Hypothesis concerning the causes of the disappearance of boreal toads from the mountains of Colorado
Abstract
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. Between 1974 and 1982, 11 populations of boreal toads ( Bufo boreas boreas ) in the West Elk Mountains of Colorado totally disappeared. The apparent cause of extinction of these toads was infection with the bacteria Aeromonas hydrophila . In this paper, the presence of disease in declining populations of these toads is used in conjunction with a variety of data from the literature to formulate a working hypothesis for explaining the cause of the decline of this species, and perhaps others: Some environmental factor or synergistic effects of more than one factor changes sufficiently to cause sublethal “stress”; This stress directly causes suppression of the immune system, or indirectly causes immunosuppression by effecting elevated secretion of adrenal cortical hormones; 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.
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