categories of threat of extinction (i.e., Vulnerable, Endangered, or Critically Endangered); 43% of species have declining populations (International Union for the Conservation of Nature, 2007). In general, greater numbers as well as proportions of species are at risk in tropical countries (e.g., Sri Lanka with 107 species, most at risk; nontropical New Zealand has an equivalent proportion, but has only 7 species) (Fig. 2.2). Updates from the Global Amphibian Assessment are ongoing and show that, although new species described since 2004 are mostly too poorly known to be assessed, >20% of analyzed species are in the top three categories of threat (Global Amphibian Assessment, 2007). Species from montane tropical regions, especially those associated with stream or streamside habitats, are most likely to be severely threatened.
We present a case study from our own work to explore the reasons underlying declines and extinctions of amphibians.
One of the most intensively studied examples of amphibian declines comes from the Sierra Nevada of California. The mountain range spans thousands of square kilometers of roadless habitat, most of which is designated as National Park and Forest Service Wilderness Areas, the most highly protected status allowable under U.S. law. The range contains thousands of high-elevation (1,500- to 4,200-m) alpine lakes, as well as streams and meadows, that until recently harbored large amphibian populations. Biological surveys conducted nearly a century ago by Grinnell and Storer (1924) reported that amphibians were the most abundant vertebrates in the high Sierra Nevada. Because large numbers of specimens were collected from well-documented localities by these early workers, the surveys provide a foundation on which current distributions can be compared. Of the seven amphibian species that occur >1,500 m in the Sierra Nevada, five (Hydromantes playcephalus, Bufo boreas, B. canorus, Rana muscosa, and R. sierrae) are threatened. The best studied are the species in the family Ranidae and include the Sierra Nevada yellow-legged frog (R. sierrae) and southern yellow-legged frog (R. muscosa) (Vredenburg et al., 2007). In the 1980s, field biologists became aware that populations were disappearing (Bradford, 1989), but the extent of the problem was not fully appreciated until an extensive resurvey of the Grinnell-Storer (1924) sites disclosed dramatic losses (Drost and Fellers, 1996). Especially alarming was the discovery that frogs had disappeared from 32% of the historical sites in Yosemite National Park. Furthermore, populations in most remaining sites had been reduced to a few individuals.
The yellow-legged frogs, which had been nearly ubiquitous in high-elevation sites in the early 1980s, are ideal subjects for ecological study.