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additional freshwater supplies. Climate change, the demise of commercial fish populations and coastal reefs, widespread soil degradation, and the re-emergence of infectious disease also reflect the strong influence of population dynamics and take a growing toll on biodiversity. These global changes threaten ecosystem function and raise the risk of future extinction. It thus makes sense to consider the prospects for human population growth.

In this article, we consider those prospects by examining the United Nations (UN) population projections—both how and what they project. The methods and meaning of UN projections are poorly understood by scientists outside the field of demography. And the recent misuses of the projections in the press have confounded the public.

Despite widespread perceptions to the contrary, there is nothing inevitable about most future human population growth. Our species now numbers 6 billion and is growing at a pace of just over 80 million per year. More than 95% of this growth is occurring in countries of the developing world. Most demographers expect human population at least to approach 8 billion in the next half-century. Beyond that expectation, however, no one can be certain that world population will ever rise to greater levels. There is equal uncertainty that population will stop growing at any particular time in the not too distant future.

We can be certain, however, that today women in most developing countries desire fewer children than their mothers or even their older sisters sought or had (Westoff 1991). Over the last 30 years, that trend, when supplemented with access to modern contraception and the information needed to use it safely and effectively, clearly has resulted in lower rates of childbearing in countries with traditionally high fertility (Robey and others 1994). In the future, changes could occur even more rapidly. Decisions made today will have an enormous influence on the demographic future. These decisions are likely to be among the most important that we can make to conserve as much as possible of the planet's remaining biodiversity.

Humanity's Place in Nature

Few scientists outside the field of ecology are aware of how ecologically unprecedented is the scale of human numbers—not just present numbers, but also those of the last several millennia. No other mammal of comparable body weight has ever attained anywhere near such abundance. By manipulating the qualities and quantities of other species through agriculture, Homo sapiens broke through the energy and nutrient constraints that limited it as a hunter-gatherer.

Statistical models relating the adult body weight of mammals to their observed abundance (Peters 1983, p 166–7) predict that the equilibrium density of mammalian species in their home ranges will vary according to the following relationships: DC = 15 W - 1.16 for carnivores and DH = 103 W - 0.93 for herbivores (grazers and browsers), where D is animal density expressed in individuals per square kilometer, and W is the adult body weight in kilograms. For a carnivorous mammal or herbivore the size of Homo sapiens (roughly 65 kg), these relationships predict 0.12 individual/km2, and 2.1 individuals/km2, respectively. The natural