Effects of the amphibian chytrid fungus on toad survival

Abstract/Summary

Chytridiomycosis is an amphibian disease linked with population declines worldwide, yet there is little information about its effects on the demography of infected populations. We used capture-recapture data from two infected populations and one uninfected population of boreal toads (Bufo boreas) in the Rocky Mountains, U.S.A. to examine a priori hypotheses about the effect of the amphibian chytrid fungus (Batrachochytrium dendrobatidis, Bd) on survival probability (Φ) and population growth rate (). Bd-positive toads had lower average annual Φ than Bd-negative individuals at sites where Bd was detected suggesting that chytridiomycosis may reduce survival by 31-42% in wild boreal toads. Bd-negative toads at infected sites had comparable survival probabilities as Bd-negative toads at the uninfected site, further supporting observed differences. Model results indicated weak evidence that environmental covariates (particularly cold temperatures during the breeding season) influenced toad survival. The diseased populations were declining by 5 - 7% per year over the 6 years of this study whereas the Bd-free population had comparatively stable ( ≈ 1) population growth. Our data suggest that Bd is a low-level chronic mortality factor in these toad populations rather than an acute factor causing rapid population declines. These results show how some amphibian populations may be coexisting with the amphibian chytrid fungus and highlight the importance of quantitative assessments of survival in wildlife populations challenged with disease.