. "2 Are We in the Midst of the Sixth Mass Extinction? A View from the World of Amphibians--DAVID B. WAKE and VANCE T. VREDENBURG." In the Light of Evolution, Volume II: Biodiversity and Extinction. Washington, DC: The National Academies Press, 2008.
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In the Light of Evolution: Volume II—Biodiversity and Extinction
were enigmatic, but eventually two primary causal factors emerged: the infectious disease chytridiomycosis and global warming (Lips et al., 2006; Pounds et al., 2006).
Chytridiomycosis was detected almost simultaneously in Costa Rica and Australia (Berger et al., 1998). From the beginning, it was perceived as a disease with devastating consequences. It quickly swept through Costa Rica and Panama, leaving massive declines and local extinctions in its wake (Lips et al., 2006). More than half of the amphibian species in lower montane forest habitats suffered declines on the order of 80%, and several disappeared. This extinction event had been predicted on the assumption that chytridiomycosis would continue its sweep southward from Monteverde, in northwestern Costa Rica, to El Cope in central Panama (Lips et al., 2006). Attention is now focused on eastern Panama and northwestern Colombia, where chythridiomycosis has not yet had evident impact.
Carcasses of animals from the Monteverde extinction event are not available, and it is not known whether Bd was responsible for frog deaths. However, Bd has been detected in many preserved specimens that were collected at different elevations along an altitudinal transect in Braulio Carrillo National Park in 1986 (Puschendorf et al., 2006). The park is in northern Costa Rica ≈100 km southeast of Monteverde. Given the high prevalence of Bd in the specimens surveyed, it seems reasonable to assume that Bd also was present at Monteverde. Of course, there are many more species present in tropical areas (67 at El Cope, Panama) (Lips et al., 2006) than in the Sierra Nevada (7 at high elevations, but 3 most commonly, only 2 of which are aquatic), and hence there are many more opportunities for the spread of Bd among tropical species. The average moisture content of the air in the tropical environments is doubtless much higher, on average, in Central America than in the Sierra Nevada, where a characteristic dry summer rainfall pattern prevails and where there is no forest canopy because of the altitude and substrate. Although we do not know the mechanism of spread, conditions in Central America appear more suitable for the spread of an aquatic fungus.
Amphibians tend to have broader ranges in temperate regions than in the tropics. Despite many population extinctions in temperate regions, there have been few extinctions. Accordingly, the tropical species of amphibians are more at risk, but not just because of their typically small geographic ranges. Because they occur in rich, multispecies communities, the species become infected simultaneously.
Climate change has been implicated in declines since the documentation of disappearances at Monteverde (Pounds et al., 1999; Still et al., 1999). Unusual weather conditions were initially implicated with amphibian declines. Large increases in average tropical air and sea surface temperatures were associated with El Niño events in the late 1980s; substan-