A population is the number of individuals of a given species , usually within a specified habitat or area. Within the science of ecology , the study of population dynamics, or the ways in which the number of individuals in a community expands and contracts, makes up an important subfield known as population biology . While ecologists and wildlife biologists rely on understanding the nature of population change and the factors that influence population size, one of the main ways population studies have been used is in projecting the growth of the human population. Since the 1960s human population growth has been an issue of fierce theoretical debate. Many scientists and social planners predict that the human population, like animal populations frequently observed in nature, is growing at a perilous rate that will ultimately lead to a catastrophic die-off in which billions of people will perish. Other scholars and planners argue that current trends may not lead to disaster and that we may yet find a reasonable and stable population level. Both sides of the debate use ecological principles of population biology in predicting human population dynamics and their effect upon the conditions in which we may live in the future.
The extent of population growth is based on fertility, or the number of offspring that individuals of reproductive age successfully produce. In some animal and plant species (flies, dandelions) every individual produces a great number of offspring. These populations grow quickly, as long as nutrients and space are available. Other species (elephants , pandas) produce few offspring per reproductive adult; populations of these species usually grow slowly even when ample food and habitat are available. The maximum number of offspring an organism can produce under ideal conditions is known as the biotic potential of that organism. For instance, if an individual housefly can produce 120 young in her lifetime, and each of those successfully produces 120 young, and so on, the fly's biotic potential in one year (seven generations) is nearly six trillion flies. Clearly biotic potentials are rarely met: reproductive failures and life hazards usually prevent the majority of houseflies, like other species, from achieving their theoretical reproductive potential.
Populations increase through reproduction, but they do not increase infinitely because of environmental limitations, including disease, predation, competition for space and nutrients, and nutrient shortages. These limits to population growth are collectively known as environmental resistance .
An important concept in environmental resistance is the idea of carrying capacity . Carrying capacity is the maximum number of individuals a habitat can support. In any finite system, there is a limited availability of food, water, nesting space, and other essentials, which limits that system's carrying capacity. When a population exceeds its environment's carrying capacity, shortages of nutrients or other necessities usually weaken individuals, reduce successful reproduction, and raise death rates from disease, until the population once again falls below its maximum size. A population that grows very quickly and exceeds its environment's carrying capacity is said to overshoot its environment's capacity. A catastrophic dieback , when the population plummets to well below its maximum, usually follows an overshoot. In many cases populations undergo repeated overshoot-dieback cycles. Sometimes these cycles gradually decrease in severity until a stable population, in equilibrium with carrying capacity, is reached. In other cases, overshoot-dieback cycles go on continually, as in the well known case of lemmings. Prolific breeding among these small arctic rodents leads to a population explosion every four to six years. In overshoot years, depletion of the vegetation on which they feed causes widespread undernourishment, which results in starvation, weakness, and vulnerability to predators and disease. The lemming population collapses, only to begin rebuilding, gradually approaching overshoot and another dieback. At the same time, related populations fluctuate in response to cycles in the lemming population: populations of owls and foxes surge when lemmings become plentiful and fall when lemmings are few. The grasses and forbs on which lemmings feed likewise prosper and diminish in response to the lemming population.
As a population grows, the number of breeding adults increases so that growth accelerates. Increase at a constant or accelerating rate of change is known as exponential growth . If each female lemming can produce four young females, and each of those produces another four, then the population is multiplied by four in each generation. After two generations there are 16 (or 24) lemmings; after three generations there are 64 (34); after four generations there are 256 (44) lemmings, if all survive. When environmental resistance (predation, nutrient limitations, and so on) causes a population to reach a stable level, without significant increases or decreases over time, population equilibrium is achieved. Generally we might consider stable (equilibrium) populations the more desirable situation because repeated diebacks involve extensive suffering and death.
These principles of population biology have strongly informed our understanding of human population changes. Over the centuries the world's human population has tended to expand to the maximum allowed by available food, water, and space. When humans exceeded their environment's carrying capacity, catastrophic diebacks (usually involving disease, famine ,or war) sometimes resulted. However, in many cases, diebacks have been avoided through emigration, as in European migrations to the Americas, or through technological innovation, including such inventions as agriculture, irrigation , and mechanization, each of which effectively expanded environmental carrying capacities. For tens of thousands of years the human population climbed very gradually, until about the year 1000, when it began to grow exponentially. Where we once needed a thousand years (200 to 1200 a.d.) to double our population from 200 million to 400 million, at current rates of growth we would require only 40 years to double our population. Since the eighteenth century, population theorists have increasingly warned that our current pattern of growth is leading us toward a serious overshoot, perhaps one that will permanently damage our environment and result in a consequent dieback. The only course to avoid such a catastrophe, argue population theorists, is to stabilize our population somewhere below our environment's carrying capacity.
Popular awareness of population issues and agreement with the principle of population reduction have spread in recent years with the publication of such volumes as The Population Bomb by Paul Ehrlich, and The Limits to Growth by Donella Meadows and others. The current population debate, however has older roots, especially in the work of Thomas Malthus, an English cleric who in 1798 published An Essay on the Principle of Population as It Affects the Future Improvement of Society. Malthus argued that, while unchecked human population growth increases at an exponential rate, food supplies increase only arithmetically (a constant amount being added each year, instead of multiplying by a constant amount). The consequence of such a disparity in growth rates is starvation and death. The remedy is to reduce our reproductive rates, where possible by "moral restraint," but where necessary by force. Malthus' work has remained well-known principally because of its conclusion about social policy: because providing food and shelter to the poor only allows them to increase their rates of "breeding," assistance should be withheld. If the poorer classes should starve as a result, argued Malthus, at least greater rates of starvation at a later date would be avoided. This conclusion continues to be promoted today by neo-Malthusians, who protest the principle of aiding developing countries. Poorer countries, neo-Malthusians point out, have especially dangerous growth rates. If wealthy countries provide assistance today because they cannot bear to watch the suffering in poorer nations, they only make the situation worse. By giving aid, donor nations only postpone temporarily the greater suffering that will eventually result from artificially supported growth rates.
Naturally this conclusion is deeply offensive to developing nations and to those who sympathize with the plight of poorer countries. Malthusian conclusions are especially harsh in situations where military and economic repression, often supported by North American and European governments, have caused the poverty. Those who defend aid to developing countries argue that high reproductive rates often result from poverty, rather than vice versa. Where poor nutrition and health care make infant mortality rates high, parents choose to have many children, thus increasing the odds that at least some will survive to support them in their old age. Assistance to poor countries, and to poor areas within a single country, argue anti-Malthusians, helps to alleviate the poverty that necessitates high birth rates. When people become confident of their childrens' survival, then birth rates will drop and population equilibrium may be achieved.
There are some ways in which ecological principles of population dynamics do not necessarily fit the human population. Improving health care and increasing life expectancies have caused much of this century's population expansion. Wars, changes in prosperity, development and transportation of resources, and social changes sometimes increase or decrease populations locally. Immigration and emigration tend to redistribute populations from one region to another, temporarily alleviating or exacerbating population excesses. Also unlike most animals, we have not historically been subject to a fixed environmental carrying capacity. The agricultural revolution , the industrial revolution and other innovations have expanded our environment's capacity to support humans. Some population theorists today, known as technological optimists, insist that, even as the human population continues to grow, our technological inventiveness will help us to feed and shelter the world's population. Even in the past 50 years, when the world's population has jumped from less than three billion to more than six billion, world food production has exceeded population growth. Technological optimists point to such historical evidence as support for their position.
Also unlike most animal populations, humans are, at least in principle, able to voluntarily limit reproductive rates. Most population theorists point to some sort of voluntary restraint as the most humane method of preventing a disastrous population overshoot. Family planning programs have been developed around the world in an effort to encourage voluntary population limits. Furthermore, human societies have been observed to go through a demographic transition as their economic and social stability has improved. A demographic transition is a period when (infant) mortality rates have decreased but people have not yet adjusted their reproductive behavior to improved life expectancies. After 20 to 30 years, people adjust, realizing that more children are surviving and that smaller families now have economic advantages. The birth rate then begins to fall. While families in poor countries often have five to ten children, the average family in developed countries has about two children.
Because of family planning programs and demographic transitions, birth rates around the world have fallen dramatically in just the last 20 years. Where the average number of children per woman was 6.1 in 1970, the average number in 1990 was only 3.4. This rate still exceeds the minimum replacement level (zero population growth level) of 2.1 children per woman, but if recent demographic trends continue the world's population will stabilize within the first decade of the twenty-first century.
No one knows exactly what the earth's carrying capacity is. Some warn that we have already surpassed our safe population size. Others argue that the earth can comfortably support far more people than it currently does. Many worry about what would happen to other species if the human population continues to expand. Most people agree that we would benefit from a decrease in population growth rates. How urgent the population question is and how best to curb growth rates are issues that continue to be hotly debated.
[Mary Ann Cunningham Ph.D. ]
Cunningham, W. P. Understanding Our Environment: an Introduction. Dubuque, IA: Wm. C. Brown, 1993.
Ehrlich, P. R., and A. H. Ehrlich. "The Population Explosion: Why Isn't Everyone As Scared As We Are?" Amicus Journal 12 (Winter 1990): 22–29.