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.
Amphibians have received much attention during the last two decades because of a now-general understanding that a larger proportion of amphibian species are at risk of extinction than those of any other taxon (SN Stuart et al., 2004). Why this should be has perplexed amphibian specialists. A large number of factors have been implicated, including most prominently habitat destruction and epidemics of infectious disease (Pechmann and Wake, 2006); global warming also has been invoked as a contributing factor (Pounds et al., 2006). What makes the amphibian case so compelling is the fact that amphibians are long-term survivors that have persisted through the last four mass extinctions.
Paradoxically, although amphibians have proven themselves to be survivors in the past, there are reasons for thinking that they might be vulnerable to current environmental challenges and, hence, serve as multipurpose sentinels of environmental health. The typical life cycle of a frog