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UV-B and water temperature unlikely cofactors in Central American amphibian declines due to pathogenic chytrid.
Murphy, Peter*,1, Lips, Karen2, 1 Idaho State University, Pocatello, ID, USA2 Southern Illinois University, Carbondale, IL, USA
ABSTRACT- The pathogen Batrachochytrid dendrobatidis has been linked with multispecies amphibian declines in Australia, the United States, and Central America. We sought to identify environmental cofactors that may predispose upland and cloud forest amphibians to an epidemic caused by this pathogen. Two cofactors, temperature and UV-B exposure, have both exhibited mean increases during the past two decades in Central America. We tested the effect of these cofactors on tadpole infection in a mesocosm experiment in Chiriqui, Panama. We measured infection in Smilisca phaeota (Hylidae) tadpoles, a species whose adults and tadpoles were susceptible to the pathogen in preliminary trials. We measured infection as the proportion of keratin lost in tadpole mouthparts due to fungal infection. Two levels of three treatments were crossed in the factorial experiment: ambient vs. reduced UV-B, ambient vs. increased water temperature, and ambient vs. increased pathogen loads. The water source for the shallow, flow-through mesocosms was a stream in which infected tadpoles were found both summer 2001 and 2002. Our results indicated that increased temperature strongly protected tadpoles from infection, UV-B did not change tadpole infection rates, and adding pathogen increased tadpole infection. Interestingly, tadpoles were protected at water temperatures at which the chytrid grew optimally under lab conditions. Our study suggests that increasing temperatures and UV-B are not linked directly to epidemic outbreaks of B. dendrobatidis, especially because warmer water protects tadpoles. The study is particularly convincing because it was conducted at a site where species declined due to this pathogen, hence UV-B and temperature treatments were balanced by ambient controls. Moreover, we used locally isolated B. dendrobatidis for the experiment, hence the pathogen strain used was that which had initially driven the declines, not an exotic with which the frogs and tadpoles had no prior exposure.
Key words: Panama, Batrachochytrid dendrobatidis, amphibian decline