Diet has been linked to parasitism, as low quality diets can cause hosts to be more susceptible to parasitic infection, possibly causing synergistic effects on host fitness and consequences for host populations. Food resource quality can influence larval amphibian growth, development, and survival and likely affects parasite susceptibility, which is concerning given recent amphibian declines. We hypothesized that higher food resource quality would increase tadpole growth, that parasite infection would decrease growth and survival, and that tadpoles fed higher quality food would experience higher infection levels. To examine the combined effects of parasite exposure and food resource quality, we performed two laboratory experiments using Rana clamitans tadpoles. In the first experiment, tadpoles were fed diets differing in protein content (high, medium, or low) and were continually exposed to either the absence or presence of trematode (plagiorchid) parasite cercariae by including either an uninfected or infected snail, comparable to exposure experienced in ponds. In the second experiment, tadpoles were fed the same three diets, but were exposed to a one-time, controlled number of parasites (20 cercariae), to assess infection success. We measured tadpole growth and survival in the first experiment, and tadpoles from both experiments were dissected to measure infection loads.
Results/Conclusions
In the first experiment, we found a significant interactive effect of diet and parasite presence on final mass (p < 0.05); the interaction resulted from a trend of increased tadpole final mass with higher protein content when parasites were absent, but decreased final mass with higher protein content when parasites were present. Diet did not significantly affect tadpole survival or infection level (p > 0.1). The presence of parasites did significantly decrease tadpole growth and survival (p < 0.05). In the second experiment, tadpoles fed the low protein diet had more than six-fold higher infection levels than tadpoles fed the high protein diet (p < 0.05), suggesting that diet does influence infection levels under a controlled exposure level. Overall, our findings suggest that diet can affect an organism’s defences against parasitic infections, and tradeoffs may exist between allocating resources to growth or to parasite defense. Understanding such tradeoffs has implications for conserving amphibians in the face of disease and changing nutrient regimes in their habitats.