Extinction can be a natural process. Indeed, the vast majority of species that ever existed are now extinct. In light of this, some people have justified the growing global impoverishment of species by arguing that because extinction is a natural process, the extinction of modern species is simply a continuation of a normal phenomenon. Yet historically, typical, or background extinction rates usually have been lower than the rate at which new species evolved (except during extinction spasms). But because modern extinction rates are roughly 1,000 times higher than background extinction rates (Myers 1988; Soule 1996), today only a handful of people deny that the planet is in, or rapidly approaching, an extinction spasm.

Lately, a new rationale to exonerate human actions holds that extinction spasms have occurred periodically throughout geologic time and that nevertheless the planet has always recovered (albeit over periods of 10 to 30 million years). Some people claim it is important only to recognize that there are periodic mass extinctions, not to know their cause (Rosenzweig 1995). However, this argument may be biologically faulty, as the subsequent discussion reveals.

There have been five historic extinction spasms, the most recent occurring at the end of the Cretaceous when dinosaurs disappeared. Although current extinctions may not have yet reached the extent of those of the Permian Period 250,000,000 years ago, the present extinction spasm has a unique aspect. The causal agent is not an external perturbation such as volcanic eruption or collision with an asteroid. The agent is one of nature's own creations, Homo sapiens (Soule 1996).

Human influence can be seen from the time humans arrived in Australia, North America, Europe, South America, and several island archipelagos (Ward 1997). In each of these regions, the arrival of people was quickly followed by a loss of 75% to 85% of the mammalian megafauna, or species larger than 44 kg (Primack 1998). The rate of loss accelerated as humans increased in number and acquired the technological capacity to alter habitat and directly exploit a wider array of animal and plant resources. It has been estimated that at present population levels, humans use 40% of the primary productivity of plants (Vitousek et al. 1986). If the human population doubles in the next half-century, that will leave little productivity for other forms of life.

In the United States alone there are now well over 1,000 species federally protected (Noss et al. 1997), and there may be another 5,000 species threatened but not included on federal lists (U.S. General Accounting Office 1992). Moreover the number of plants and animals formally listed as threatened or endangered continues to rise; only a small handful of species have been removed from the U.S. Endangered Species List because their numbers have recovered (U.S. General Accounting Office 1992; Murphy et al. 1994; Wilcove et al. 1996). In a recent analysis of the causes for these astounding numbers, 85% were linked to habitat loss, 49% to invasion of alien species, 24% to pollution, 17% to overexploitation, and 3% to disease 1 (Wilcove et al. 1998). The main causes of habitat destruction included agriculture, livestock grazing, mining, logging, infrastructure development, road construction, military activities, outdoor recreation, water development, urban and commercial development, and disruption of fire ecology (Wilcove et al. 1998).

Habitat changes reduce biotic integrity (i.e., ecosystem health), deplete native species, and greatly simplify the system and its habitats (e.g., crop agriculture). The process of habitat destruction is incremental. Each piece of habitat may not seem important individually, but there are cumulative effects. The process is more insidious than direct overexploitation. No one holds a 'smoking gun.' The native species simply vanish.

The effects of these changes can be predicted mathematically. Roughly, when 90% of the habitat is eliminated, 50% of the species will be lost (Wilson 1992). Selection of the lost species, however, is not random. The larger, wide-ranging species, such as large carnivores, suffer first. Because those groups often contribute to healthy ecosystem processes, a wave of secondary losses may follow their decline (see Terborgh et al. 1999). Animals that conflict with humans are also the victims of concerted eradication efforts. Species with a narrow geographic range, or species that were never common, are vulnerable as well. Species that are not effective dispersers are limited when their habitat is disrupted. Species with narrow niche requirements may see that niche disappear quickly. And species that live in colonies, or social groups, are often affected when numbers decline.

When habitat is fragmented, some species die as a direct result of lost resources. Other species survive in reduced numbers in the habitat fragments, but their vulnerability to extinction increases. Populations existing in fragments become susceptible to genetic disorders, demographic problems, environmental variability, and catastrophic events. Fragmented populations are especially vulnerable to deterministic events, such as susceptibility to poaching, as border areas become population sinks, where population death rates exceed birth rates. Woodroffe and Ginsberg (1998) reported that 74% of known mortality of large carnivores living in protected areas occurred at the boundary and was caused by humans. When deterministic factors (e.g., habitat destruction) reduce populations such that random genetic, demographic, and environmental forces are able to drive them to extinction, the condition is called an extinction vortex; it is usually extremely difficult to recover species entering such a whirlpool (Gilpin&Soule 1986).

In short, human impacts have triggered the present extinction spasm. During the other five spasms the causal agent was a temporary disturbance. When the causal agent subsided, recovery proceeded. Some people have speculated that human activities may ultimately result in the elimination of Homo sapiens; and if that happens, recovery over geologic time may not be very different from the other five spasms. However, the causal agent of this current extinction episode may well endure. After all, humans are the ultimate weed species (Quammen 1998). We have shown an incredible ability to invade, change, and inhabit every habitat type on the planet. Sheer numbers of humans alone can overwhelm our co-species, and technology heightens our capabilities. The only other species that will likely survive our impact will be those adapted to living with humans—in short, the species that can utilize the pauperized systems left in our wake. Because evolution, and recovery from an extinction spasm, rely on vacant niches, the human species will do more than just eliminate other species. By simplifying habitat, humans will also limit other species' capacity to recover from that blow (Soule 1996). . .