The possibility that a sixth mass extinction spasm is upon us has received much attention (Novacek, 2007). Substantial evidence suggests that an extinction event is underway.
When did the current extinction event begin? A period of climatic oscillations that began about 1 Mya, during the Pleistocene, was characterized by glaciations alternating with episodes of glacial melting (Barnosky, Chapter 12, this volume). The oscillations led to warming and cooling that impacted many taxa. The current episode of global warming can be considered an extreme and extended interglacial period; however, most
30 / David B. Wake and Vance T. Vredenburg geologists treat this period as a separate epoch, the Holocene, which began «11,000 years ago at the end of the last glaciation. The Holocene extinctions were greater than occurred in the Pleistocene, especially with respect to large terrestrial vertebrates. As in previous extinction events, climate is thought to have played an important role, but humans may have had compounding effects. The overkill hypothesis (Martin, 2005) envisions these extinctions as being directly human-related. Many extinctions occurred at the end of the Pleistocene, when human impacts were first manifest in North America, in particular, and during the early Holocene. Because naive prey were largely eliminated, extinction rates decreased. Extinctions were less profound in Africa, where humans and large mammals coevolved. Most currently threatened mammals are suffering from the effects of range reduction and the introduction of exotic species (MacPhee and Marx, 1997). In contrast to the overkill hypothesis, an alternative explanation for the early mammalian extinctions is that human-mediated infectious diseases were responsible (MacPhee and Flemming, 1999).
Many scientists think that we are just now entering a profound spasm of extinction and that one of its main causes is global climate change (Thomas et al., 2004; Parry et al., 2007; Jackson, Chapter 1, this volume). Furthermore, both global climate change and many other factors (e.g., habitat destruction and modification) responsible for extinction events are directly related to activities of humans. In late 2007, there were 41,415 species on the International Union for Conservation of Nature Red List (International Union for the Conservation of Nature, 2007), of which 16,306 are threatened with extinction; 785 are already extinct. Among the groups most affected by the current extinction crisis are the amphibians.
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