Ecology and Population Growth



Population Ecology: How Do Organisms Interact to Form Populations?

Ecology is the study of all those complex interactions referred to by Darwin as the conditions of the struggle for existence.

—Ernst Haeckel, 1870

Chapter opening photo Humans seem to live everywhere.

Overview Until now, we have viewed

biology by peering deep

inside individual organisms.

We have focused on complex

cells, chemicals, and

processes that, in concert,

produce, maintain, and

reproduce individuals. What

we have seen thus far is a

series of hierarchical levels

leading to individuals. Atoms

and elements are organized

into molecules and

compounds, the largest and

most complex of which are

biochemicals. These, in turn,

are organized into cells,

complex entities often made


c15.qxd 12/10/04 9:40 AM Page 487

488 CHAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations?

Figure 15-1 ■ Any situation in which organisms interact with their environment is of potential interest to ecologists. This includes (a) vultures feeding at a lion kill and (b) rats feeding in an urban trash can.

(a) (b)

up of smaller parts called organelles. Cells, we have seen, are the

building blocks of life. As with buildings, what is important to life is

not only their blocks but how they are arranged. Cells are

organized into individuals.

In the next two chapters, we will focus on the discovery that

life’s hierarchy doesn’t stop at three levels. Individuals are

organized into populations, which are organized into biological

communities, which are organized into even higher levels,

ecosystems. Studying these higher levels of organization and the

processes and factors that explain them is the subject of ecology,

one of biology’s youngest and most complex branches.

15-1 What Is Ecology? For much of the 20th century, ecology was little noted or understood outside of academic circles. Then came the 1970s. Within the course of a few years, environmental awareness blossomed and ecology was swept into the limelight. Books, television programs, newspaper articles, and even movies appeared, claiming to be ecological. “Save the Ecology” became a theme at public demonstrations, often with the underlying assump- tion that the primary goal was to save pristine wilderness.

But environmental concern is not ecology. Certainly, ecological principles are at work in wilderness, but they are equally at work in situations dominated by human activities. Vultures scavenging a lion kill in east Africa are of interest to ecologists. So are rats scavenging dumpsters in urban America (Figure 15-1).

So, what exactly is ecology? And what is it not?

c15.qxd 12/10/04 9:40 AM Page 488

15-1 What Is Ecology? 489

Figure 15-2 ■ In Denali National Park, Alaska, in summer grizzly bears spend much of their time digging out Arctic ground squirrels from rocky burrows.

Ecology Is a Branch of Biology Ecology involves the study of organisms in relation to their environments. Organisms cannot exist as isolated entities; organisms interact. They interact with their physical environment (soil, climate, light and dark, everyday weather). They interact with others of their kind. They interact with other species, including those they try to eat, those that try to eat them, those with which they compete, and those with which they cooperate. Ecology also involves the abundance and distribution of animals and plants. This description of ecology may come as a surprise to some. In everyday usage, “ecology” appears to mean something else. It is important to remember several key factors:

Ecology is not a social cause. Although the word “ecology” may be in mainstream societal consciousness, ecology is not a movement.

“Ecology” is not the same as “environment.” Ecology is a branch of biology; environ- ment means surroundings. Does this mean that ecology, the science, has nothing to con- tribute to environmental issues? Of course not. But it’s important to remember the limits of the contributions that ecology can make. How would other organisms and the envi- ronment as a whole be changed if California condors become extinct in southern Cali- fornia? Ecology can answer that. Should we spend money to save them? Such political or moral questions cannot be answered by ecology.With many topics of interest to both environmentalists and ecologists, science and politics overlap, or at least butt up against one another, but they are not the same. Our goal here is to keep separate that which is sci- entific from that which is political; we want to keep the factual separate from the emotional.

Ecology is not natural history. Fascinating stories of nature have captivated people for thousands of years. Usually they stress organisms living in and reacting to particular environments. Isn’t that ecology? Not really. There is a difference between natural history and ecology. Let an example illustrate: One of the most conspicuous large mammals in Denali National Park in central Alaska is the grizzly bear. Roads allow visitors and scientists to sit comfortably and safely in buses or observation huts and make extensive observations on bears in their natural habitat. Frequently, especially late in summer, grizzlies are seen digging up ground squirrel burrows.They will spend half a day moving room-sized mounds of soil, rocks, and plant material to catch a small meal. All the while, caribou move past the slaving bears, neither seemingly noticing the other (Figure 15-2).

This is natural history—interesting stories of animals in nature. It leads to an inter- esting question: Why would a grizzly bear spend so much time hunting ground squirrels when much larger prey is seemingly at hand? Can this question be answered scientifically?

What Is Ecology? Observe the relationships among ecosystems including the biosphere.

c15.qxd 12/10/04 9:40 AM Page 489

490 CHAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations?

Yes. Ecologists might approach the question by considering energy requirements. In the Denali area, the weight of the average grizzly bear is about three times that of the average human. Obviously, larger animals need more energy than smaller ones, but it turns out that grizzly bears need considerably less than three times more energy than humans. In 1945, the animal physiologist Samuel Brody reported a relationship between a mam- mal’s size and its minimal daily energy needs in the form of an equation, where E is the energy needed for one day and W is the animal’s weight. Plug in the average human’s weight (60 kg), and E is nearly 1500 kcal—not a bad approximation for our average minimal daily energy needs. Plug in the average Denali Park grizzly bear’s weight (200 kg), and the energy required is about 4000 kcals per day.

The average Arctic ground squirrel weighs in at around 2400 g, nearly three-fourths of which is water. In late summer, when they are preparing for a long winter’s hiberna- tion, half of the remaining grams are pure fat while the rest are mainly proteins and car- bohydrates. Each gram of fat contains 9 kcal of energy. Each gram of protein and carbohydrate has about 5 kcal of energy. Run these figures through a calculator to see that the average Arctic ground squirrel is packed with about 4200 kcals of energy. All the average grizzly bear needs to do is catch one Arctic ground squirrel each day and its basic, daily energy needs are filled.

Still, one caribou weighing 130 kg could provide a grizzly bear’s energy needs for several days. Predators often have a choice of prey. Usually the environment offers a number of potential prey species. Ecologists evaluate how predators choose prey in terms of energy efficiencies. What is the ratio, for example, between energy expended catching a caribou compared with the energy payoff once it is captured? While it is true that a caribou would supply the bear’s energy requirements for several days, caribou are not easy to kill. If a digging bear even glances up, a nearby caribou can easily bound away. To chase a caribou, assuming that it is not sick, wounded, or very young, is to expend a lot of energy on an effort that probably will not be successful.

Hunting Arctic ground squirrels, it turns out, is much more efficient. Burrows have only one entrance. If the bear just keeps digging it will eventually be successful. At the end of the burrow is a 4200 kcal tidbit. Now we have a plausible explanation for what at first appeared to be bizarre behavior in grizzly bears.

Ecology is science. Notice that, until we got quantitative and started thinking in terms of energy needs, efficiencies, and balances, we had little more than an interesting story. Modern ecology, like other modern sciences, is quantitative. Ecological research often uses the same methods, steps, and processes as other branches of biology. It usu- ally starts with observations gathered in the field, but moves quickly from description to experimentation. Testable hypotheses are proposed. At its best, ecology takes mea- surements for testing hypotheses. Measuring specimens is easy enough—length, weight, degree of coloration. But how does one measure an organism’s surroundings? Doing so is the heart of ecology. Data are collected, analyzed, and related to what is already known from previous studies. Ecology strives to get to the point where predictions can be reli- ably made. This step often pushes existing ecology to its limits. Great progress has been made in recent years and ecology’s track record is constantly improving.

Ecologists need at least a basic understanding of nearly every other branch of sci- ence. Biochemistry, taxonomy, physiology, geology, and statistics are especially important to ecologists. In addition to understanding other sciences and branches of biology, ecol- ogists have gathered their own store of knowledge, some of which will engage us for the rest of the chapter.

Ecology Grew from Natural History Ecology as a science has a relatively brief history, encompassing only the 20th century. In a sense, however, interest in organisms and their surroundings is as old as humanity itself. The earliest humans needed to know when and where to find game, edible plants,

E = 72W3>4,

c15.qxd 12/10/04 9:40 AM Page 490

15-1 What Is Ecology? 491

and other life-supporting materials. Later, at the dawn of history, as humans switched to pastoral lifestyles, knowledge we would call ecological was probably essential: Where will crops grow best? Where are reliable sources of water? How can we discourage pests? What would it take to domesticate chickens, pigs, and cattle? These were the kinds of questions that captivated early farmers. By the Middle Ages, a vast store of ecological folk wisdom existed throughout the world.

Starting in the 1300s, opportunities to study natural history flourished, particularly among Europeans.World travelers brought back to Europe extensive collections of specimens. These collections launched Linnaeus, Darwin, and others on their life’s work. Interest went beyond the specimens themselves. Where did they come from? Under what conditions did they live? How did they behave? Travelers and explorers were eager to supply answers. By the time of Darwin, Europeans and North Americans had an extensive literature of natural history. Unfortunately, accuracy varied among these accounts, and readers sometimes had no way of distinguishing between whales and sea monsters, elephant seals and mermaids, and kangaroos and unicorns.

By the last half of the 19th century, the stage was set to move beyond folk wisdom, stories, and traveler’s tales. Darwin had interpreted differences between populations of organisms as arising in response to natural selection. Soon after he wrote, others, including Haeckel, whose quote heads this chapter, began intellectually to build on Darwin’s foun- dation. What aspects of the environment frame and influence natural selection? This question, rooted in a rich tradition of natural history, led to the science of ecology.

The growth of many important ideas in biology can be likened to a tree, with one unifying idea emerging from an extensive historical system of roots. The trunk is a sin- gle individual—Darwin, Mendel—or a small group—Schlieden, Schwann, and Virchow. No such individual emerges as the founder of ecology. Rather, ecology can be likened to a bush (Figure 15-3). Near the end of the 19th century, out of a tangled root system of natural history, anecdotes, and folk wisdom, four main stems emerged that led to modern ecology:

1. Early botanists developed plant biogeography. Along with the extensive plant col- lections in Europe’s museums and universities, gathered over a 500-year period, were extensive notes on where each specimen was collected and the environmental conditions in that place. Over the years, accuracy and completeness improved. From these collections, predictions followed. Expect to find cactuses in deserts. Expect to find few trees in areas receiving less than 12 inches of rain per year. Expect to find orchids sprouting from branches on tropical trees. There were exceptions, of course, just as there are for many generalizations in biology. But as generalizations, the statements held true.

2. Meanwhile, other early ecologists focused on what we would today call environ- mental physiology. How does living in a particular environment affect an animal’s structure and function? Most polar animals are white. Many desert plants store water and have extensive root systems. Marine mammals use tear glands to get rid of excess salt. These are examples of how physiology is attuned to environment.

3. In Europe, during the early 1900s, a group of biologists launched limnology, the extensive study of freshwater ponds and lakes. They introduced the idea that whole systems, comprising many different species and many different physical factors, interacted and could be simultaneously studied. These studies became another branch of early ecology.

4. Still another group of biologists was interested in behavioral ecology. How does liv- ing in a particular area with a particular set of characteristics affect the way animals behave? Within a few decades of the 20th century, such studies focused on a diverse group of insects, birds, and mammals.

c15.qxd 12/10/04 9:40 AM Page 491

492 CHAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations?

By the end of the 19th century, these four somewhat dissimilar kinds of studies, along with certain studies of marine biology, economic biology, and latter-day natural history, became lumped into ecology, a new branch of biology. The scientists conducting these diverse inquiries were themselves diverse, coming from different backgrounds, national- ities, and scientific traditions. It may not be surprising, then, that they failed to share a com- mon vision. What is this new science of ecology? There was, at first, little agreement. Indeed, one of the first serious disagreements was on how the word should be spelled, ecology or oecology. Discussions became so heated that the field nearly split in two.

Behind this seemingly trivial argument was a more important one. How should ecol- ogy best be studied? One group took a holistic approach, feeling that whole systems should be studied as units. By and large, these were the botanists and limnologists. Their studies of plants, particularly plant biogeography, and of water systems emphasized important associations between organisms and environments. Zoologists, on the other hand, tended to be more reductionist in approach. They believed that the best way to understand complex biological systems was to gain intimate and complete understanding of each component population of the system. These early ecologists studied individual species; their work grew out of the tradition of natural history studies.

Arguments between the two camps raged and continue to do so. We might like to think that disputes in science are worked out in sophisticated, intellectual ways. Alas, such is often not the case. Scientists are nothing if not human, and disputes often get rancorous and personal. Examining these disputes, while an amusing enterprise, teaches us little about ecology. One important point can be made: Ecology is no stranger to controversy.

At the beginning of the 21st century, three of the main branches of ecology are pop- ulation ecology, systems ecology, and applied ecology.

P la

nt biogeo graphy


ehavioral ec o

lo g


Lim no

l o g y

En vir

on m

en ta

l p hy

si o

lo g


P op


at io

n e co

log y

Systems ecology

Applied ecology

A n

e c

d otes

Natur al his

tor y

Folk w is dom



Figure 15-3 ■ The history of ecology is more like a bush than a tree. Its origin cannot be attributed to any single person or idea. Rather, the science of ecology has been the work of numerous scientists, primarily of the 20th century.

c15.qxd 12/10/04 9:40 AM Page 492



15-1 What Is Ecology? 493

Figure 15-4 ■ Restoration ecology is the youngest branch of contemporary ecology. Here, a wetlands, damaged by development, is being restored.

1. Population ecology focuses on dynamic changes occurring in one population or species. Here, relatively narrow questions are answered in great detail. These spe- cialists might, for example, follow and predict changes in specific deer populations in temperate deciduous forests of North America. This chapter will focus on factors important to population ecologists.

2. Systems ecology studies the dynamics of complex ecological communities and is holistic in approach. Complex systems, with all their many interacting parts, are studied as units by systems ecologists concerned with “big picture” concepts and questions. They might, for example, be interested in looking at the flow of energy through a temperate deciduous forest community ranging from browse plants to deer to wolves and other predators. In Chapter 16, we will stress factors particularly important to systems ecologists.

3. Applied ecology is modern ecology’s newest branch. This specialty predicts outcomes of human activities and recommends courses of action to mitigate certain of those activities (Figure 15-4). Within the field are several subspecialties, the best known, perhaps, being restoration ecology. Applied ecologists may be in the limelight, working with economists, politicians, business leaders, and community groups. They might, for example, study what could be done to restore deer habitat in an area that was once temperate deciduous forest.We will discuss specific examples in both this chapter and Chapter 16.

Applied Ecology in Action Find examples of applied ecology projects that are currently being conducted. What are the overall goals of the project? How are the goals being pursued? Are any applied ecology projects being conducted in your area?

The science of ecology continues to change and grow. The Gaia hypothesis views the entire planet as a self-regulating entity that has been likened to a supraorganism. Spiritual ecology attempts to bridge the gap between ecology, environmental awareness, and spiri- tuality. The proper role that these ideas may play in the future science of ecology is being hotly debated among ecologists. Visit our Web site for a discussion of these controversies.

More on Gaia What is the current status of the Gaia hypothesis? What are the pros and cons of spiritual ecology?

c15.qxd 12/10/04 9:40 AM Page 493


494 CHAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations?

Piecing It Together

Ecology, that branch of biology dealing with relationships between organisms and their environments, including other organisms, differs from other branches of biology in at least three ways:

1. It is relatively young and, unlike other branches of biology, draws on the work of many founders rather than one or a few.

2. It has captured the imagination of the general public.

3. It is often confused with things it is not. There is a difference between “ecology” and “environment.” There is also a difference between “ecology” and “natural history.” Both environmentalism and natural history are important; they fulfill our everyday need to know about and respect the organisms with which we share habitat and a planet. But ecology is something different. Ecology is science.

15-2 How Do Ecologists Study Populations? In a meditation about human life, John Donne wrote, “No man is an island.” The same is true for any individual plant or animal. Individuals are always part of something larger, namely populations. In some respects, populations act like organisms. They require space and nutrients. They move through definite daily and seasonal cycles. They grow, reproduce, and die. But populations have other properties that are unique. They have an age structure, a sex structure, and death and birth rates. They interact with other popu- lations and with environmental pressures.

When ecologists study populations, they ask certain questions: How large is the pop- ulation and how is it changing? How do populations interact? Can we control changes in a population? Before addressing these questions, let’s start with a bit of history.

See How We Grow Growth of the human population. What is the estimated size of the worldwide human population today? Watch how that number changes over the next few days.

The Study of Populations Has Been One of Ecology’s Major Tasks Ecology inherited concerns about populations. In 1798, Thomas Malthus, a British cler- gyman and economist, published An Essay on the Principle of Population. His major thesis was an observation: Humanity has an innate and almost unlimited ability to pro- create, but a limited ability to produce food. Thus, human populations tend to grow and outstrip their ability to feed themselves, which inevitably leads to problems. His rather pessimistic prediction was that if human population growth were left unchecked, the result would be pestilence, war, and famine.

If a person’s worth can be measured by the amount of controversy his works gener- ate, Thomas Malthus was one of the greatest writers of all times. His essays were instant- ly controversial because he challenged head-on the prevalent view of his day that humanity was moving inexorably toward social perfection, becoming a Utopian state in which all would be fully provided. Heaven on Earth was just a matter of time and continued progress. No, said Malthus, humanity is moving inexorably toward misery—misery that can only grow and continue indefinitely. The Establishment did not take his challenge lightly.

c15.qxd 12/10/04 9:40 AM Page 494

15-2 How Do Ecologists Study Populations? 495

Figure 15-5 ■ Modern farms are large, expensive to operate, and highly efficient. They have been described as food factories.

1 Why rates and not absolute numbers? Rates work well in calculations and make comparisons between populations easier. Rates are always expressed over an appropriate time interval, as in “births per year.”

Nor do they do so today. Two hundred years after publication, his works continue to be widely quoted, to challenge established policies, and to spark controversy. Cer- tainly, he underestimated humanity’s ability to grow food (Figure 15-5). But his overall thesis—that our ability to procreate is nearly unlimited while our ability to produce food sooner or later will meet its limits—concerns some policy makers today.

More important to our story, his concerns went beyond the populations of humans. He observed, “Through the animal and vegetable kingdoms, nature has scattered the seeds of life abroad with the most profuse and liberal hand. She has been comparatively sparing in the room and nourishment necessary to rear them.” In this and other passages, he focused attention on populations of animals and plants. Malthus’s works were well known to Darwin. Remember that, nearly half a century after the essay’s first publication, Darwin observed that “all populations’ ability to increase is greater than required,” which is very nearly a restatement of one of Malthus’s major points.

After Darwin, population studies, particularly of animals, flourished. In the 20th cen- tury, there have been many studies of invertebrates, especially insects (because of their eco- nomic importance), farm animals (for the same reason), and game animals (because of an intense need to manage and predict changes in their populations). These studies continue today. Only recently have ecologists begun to study population dynamics in plants.

The Size of a Population Is Determined by Natality, Mortality, Immigration, and Emigration The size of any population is in large part determined by a balance between several factors, some obvious, some less so. One obvious factor is the number of new individuals being born into the population. This is usually expressed as its birth rate, or natality.1

Another factor, working in the opposite direction, is the population’s death rate, or mortality. Theoretically, any population is stable—it neither grows nor shrinks—if these rates are balanced. If not, the population changes. That is, the population’s growth rate is determined by its birth and death rates. Said more succinctly, where r isr = 1b - d2,

Growth Rate: Determine the growth rate of two populations.

c15.qxd 12/10/04 9:40 AM Page 495

496 CHAPTER 15 Population Ecology: How Do Organisms Interact to Form Populations?




Forty-Mile Herd

Nelchina Herd


Bering Sea

Figure 15-6 ■ The Nelchina caribou herd ranges over a large area of central Alaska. In the early 1980s, segments of the herd joined the Forty-mile herd to the northeast in a large emigration that significantly reduced the size of the Nelchina herd. (Map adapted from J. E. Hemming, 1971. The distribution movement patterns of caribou in Alaska. Wild Tech Bul 1, APFG p2.)

the population’s growth rate (also referred to as “little r”), b is its birth rate, and d is its death rate. (What happens to a population if its ? In each case, what are the values of its “little r”?)

Simple, right? Let’s see how things work out in real life. In south central Alaska, about equally distant from its two largest cities, Anchorage and Fairbanks, lives the Nelchi- na caribou herd (Figure 15-6). During its fall migration, the bulk of the herd comes close to a well-traveled highway, and sport hunters kill many of the animals each year. In the early 1960s, game managers, charged with the responsibility of managing the herd, wor- ried about how this hunting pressure might affect the herd’s size. They launched an ex- tensive study of the herd’s population dynamics. They determined the birth rate by monitoring the calving grounds in early summer. They already knew a great deal about the natural mortality rate of the population, caused mainly by wolves, bears, wolverines, and severe winter weather. Their results revealed that the population’s “little r” was in fact quite large, indicating that the herd’s numbers would probably increase. A large harvest was thought to be not only possible, but desirable to keep the population from growing too large. For a time, hunter success was consistently high. But, in the early 1980s, when sport hunters took to the field, few animals were found. What had happened?

Another factor important to population dynamics is movements of individuals be- tween populations. There are two types: Immigrations bring new individuals into a pop- ulation, while emigrations remove individuals. Some such movements are regular and easy to predict. At the start of the breeding season, nearly all members of certain song- bird populations leave winter homes in Central and South America and migrate to nest- ing grounds in North America. In other cases, predictions are not so easy. Apparently, large segments of the Nelchina caribou herd emigrated to an adjacent herd—the Forty- mile. Mass movements from the Nelchina had not been observed before and have not occurred since, but have been observed in other herds of caribou. Such movements make predicting future changes in populations difficult at best. At least we can rewrite our mathematical formula relatively easily as:

where the new terms are i, the population’s immigration rate, and e, its emigration rate.

r = 1b - d2 + 1i - e2,

b 7 d; b 6 d; b = d

c15.qxd 12/10/04 9:40 AM Page 496

15-2 How Do Ecologists Study Populations? 497

Population Characteristics Are Expressed in a Variety of Ways A word of caution: Beware when reading about populations. Relevant characteristics such as size and mortality each can be expressed in more than one way. The most straight- forward expression of population size is the absolute number. Count all individuals in the population and express the total. This is almost never done, because it is nearly im- possible to count each individual. A notable exception is the U.S. population census taken every 10 years—a massive effort that is fraught with errors and assumptions.

More frequently, population size is expressed in terms of density, the number …