ECOLOGICAL PERSPECTIVES ON POPULATION

Human population size profoundly influences the state of the environment, which in turn has sweeping effects on human vital rates, as well as on the health, standards of living, and personal satisfaction of individual human beings. For most of the approximately five million years of human history, populations exerted mostly local and reversible influence on the environment. But beginning with the agricultural revolution some 10,000 years ago, that changed dramatically.

Human Domination of the Biosphere

From about 1850 to 2002, the human population increased some five-fold and consumption per person about four-fold. The scale of the human enterprise, then, has expanded roughly twenty-fold; as a result, Homo sapiens has become a truly global ecological force.

There is no significant area of the biosphere that has not been altered by human activities. Synthetic pesticides and radioactive materials from nuclear weapons tests have been dispersed globally, and humans have dramatically altered the distribution of key elements such as carbon and nitrogen throughout the land, atmosphere, and oceans. Only a few extremely deep parts of the ocean may still be relatively pristine. More obviously, humans have cut down forests, plowed plains, exterminated many species and populations, paved over large areas, and otherwise transformed most of the land surface. People have released a great variety of novel poisons into the environment, and have degraded oceans and most freshwater bodies by overfishing and pollution.

Ecosystems, Ecosystem Services, and Natural Capital

An ecosystem is defined simply as the community of organisms (plants, animals, microbes) that reside in a given area and the physical environment with which they interact. Human beings are always elements of the ecosystems in which they live, be it the ecosystem of a village, a city, a watershed, a region, a country, or the entire planet. Ecosystems supply human beings with the material basis of life, including food, water, oxygen, wood, energy sources, and metal ores. They also supply an array of indispensable "ecosystem services." These services include climate stabilization; provision of fresh water; control of floods; generation and replenishment of soils essential to agriculture and forestry; detoxification and disposal of wastes; recycling of nutrients; control of the majority of potential crop pests; pollination of crops; provision of forest products and seafoods; and maintenance of a vast genetic library from which humanity has already drawn its crop plants, domestic animals, and a critical portion of its medicines. In extracting benefits from ecosystems, humans also inevitably modify those ecosystems.

The organisms, soils, aquifers, ores, and other features of ecosystems can be thought of as "natural capital": a more fundamental ingredient of economic well-being than the human-made physical and human capital that modern economies so carefully evaluate and husband. The loss of natural capital is all too often irreversible on a time scale relevant to human societies.

Human Impacts on Ecosystem Services

Humans alter ecosystems not only by extracting materials and services from them but also by returning materials and energy (e.g., carbon dioxide, heat, other wastes, synthetic chemicals) to them. In the course of becoming the dominant animal on Earth, human beings also have destroyed outright many natural components of ecosystems through habitat destruction, overexploitation, and depletion, reducing the ability of many natural systems to supply goods and services in the future. On the positive side, the goods and services that are being extracted sustain, at least for the present, a population of more than 6 billion individuals.

A trade-off that is often ignored is the cost in ecosystem degradation that is often a counterpart to the benefits people derive from ecosystem services. Locally, destruction of a wetland may damage its waste disposal service to the degree that artificial treatment is required to protect people from polluted water. Paving over recharge areas may disrupt the flow of clean water into local wells, and overexploiting a local fishery may drive the resource to economic extinction–that is, to the level where harvesting is no longer worth the effort.

Regionally, deforestation of a watershed that formerly discharged a relatively constant surface flow of water may lead to a series of alternating droughts and floods. In Honduras, a combination of population growth and inequitable land tenure arrangements led to forest loss and numerous people living in areas lacking natural protection from floods. In 1998 Hurricane Mitch, an unusually severe storm, killed thousands of people and left hundreds of thousands more homeless in Honduras–illustrating the ecological principle that high population density can produce disproportionate percapita impacts of climatic and other factors that on the surface might seem independent of demography.

There is a small but growing trend toward investing in the restoration of ecosystem assets. In another regional example, New York City's population growth required it to seek more and more distant water sources, and the city eventually developed the Catskill/Delaware Watershed as its main source. The watershed is 100 miles to the north of the city, and it originally supplied water of fabled purity. By the late 1980s, though, water quality had declined to the point where it no longer met Environmental Protection Agency (EPA) standards. The cause was itself indirectly connected with the growth and affluence of New York's population: Suburban sprawl and rising demands for recreation and second homes placed higher demands on the watershed. The city was faced with the choice of building costly new treatment facilities or controlling development in the watershed ecosystem. Economic analyses showed that controlling development for improving water quality would cost far less, even without taking into account the other benefits of preserving the watershed, and that was the solution chosen.

Globally, climate change–traceable primarily to the anthropogenic injection of greenhouse gases into the atmosphere, itself strongly correlated with both human population size and consumption of fossil fuels for energy–threatens to damage agricultural production in many areas, inundate low-lying coastal areas, and spread tropical diseases into temperate regions. Population-related global warming also endangers coral reefs through bleaching, and threatens the persistence of ecologically and economically important tropical reef fisheries.

Loss of biodiversity–the plants, animals, and microorganisms with which humanity shares Earth–is an important element in the degradation of ecosystems related to increasing human numbers. It is especially critical because losses of species diversity, including genetically diverse populations of those species, are irreversible. In addition, it is often very difficult to find substitutes that can play the same roles as the organisms that are gone, so that restoration of certain ecological services may be difficult or impossible. Areas of high biodiversity and high human population density frequently coincide, which puts the living parts of local ecosystems at risk.

Overexploitation and resulting degradation reduce the productivity of ecosystems. Major fisheries such as cod and Atlantic swordfish are threatened with economic extinction by overharvesting in response to growing demand, especially from increasingly affluent consumers. In poor countries, tropical forests and their precious stores of biodiversity are threatened with destruction by overexploitation for timber, combined with the need for farmland to feed growing populations.

Generally, the more people there are the more they extract from, emit pollutants into, and disrupt ecosystems. But numbers of people alone do not tell the whole story; how they behave also contributes to the resultant loss of ecosystem products and services. Environmental scientists generally divide that behavior into two factors: affluence (how much each person consumes on average) and technology, a complex factor that includes both the technologies of production and the sociopolitical and economic arrangements necessary for production and distribution to take place. This is the basis of the I = PAT equation: I mpact on life-support systems is the product of P opulation size, A ffluence, and T echnology. The equation suggests that, from an ecological viewpoint, the worst problems of overpopulation are likely to be found not in populous poor nations like India and China, but in populous rich ones like the United States because of the latter's high levels of per-capita consumption.

It is important to note that, although population growth is an important driver of environmental deterioration, in some cases it is possible to ameliorate its impact. For instance, growing populations contributed to an increasing flow of chlorofluorocarbons (CFCs) into the atmosphere, threatening the destruction of the important ozone layer. But technological changes, entailing substituting less destructive chemicals for CFCs, largely removed the threat. However, in many other areas such as the provision of water to homes, industry, and agriculture, opportunities for substitution are more limited and often involve high costs. Locally, environmental outcomes are complicated by the existence of positive feedback involving population growth. As a favorable example, additional people moving into a forested area may supply capital to provide jobs that do not depend on logging, to install sewage treatment plants, and to take other measures that help protect the environment. The better conditions that result in turn attract more people, who (up to a point) may further improve conditions.

The Epidemiological Environment

The human epidemiological environment determines susceptibility to disease. It is shaped by a complex of biophysical, economic, sociocultural, and political factors in which the size and structure of the population are key variables. Demographic factors have been important in increasing susceptibility to epidemic disease. Many diseases require a certain host population size in order to maintain themselves; for instance, measles requires agglomerations of 200,000 to 500,000 in order to persist. The growing world population brings ever larger groups into contact with the animal reservoirs of pathogens potentially able to colonize Homo sapiens. This increases the probability that more HIV-like or "killer flu"-type epidemics will occur in the future.

Factors contributing to higher risks to public health include greater geographic mobility, urbanization (especially the growth of urban fringe settlements), large numbers of malnourished (and thus immune-compromised) people, declining water quality, misuse of antibiotics (leading to increasing problems with resistant pathogens), widespread distribution of recreational intravenous drugs with sharing of needles, and bioterrorism. While these risks are relatively well understood and advances in molecular biology should strengthen the human drug and vaccine armamentarium, and there is little question that the present perilous state of the epidemiological environment could be substantially improved despite the opposing effects of demographic pressures and mobility. Substantial advances, however, will require intensification of medical effort, especially better provision of public health services in developing nations.

Defining Overpopulation

Overpopulation (or population overshoot) is much discussed but rarely defined. From an ecological standpoint, the biophysical carrying capacity–that is, the maximum population size that can be long sustained under given technological capabilities–has no direct connection to population density. Overpopulation may best be defined as occurring when the number of people is larger than can be supported over the long term by the flow of income from natural capital, since depletion of that capital will constrain future generations. There are complexities in this definition, such as accounting for depletion of nonrenewable resources (economists generally do this by considering possibilities for substitution) or gauging overpopulation for countries heavily involved in international trade, but more precision is rarely required. The basic point is that remaining somewhat below carrying capacity is essential in order to avoid excessive damage to ecosystems and thus reduce negative feedbacks from ecosystems to human populations. Where no such margin for error exists, the result too often is a "natural disaster" as exemplified by Hurricane Mitch's devastation in Honduras.

Population Structure

Population growth is not the only demographic factor important in the human impact on ecosystems. The age structure of the population and its spatial distribution can also be important. With population aging, for example, there is often a rising proportion of single-person households, with greater per capita demands on fuel. In southwestern China, for instance, the aging population requires more home heating than was required when the average age was lower, increasing the consumption of fuel-wood from disappearing forests. In many countries there is steady migration into coastal areas with damaging consequences for the marshes and mangroves that serve as nursery areas for many marine fishes, and, in the longer run, where people are increasingly vulnerable to sea-level rise. Urbanization and international migration also have ecosystem effects, which can be very complex.

Ecological Sustainability and Environmental Ethics

An important ecological issue is that of sustainability: whether supporting the human population today might limit the ability of future generations to sustain themselves. This raises complex questions. Some are technical: How much reliance can be placed on technological progress to find substitutes for the natural capital now being depleted? Other, more contentious questions are ethical: What are a population's obligations to future generations, given that their reproductive decisions also influence the size of those generations? What duties of stewardship does the human population owe to other species and to the natural environment? Such questions are too rarely systematically explored.

The Scientific Consensus

The consensus of the scientific community on the interrelationship of demographics and the environment was well expressed in a 1993 statement by the world's scientific academies. This said, among many things:

Throughout history and especially during the twentieth century, environmental degradation has primarily been a product of our efforts to secure improved standards of food, clothing, shelter, comfort, and recreation for growing numbers of people. The magnitude of the threat to the ecosystem is linked to human population size and resource use per person. Resource use, waste production and environmental degradation are accelerated by population growth…. As human numbers further increase, the potential for irreversible changes of farreaching magnitude also increases. Indicators of severe environmental stress include the growing loss of biodiversity, increasing greenhouse gas emissions, increasing deforestation worldwide, stratospheric ozone depletion, acid rain, loss of topsoil, and shortages of water, food, and fuel-wood in many parts of the world. (National Academy of Sciences)

See also: Carrying Capacity; Environmental Ethics; Environmental Impact, Human; Natural Resources and Population; Sustainable Development.

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Paul R. Ehrlich

Anne H. Ehrlich