9400333 Carey Recent reports of world-wide declines and extinctions of amphibian populations have raised questions about the relation of environmental change to the demise of certain amphibian species. If these species are being harmed by man-made change, amphibians may prove to be excellent "harbingers of decay" and indicators of ecosystem stress. The causes of amphibian populational declines have not been conclusively demonstrated. %%% The following working hypothesis is presented to explain how environmental change might cause the demise of amphibian populations. 1. A sublethal change in one factor, or multiple change(s) in several factors, acting singly of synergistically, occurs in the environment of these amphibians. 2. The environmental change either directly suppresses function of immune system, or indirectly affects immune function by eliciting an immunosuppressive neuroendocrine response. 3. This depression of immune function, coupled with the additional suppressive effects of seasonally-associated cold body temperatures on the abilities of ectothermic immune systems to resist disease, leads to increased susceptibility to opportunistic pathogens such as Aeromonas hydrophila. In turn, susceptibility results in subsequent disease, death of the individual, and demise of the population. %%% Parts 2 and 3 of the working hypothesis will be tested in the project period in Bufo b. boreas, Rana yavapiensis, Bufo marinus, and Ambystoma tigrinum. The difference between our approach and that of others working to find the causes of amphibian declines is that others are looking to find an environmental change severe enough to kill amphibians directly, while our approach evaluates the role of smaller, sublethal changes sufficient to cause suppression of immune function and disease.
