Effects of Disease and Pond Drying on Gray Tree Frog Growth, Development, and Survival

Pathogens have important effects on host growth, behavior, and population dynamics. Nevertheless, the impact of parasitic infection on host populations may be strongly context dependent. For example, the outcome of host–pathogen interactions may be subject to change based on the level of abiotic stress experienced by the host. In northeastern Connecticut, USA, larvae of the gray treefrog (Hyla versicolor) co-occur in temporary and permanent ponds with a snail (Pseudosuccinea columella) that is frequently infected with a digenetic trematode (Telorchis sp.) whose free-swimming cercariae subsequently infect H. versicolor tadpoles. Field collections of H. versicolor tadpoles suggest that the prevalence of infection by trematodes can be as high as 71%. We measured survival, mass, and time to metamorphosis of larval gray treefrogs in an artificial pond experiment designed to determine how infection with trematode cercariae and the stress of pond drying interact to influence gray treefrog performance. We exposed gray treefrog tadpoles to infected snails, uninfected snails, or no snails, within artificial ponds that were either permanent or subjected to a 49-d drying regime. The presence of infected P. columella had strong negative effects on the performance of gray treefrog larvae. However, this effect depended on whether ponds were temporary or permanent. When tadpoles were exposed to infected snails in temporary ponds, survivorship to metamorphosis decreased by �30%, and mass at metamorphosis was reduced by 40%. The presence of infected P. columella had no detectable effect on larval performance of gray treefrogs in permanent ponds. Assays of infection level (mean number of cysts per individual) indicate that temporary pond animals were exposed to higher rates of infection. In contrast, uninfected snails had very little effect on H. versicolor in either temporary or permanent pools. There was, however, a small increase in time to metamorphosis for H. versicolor in permanent pools that were exposed to increased densities of uninfected snails. Our results indicate that an endemic parasite can have large effects on the performance of its native host, but that these effects may be strongly context dependent. Given this strong context dependence, an important challenge is to assay these effects within the natural environment.