CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

17. Population Ecology

17.7. Limiting Factors to Human Population Growth

Today we hear differing opinions about the state of the world’s human population. On one hand we hear that the population is growing rapidly. By contrast we hear that some countries are afraid that their populations are shrinking. Other countries are concerned about the aging of their populations, because birthrates and death rates are low. In magazines and on television, we see that there are starving people in the world. At the same time, we hear discussions about the problem of food surpluses and obesity in many countries. Some have even said that the most important problem in the world today is the rate at which the human population is growing. Others maintain that the growing population will provide markets for goods and be an economic boon. How do we reconcile this mass of conflicting information?

It is important to realize that human populations follow the same patterns of growth and are acted on by the same kinds of limiting factors as are populations of other organisms. The growth of the human population over the past several thousand years follows a pattern that resembles the lag and exponential growth phases of a population growth curve. It is estimated that the human population remained low and constant for thousands of years but has increased rapidly in the past few hundred years (figure 17.14). For example, it has been estimated that before European discovery, the Native American population was at or near its carrying capacity. Although it is impossible to know the Native American population at that time, various experts estimate it was between 1 million and 18 million. Today, the population of the United States is nearly 310 million people. Does this mean that humans are different from other animal species? Can the human population continue to grow forever?

FIGURE 17.14. Human Population Growth

The number of humans doubled from a.d. 1800 to 1930 (from 1 billion to 2 billion), had doubled again by 1975 (4 billion), and is projected to double again (8 billion) by about 2025. How long can the human population continue to double before the Earth’s ultimate carrying capacity is reached?

The human species has an upper limit set by the carrying capacity of the environment, as does any other species. However, the human population has been able to increase astronomically because technological changes and the displacement of other species have allowed us to shift the carrying capacity upward. Much of the exponential growth phase of the human population can be attributed to improved sanitation, the control of infectious diseases, improvements in agricultural methods, and the replacement of natural ecosystems with artificial agricultural ecosystems. But even these conditions have their limits. Some limiting factors will eventually cause a leveling off of our population growth curve. We cannot increase beyond our ability to get raw materials and energy, nor can we ignore the waste products we produce and the other organisms with which we interact.

Availability of Raw Materials

To many of us, raw materials consist simply of the amount of food available, but we should not forget that, in a technological society, iron ore, lumber, irrigation water, and silicon chips are also raw materials. However, most people of the world have much more basic needs. For the past several decades, large portions of the world’s population have not had enough food (figure 17.15). Although it is biologically accurate to say that the world can currently produce enough food for everyone, there are complex political, economic, and social issues related to food production and distribution. Probably most important is the fact that the transportation of food from centers of excess to centers of need is often very difficult and expensive. Societies with excess food have been unwilling to bridge these political and economic gaps.

FIGURE 17.15. Food Is a Raw Material

However, a more fundamental question is whether the world can continue to produce enough food. In 2010, the world population was growing at a rate of 1.2% per year. This amounts to about 150 new people added to the population every minute. This would result in an increase from our current population of nearly 6.9 billion to about 9.5 billion by 2050. With a continuing increase in the number of mouths to feed, it is unlikely that food production will be able to keep pace with the growth in human population (How Science Works 17.1).

A primary indicator of the status of the world food situation is the amount of grain produced for each person in the world (per capita grain production). World per capita grain production peaked in 1984.

HOW SCIENCE WORKS 17.1

Thomas Malthus and His Essay on Population

In 1798, Thomas Robert Malthus, an Englishman, published an essay on human population, presenting an idea that was contrary to popular opinion. His basic thesis was that the human population increased in a geometric, or exponential, manner (2, 4, 8, 16, 32, 64, etc.), whereas the ability to produce food increased only in an arithmetic manner (1, 2, 3, 4, 5, 6, etc.). The ultimate outcome of these different rates would be that the population would outgrow the land's ability to produce food.

He concluded that wars, famines, plagues, and natural disasters would be the means (limiting factors) of controlling the size of the human population. His predictions were hotly debated by the intellectual community of his day, and his assumptions and conclusions were attacked as erroneous and against the best interest of society. At the time he wrote the essay, the popular opinion was that human knowledge and "moral constraint" would be able to create a world that would supply all human needs in abundance. One of Malthus's basic postulates was that "commerce between the sexes" (sexual intercourse) would continue unchanged. Other philosophers of the day believed that sexual behavior would take less procreative forms and the human population would be limited. Only within the past 50 years, however, have effective conception-control mechanisms become widely accepted and used. However, even today they are used primarily in the more developed countries of the world.

Malthus did not foresee the use of contraception, major changes in agricultural production techniques, or the exporting of excess people to colonies in the Americas. These factors, as well as high death rates, prevented the most devastating of his predictions from coming true. However, in many parts of the world, people are experiencing the forms of population control (famine, epidemic disease, wars, and natural disasters) that Malthus predicated in 1798. Many people believe that his original predictions were valid—only his time scale was not correct—and that his predictions are coming true today.

Another important impact of Malthus's essay was its effect on young Charles Darwin. When Darwin read it, he saw that what was true for the human population could be applied to the whole of the plant and animal kingdoms. As over reproduction took place, there would be increased competition for food, resulting in the death of the less-fit organisms. This was an important part of his theory of natural selection.

Thomas Robert Malthus

Availability of Energy

The availability of energy also affects human populations. All species on Earth ultimately depend on sunlight for energy. All energy—whether power from a hydroelectric dam, fossil fuels, or solar cell—is derived from the Sun. Energy is needed for transportation, the construction and maintenance of homes, and food production. It is very difficult to develop unbiased, reasonably accurate estimates of global energy reserves in the form of petroleum, natural gas, and coal. Therefore, it is difficult to predict how long these reserves might last. However, the quantities are limited and the rate of use has been increasing (figure 17.16).

FIGURE 17.16. Humans' Energy Use

If the less-developed countries were to attain a standard of living equal to that of the developed nations, the global energy reserves would disappear overnight. People should realize that there is a limit to our energy resources; we are living on solar energy that was stored over millions of years, and we are using it at a rate that could deplete it in hundreds of years.

Accumulation of Wastes

One of the most talked about aspects of human activity is the problem of waste disposal. We have normal biological wastes, which can be dealt with by decomposer organisms. However, we also generate a variety of technological wastes and by-products that cannot be degraded efficiently by decomposers (figure 17.17). Most of what we call pollution results from the waste products of technology. The biological wastes usually can be dealt with fairly efficiently by building wastewater treatment plants and other sewage facilities. Certainly, these facilities take energy to run, but they rely on decomposers to degrade unwanted organic matter to carbon dioxide and water. Earlier in this chapter, we discussed the problems aquarium organisms face when their metabolic waste products accumulate. In this situation, the organisms so “befoul their nest” that their wastes poison them. Are humans in a similar situation on a much larger scale? Are we dumping so much technological waste, much of it toxic, into the environment that we are being poisoned? Is carbon dioxide from the burning of fossil fuels changing our climate?

FIGURE 17.17. Human Waste Production

Interactions with Other Organisms

Humans interact with other organisms in as many ways as other animals do. We have parasites and, occasionally, predators. We are predators in relation to a variety of animals, both domesticated and wild. We have mutualistic relationships with many of our domesticated plants and animals, because they could not survive without our agricultural practices, and we would not survive without the food they provide. Competition is also very important. Insects and rodents compete for the food we raise, and we compete directly with many other kinds of animals for the use of ecosystems.

As humans convert more and more land to agriculture and other purposes, many other organisms are displaced (figure 17.18). Many of these displaced organisms are not able to compete successfully and must leave the area, reduce their populations, or become extinct. The American bison (buffalo), African and Asian elephants, panda, and grizzly bear are a few species that have reduced populations because they were not able to compete successfully with the human species. The passenger pigeon, Carolina parakeet, and great auk are a few that have become extinct. Our parks and natural areas have become tiny refuges for plants and animals that once occupied vast expanses of the world. If these refuges are lost, many organisms will become extinct. What today might seem to be an insignificant organism may tomorrow be seen as a link to our very survival. We humans have been extremely successful in our efforts to convert ecosystems to our own uses at the expense of other species.

FIGURE 17.18. Interaction with Other Organisms

Competition with one another (intraspecific competition), however, is a different matter. Because competition is negative for both organisms, competition between humans harms humans. We are not displacing another species; we are displacing our own kind. Certainly, when resources are in short supply, there is competition. Unfortunately, it is usually the young who are least able to compete, and high infant mortality is the result.

17.7. CONCEPT REVIEW

18. As the human population continues to grow, what should we expect to happen to other species?

19. How does the shape of the population growth curve of humans compare with that of other kinds of animals?