Mass extinctions: what can the past tell us about the present and the future?

Abstract

The biosphere is undergoing a series of environmental perturbations whose impacts are collectively exceeding all but the greatest geologic upheavals of the prehistoric past. Among these current impacts is a mass extinction that portends the elimination of a large proportion, perhaps as many as half, of all species now extant. The predominant driving force behind the present-day perturbations is made up of two factors: growth in human numbers and growth in human consumerism. Since the start of this century human numbers have increased threefold, growth in human consumption of fossil-fuel energy has increased 12-fold, and growth in the global economy has increased 29-fold (Brown et al., 1989; World Resources Institute, 1989). The most rapid growth has occurred in just the past few decades. Of this century's economic growth, four-fifths has taken place since 1950; and the expansion of the global economy in 1988 and 1989, $600 billion, has been as great as was the entire economy in the year 1900. As a result of these various forms of growth in the human enterprise, along with inappropriate technologies and deficient economic policies, the biosphere now suffers from a lengthening list of environmental assaults: widespread pollution such as acid rain, soil erosion, tropical deforestation, spread of deserts and decline of water supplies, to name but the main forms of environmental degradation and biospheric impoverishment. Also underway are depletion of the ozone layer and the greenhouse effect. To comprehend the scope of our overburdening of the biosphere, consider the capacity of humankind to feed its present total of over 5 billion people. If we all lived off a basically vegetarian diet and shared our food supplies equally around the world, the biosphere could support roughly 6 billion people. If the diet were upgraded until 15% of calories were derived from animal products, making it the equivalent of what many South Americans consume today, the global total would be only 4 billion. Were the diet to be further improved to a full but healthy level with 25% of calories derived from animal products, being the equivalent of what many people in southern Europe consume today, the total would fall to 3 billion; and if the level included 35% of calories from animal products, or the equivalent of today's average North American diet, the total would be a mere 2.5 billion (Kates et al., 1989). Of course we can hope that better agro-technologies will improve the prospect. But despite the exceptional technological feats of the past few decades, the 1988 harvest worldwide was about 5% smaller than the 1985 harvest--while the global population was 5% percent larger (Ornstein and Ehrlich, 1989). Hence the present throngs of humankind press ever-harder upon the planetary ecosystem. To summarize the situation through an alternative analysis, humankind now co-opts some 40% of