In a drying pond, larval amphibians experience a reduction in foraging space as both the volume of water decreases and the density of conspecifics increases. These aspects of pond drying may reduce the quality of the larval habitat and serve as environmental cues to which amphibians respond plastically by reducing the length of the larval period. This accelerated development may allow a larval amphibian to escape a drying pond, but it may also result in physiological consequences that can influence post-metamorphic survival in the long term.
In an attempt to tease apart the relative importance of these cues in terms of plasticity of the larval period, we examined two rates of drying and two conspecific densities in a mesocosm experiment in Northern Pennsylvania. Utilizing the temperate pond breeder Rana pipiens, we examined how differences in the larval environment affected size at and time to metamorphosis, the thermal dependence of jumping performance, innate immune function, and survival post-metamorphosis. We hypothesized that the low density, fast drying treatment would result in metamorphic animals with the shortest development times, but that there would be a corresponding reduction in post-metamorphic performance in this group.
Results/Conclusions
Interestingly, there was no effect of pond drying rate on larval period and post-metamorphic traits. Density had strong effects, however, with animals from the low-density treatments demonstrating drastically shorter larval periods and larger sizes at metamorphosis. Animals from the low-density treatments also demonstrated reduced survival post-metamorphosis, and significantly reduced size-adjusted jumping ability across all test temperatures. But there was no effect of density or pond drying on the bacterial killing ability of whole blood of metamorphic animals. With abundant food resources, animals in the low-density treatments were able to metamorphose quickly and at larger sizes. However, this work demonstrates that there are consequences to rapid larval growth and accelerated development, resulting in reduced performance and survival post-metamorphosis.