Amphibian Declines in the Greater Yellowstone Ecosystem: Toads with Protection from Bacterial Disease

Abstract

During the past forty years, biologists have become increasingly concerned about the decline and disappearance of various amphibian species throughout the world (Wyman 1990, Wake 1991). An example of an amphibian decline in western North America is that of the Boreal Toad (Bufo boreas). A previously widespread and abundant species, the Boreal Toad has undergone large population declines and is now a candidate for threatened and endangered status in the eastern half of its range (Stebbins and Cohen 1995, Federal Register 1995). Decreases in Boreal Toad abundance and distribution have been observed in Colorado, eastern Wyoming, and eastern Utah (Com et al. 1989). From 1971-82, eleven Boreal Toad populations disappeared from the West Elk Mountains of Colorado (Carey 1993). During this eleven-year period, numerous individuals were observed exhibiting symptoms of redleg, a well­documented disease in amphibians caused by the bacterium Aeromonas hydrophila (Russel 1898, Emerson and Norris 1905, Kulp and Borden 1942, Reed and Toner 1942, Dusi 1949, Hunsaker and Potter 1960, Hird et. al. 1981, Nyman 1986, Carey 1993). This disease was considered responsible for the decline and ultimate extinction of these eleven toad populations. These extinctions led Carey to propose a hypothesis regarding the disease process which is stated as follows: (1) Some environmental factor or synergistic effects of more than one factor changes sufficiently to cause sublethal "stress;" (2) This stress directly causes suppression of the immune system, or indirectly causes immunosuppression by effecting elevated secretion of adrenal cortical hormones; (3) Immunosuppression, coupled with the apparent effect of cold body temperatures on the ability of immune systems of ectothermic animals to fight disease, leads to infection by Aeromonas or other infectious agents, and to subsequent death of individuals and extinction of populations.