Factors influencing recovery of boreal toads from mass population mortalities in the Colorado Rockies

Boreal toads (Bufo boreas) are a widespread species throughout the western United States. In the Colorado Rockies, they were once very common in mountain valleys above 2500 m. Mass mortalities and population extinctions, beginning in 1973, reduced the number of breeding populations to probably less than 10% of the pre-1970s number. The patterns of mortality in the 1970s (all adults and metamorphic young die) is similar to that observed now in populations in which Batrachochytrium dendrobatidis is causing mortalities. The Colorado Division of Wildlife initiated a Boreal Toad Recovery Project, which included, among other activities, rearing of young to be reintroduced into historical habitat. These reintroductions, sometimes involving thousands of individuals, have failed to establish new populations. Since a few other amphibian species persist in boreal toad habitats, Bd cannot be eradicated from these localities. Furthermore, the prolonged cold and short growing seasons of the montane environment, coupled with life history characteristics such a very rates of growth of young and alternate year breeding in females, have made it very difficult for small, remnant populations to grow. When Bd passes through a population, differences in gene frequencies occur that may, ultimately, result in selection for improved survival with the persistence of this pathogen in the environment.