Evidence for disease-related amphibian decline in Colorado

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The recent discovery of a pathogenic fungus (Batrachochytrium dendrobatidis) associated with declines of frogs in the American and Australian tropics, suggests that at least the proximate cause may be known for many previously unexplained amphibian declines. Beginning in 1991, the authors monitored boreal toads in Colorado at four sites using capture–recapture of adults and counts of egg masses to examine the dynamics of this metapopulation. Numbers of male toads declined in 1996 and 1999 with annual survival rate averaging 78% from 1991 to 1994, 45% in 1995, and 3% between 1998 and 1999. Numbers of egg masses also declined. An etiological diagnosis of chytridiomycosis consistent with infections by the genus Batrachochytrium was made in six wild adult toads. Characteristic histomorphological features (i.e., intracellular location, shape of thalli, presence of discharge tubes and rhizoids) of chytrid organisms, as well as host tissue response (acanthosis and hyperkeratosis), were observed in individual toads. These characteristics were indistinguishable from previously reported mortality events associated with chytrid fungus. The authors also observed epizootiological features consistent with mortality events associated with chytrid fungus: an increase in the ratio of female:male toads captured, an apparent spread of mortalities within the metapopulation, and mortalities restricted to post-metamorphic animals. The authors concluded that 11 years of population data suggest that this metapopulation of toads is in danger of extinction and that pathological and epizootiological evidence indicates that B. dendrobatidis has played a proximate role in this process.

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ELECTRONIC FILE - Zoology