDCs, such as those in Europe and North America, have modest population growth and a good standard of living. LDCs, such as those in Latin America, Asia, and Africa, have a fast rate of population growth, and most of the people live in poverty.

Figure 30.4Age structure diagrams (1998).

The diagrams predict that (a) the MDCs are approaching stabilization, whereas (b) the LDCs will expand rapidly due to their age distributions. c. Improved women’s rights and increasing contraceptive use could change this scenario. Here a community health worker is instructing women in Bangladesh about the use of contraceptives.

(a,b) Data from the United Nations Population Division, 1998, p. 536.

Figure 30.5Environmental impacts.

a. The combined population of the MDCs is much smaller than that of the LDCs. b. MDCs produce most of the world’s hazardous wastes. c–e. MDCs consume much more fossil fuel, metals, and paper than do LDCs.

Check Your Progress

1. The spatial distribution of individuals in a population can be described in what three ways?

2. List two major factors that influence population density.

Answers:1. Clumped, random, and uniform.2. Resource availability and body size.

Figure 30.6Patterns of distribution within a
population.

a. A population of mature desert shrubs. b. Young, small desert shrubs are clumped. c. Medium shrubs are randomly distributed.
d. Large shrubs are uniformly distributed.

Figure 30.7Age structure diagrams.

Typical age structure diagrams for hypothetical populations that are (a) increasing, (b) decreasing, or (c) stable. Different 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.

Figure 30.8Life table and survivorship curves.

a. A life table for Dall sheep. b. Three typical survivorship curves. Among Dall sheep, with a type I curve, most individuals survive until old age, when they gradually die off. Among hydras, with a type II curve, there is an equal chance of death at all ages. 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.

Check Your Progress

1. What is the relationship between survivorship and population size?

2.
List the four demographic characteristics of a population that determine its biotic potential.

3. Contrast exponential growth with logistic growth.

Answers:1. If members of a population typically die young, the size of the population will be lower than expected.2. Usual number of offspring per reproduction; chances of survival until age of reproduction; how often each individual reproduces; age at which reproduction begins.3. Exponential growth produces a J-shaped curve and occurs when resources are unlimited. Logistic growth produces an S-shaped curve and occurs when resources
are limited.

Figure 30.9Biotic potential.

A population’s maximum growth rate under ideal conditions—that is, its biotic potential—is greatly influenced by the number of offspring produced in each reproductive event. a. Pigs, which produce many offspring that quickly mature to produce more offspring, have a much higher biotic potential than (b) the rhinoceros, which produces only one or two offspring per infrequent reproductive events.

Figure 30.10Exponential growth.

Exponential growth results in a J-shaped growth curve because the growth rate is positive.

Figure 30.11Logistic growth.

Logistic growth produces an S-shaped growth curve because competition for resources increases as the population approaches the carrying capacity.

Figure 30.13Density-independent effects.

The impact of a density-independent factor, such as weather or a natural disaster, is not influenced by population density. Two populations of field mice are in the path of a flash flood. (a) In the low-density population, 3 out of 5 mice drown, a 60% death rate. b. In the high-density population, 12 out of 20 mice drown—also a 60% death rate.

Figure 30.14Density-dependent effects—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 available resources, and some fail to gain access.

Figure 30.15Density-dependent effects—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 finding the prey are low, resulting in little predation (mortality rate of 0/2, or 0%). b. In the higher-density population, there is a greater likelihood of the predator locating potential prey, resulting in a greater predation rate (mortality rate of 50/100, or 50%).

Figure 30.16Predator-prey cycling of a lynx and a snowshoe hare.

The number of pelts received yearly by the Hudson Bay Company for almost 100 years shows a pattern of ten-year cycles in population densities. The snowshoe hare (prey) population reaches a peak before that of the lynx (predator) by a year or more.

Data from D. A. MacLulich, Fluctuations in the Numbers of the Varying Hare (Lepus americanus), University of Toronto Press, Toronto, 1937, reprinted 1974, p. 543.

Check Your Progress

Explain why competition and predation are considered density-dependent factors regulating population growth.

Answer:With both factors, the percentage of the population affected increases as the density of the population increases.

Check Your Progress

1. Contrast opportunistic populations with equilibrium populations.

2.
List five factors that can determine whether an equilibrium population is in danger of extinction.

Answers:1. Opportunistic populations exhibit exponential growth and are regulated by density-independent effects. Equilibrium populations exhibit logistic growth and remain near carrying capacity.2. Time to maturity; number of offspring produced; size of geographic range; degree of habitat tolerance; and size of local populations.

Figure 30.17Life history patterns.

a. Dandelions are an opportunistic species, whereas (b) mountain gorillas are an equilibrium species.

Figure 30.18Vulnerability to extinction.

Vulnerability is particularly tied to range, habitat, and size of population. The amount of white-highlighted text in the boxes indicates the chances and causes of extinction.

Check Your Progress

1. Which levels of biological organization are of interest to ecologists?

2.
How is conservation biology like environmental science?

Answers:1. Ecologists study organisms, populations, communities, ecosystems, and the biosphere.2. Both of these fields apply the principles of ecology to matters of practical concern to humans.

Figure 30.19Levels of organization.

The study of ecology encompasses the organism, population, community, and ecosystem levels of biological organization.

Figure 30.20Studying the effects of fire.

Fire is a natural occurrence in lodgepole pine forests, such as these in Yellowstone National Park. Over time, the recently burned forest (a) will mature and look like the unburned forest (b).

a. After a burn.

b. Before a burn.

Figure 30.3More-developed versus less-developed countries.

People in the (a) more-developed countries have a high standard of living and will contribute the least to world population growth, while people in the (b) less-developed countries have a low standard of living and will contribute the most to the world population growth.

etc.

801–12

1,000–199

Age (years)	Number of survivors at beginning of year	Number of deaths during year
0–1	1,000	199
1–2	801	12
2–3	789	13
3–4	776	12
4–5	764	30
5–6	734	46
6–7	688	48
7–8	640	69
8–9	571	132
9–10	439	187
10–11	252	156
11–12	96	90
12–13	6	3
13–14	3	3
14–15	0

Figure 30.12Biotic potential.

The ability of many populations, including apple trees, to reproduce exceeds by a wide margin the number necessary to replace those that die.