Unlike the great species extinctions of Earth's past, the one occurring today is less an episode than a process, whose full results will not be known for hundreds of years. Between the linked human-induced phenomena of global climate change and biodiversity loss, the planet could be passing into the equivalent of an entirely new geological epoch in just a few human generations. Or it could be that biodiversity loss will amount to little more than a manageable depletion, incurring regrettable scientific and economic losses but leaving the basic services provided by most major ecosystems largely intact.
The size and distribution of human population over the near and distant future will surely be a dominant factor in determining whether the loss of biodiversity that the world faces turns into merely a source of wistful regret for future generations, a planetary catastrophe, or something in between. Population growth enlarges the scale and extent of the human enterprise and hence inflates the likelihood that human activities will push native nonhuman populations and biotic communities past critical thresholds of tolerance and renewal.
Demands for housing (Mason 1996), food energy and arable land (Bongaarts 1994; Engelman and LeRoy 1995; Smil 1994), freshwater (Engelman and LeRoy 1993; Falkenmark and Widstrand 1992), and industrially fixed nitrogen (Howarth and others 1996; Smil 1991; Vitousek and others 1997) appear more sensitive to the growth of human population than to the growth of per capita income or even to recent changes in technological efficiency. Habitat conversion, historically the greatest threat to biodiversity, has been driven by these very demandsby housing needs, pressures to expand and intensify agriculture, and the quest to harness