Discussion

Parasites compete for host access and as a result may alter growth and reproductive rates in response to competitive interactions. Batrachochytrium dendrobatidis (Bd), a fungal pathogen of amphibians, exhibits phenotypic variation in response to environmental conditions that should optimize probability of infection. We have shown that probability of infection with the global panzootic lineage of the fungus (BdGPL) increases with increasing host density and when competed against another, less virulent lineage BdGPL increases growth and/or reproductive rates in response. These results indicate that the global success of BdGPL and its impact on a wide range of amphibian hosts have been facilitated by phenotypically plastic growth and reproduction. However, some amphibian hosts exhibit strong intraspecific variation in probability of infection and pos-metamorphic survival, suggesting variation of host responses may under some circumstances afford resistance to or tolerance of BdGPL infections. We have found that growth rates of common toads (Bufo bufo) do indeed vary in response to exposure to BdGPL later in larval development, but not soon after hatching. Tadpole growth rates are dictated by food intake, but in a manner that converges with regards to time to metamorphosis, which in turn strongly influences both probability of infection and post-metamorphic survival. These findings suggest that coexistence between lethal variants of Bd and susceptible amphibian host species can occur without the benefit of natural selection on host immunity.