Amphibian Declines: The Conservation Status of United States Species

28 Lessons from the Tropics

198 Abrupt declines in amphibian populations have been reported in the media and the scientific literature for more than a decade. Scientists have detected declines in amphibian populations in North America (Corn and Fogleman, 1984; Blaustein and Wake, 1990; Bishop and Pettit, 1992; Carey, 1993; Kagarise -Sherman and Morton, 1993; Scott, 1993; Drost and Fellers, 1996; Green, 1997b; Lannoo, 1998b), Central America (Crump et al., 1992; Pounds et al., 1997; Lips, 1998, 1999; Wilson and McCranie, 1998), and South America (Heyer et al., 1988; Weygoldt, 1989; LaMarca and Reinthaler, 1991; Lynch and Grant, 1998), as well as in Australia (Richards et al., 1993). Declines may have resulted in the extirpation of up to 50% of the anuran species (Pounds et al., 1997) at a site and reduced the abundance of surviving species. There are two major types of decline—those in which obvious causes such as habitat destruction or environmental contamination can be identified and those “mysterious” declines that have no obvious cause. Most scientists generally agree that the majority of historical and current amphibian declines are due to habitat destruction or habitat alteration. The causes of mysterious declines are more intractable. These declines have tended to occur at mid- to high-elevation sites and in protected regions; they happen rapidly and are selective (i.e., not all species at a given locality are affected). Declines with no apparent cause generate the greatest concern, as do declines occurring in protected reserves, such as those from Costa Rica (Crump et al., 1992; Lips, 1998), Panama (Berger et al., 1998; Lips, 1999), Colombia (Lynch and Grant, 1998), Brazil (Heyer et al., 1988; Weygoldt, 1989), Venezuela (LaMarca and Reinthaler, 1991), and Australia (Richards et al., 1993). These declines suggest global factors that could threaten human welfare. There are several differences between temperate and tropical amphibian declines. While temperate declines generally occur more slowly, affect mostly pond-breeding species, and include salamanders as well as anurans (Stebbins and Cohen, 1995; Jancovich et al., 1997), tropical declines have tended to involve entire anuran faunas that abruptly crash (the “faunal collapse” of Drost and Fellers, 1996). One of the biggest differences between declines in the two regions is the occurrence of malformities, which have been found in many temperate areas (California, Oregon, all states bordering the Great Lakes, the St. Lawrence River, and Lake Champlain), but have not been reported from tropical areas. Nor have deformities been associated with declines at any site. Hypothesized causes for these recurring malformations in temperate frogs include parasites (Sessions and Ruth, 1990; Johnson et al., 1999; Sessions et al., 1999), chemical contaminants (Ouellet et al., 1997a), and ultraviolet B (UV-B) radiation (Ankely et al., 1998; summarized in Souder, 2000). Most mysterious North American declines occurred in the 1970s–1980s, primarily in the montane areas of the western United States and Canada (Carey, 1993; Stebbins and Cohen, 1995), although more recent declines in upper midwestern northern cricket frog (Acris crepitans) populations have raised serious concerns. Similarities between historic disappearances in North America (Corn and Fogleman, 1984; Carey, 1993; Kagarise-Sherman and Morton, 1993; Scott, 1993; Blaustein, 1994; Drost and Fellers, 1996) and those now happening in Central America suggest that similar agents might have affected amphibians in both sites (Carey et al., 2003). At least 13 countries in Latin America and the Carribbean have experienced declines or extinctions of protected anuran populations over the past ten years (Young et al., 2001). Each of these unexplained tropical declines has occurred in upland areas, suggesting a possible synergistic interaction of the causative agent with some environmental condition that varies with elevation (e.g., temperature, UV-B radiation, precipitation , and wind patterns). Tropical declines especially involve species associated with streams during some portion of their life cycle (Pounds et al., 1997; Lips, 1998, 1999; Lynch and Grant, 1998), although some tropical declines have been reported in terrestrial salamander populations (Parra-Olea et al., 1999; see below), pond- or pool-breeding frogs (Pounds et al., 1997), and terrestrial frogs (Stewart, 1995; Lips, 1999). Few tropical declines have actually been observed as they occurred, so it is not always clear how or in which life history stage the amphibians are affected. The rapidity of most declines suggests that adults are being killed. In some tropical declines , life stages other than adults are known to be affected. Additionally, deformities of tadpole mouth parts and dying tadpoles or newly metamorphosed animals have been reported from some tropical sites (J. Campbell, personal communication ; J. Mendelson, personal communication; Weygoldt...