Effects of disease and pond drying on gray tree frog growth, development, and survival

Pathogens have important effects on host growth. behavior, and population d ynamics. Nevertheless, the impact of parasitic infection on host population s may be strongly context dependent. For example, the outcome of host-patho gen interactions may be subject to change based on the level of abiotic str ess experienced by the host. In northeastern Connecticut. USA, larvae of th e 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 subseque ntly infect H. versicolor tadpoles. Field collections of H. versicolor tadp oles suggest that the prevalence of infection by trematodes can be as high as 71%. We measured survival, mass, and time to metamorphosis of larval gra y treefrogs in an artificial pond experiment designed to determine how infe ction with trematode cercariae and the stress of pond drying interact to in fluence gray treefrog performance. We exposed gray treefrog tadpoles to inf ected snails, uninfected snails. or no snails, within artificial ponds that were either permanent or subjected to a 49-d drying regime. The presence o f infected P. columella had strong negative effects on the performance of g ray treefrog larvae. However. this effect depended on whether ponds were te mporary or permanent. When tadpoles were exposed to infected snails in temp orary ponds. survivorship to metamorphosis decreased by similar to 30%. and mass at metamorphosis was reduced by 40%. The presence of infected P. colu mella had no detectable effect on larval performance of gray treefrogs in p ermanent ponds. Assays of infection level (mean number of cysts per individ ual) indicate that temporary pond animals were exposed to higher rates of i nfection. In contrast. uninfected snails had very little effect on H. versi color in either temporary or permanent pools. There was. however, a small i ncrease in time to metamorphosis for H. versicolor in permanent pools that were exposed to increased densities of uninfected snails. Our results indic ate that an endemic parasite can have large effects on the performance of i ts native host, but that these effects may be strongly context dependent. G iven this strong context dependence, an important challenge is to assay the se effects within the natural environment.