Degree of Threat: Medium
Comments: The extent of threats range-wide is not known with certainty, but there appear to be multiple causes contributing to the range-wide trend.
Disease and parasites appear to be contributing factors leading to population declines. Carey (1993) hypothesized that some environmental factor or synergistic effects of more than one factor may stress toads, causing suppression of the immune system or indirectly causing immunosuppression by effecting elevated secretion of adrenal cortical hormones. Immunosuppression, coupled with the apparent effect of cold body temperatures on the ability of the immune system to fight disease, may lead to infection by Aeromonas hydrophila bacteria (which causes "red-leg") or other infectious agents and subsequently to death of individuals and extirpation of populations. Die-offs in the Southern Rockies have been associated with chytrid fungus (Batrachochytrium dendrobatidis) infections (Daszak et al. 2000), which attacks keratinized tissue and is especially detrimental to recently metamorphed toadlets. Also, toad eggs are highly susceptible to the pathogenic fungus Saprolegnia ferax (which may be introduced during fish stocking), and mortality of eggs due to this fungus has been documented in Oregon (Blaustein et al. 1994; Kiesecker and Blaustein 1997; Kiesecker et al. 2001). Kiesecker et al. (2001) observed catastrophic embryo mortality from S. ferax infection in shallow water that was protected from UV-B but not in water protected from UV-B.
Limb malformations in toads have been linked directly to trematode infections by Ribeiroia ondatrae (Johnson et al. 2001; Johnson et al 2002), although the impacts of these infections on reproduction, and the magnitude of the infections across the breeding range, require further study. Preliminary analysis suggests that limb malformations may increase mortality in larval amphibians prior to and during metamorphosis.
Some have proposed that declines are related to sensitivity of eggs to increased levels of ultraviolet radiation (Blaustein et al. 1994), but studies by Corn (1998) yielded no support for UV-B alone as the cause of declines. Also, spectral characteristics of natural waters likely shield eggs from detrimental physiological effects in all but the clearest waters (Palen et al. 2002). Corn and Muths (2002) proposed that temperature stress is as plausible a hypothesis as increased UV-B to explain episodes of high mortality observed in Oregon (Kiesecker et al. 2001).
Increased acid deposition in natural habitats is a potential threat to amphibians, but this does not appear to be a major threat to Bufo boreas. For example, declines in the Southern Rocky Mountains are not due to acidification of breeding habitats (Corn and Vertucci 1992).
In the Cascade Range of Oregon, persistent predation on adult toads by Common Ravens during the breeding season appears to have contributed significantly to declines of some populations (Olson 1992). Possible significant predation by birds also has been observed in Colorado and Idaho (Hammerson 1999).
Declines may be related at least in part to habitat destruction and degradation, water retention projects, predation by and competition with native and non-native species, fishery management activities, or other factors, but these factors have not been adequately assessed. In Idaho, several hundred toadlets were trampled when domestic sheep were herded through the dried breeding pond (Bartelt 1998).