Trophic cascades are ubiquitous in natural systems and their role in maintaining many ecosystems is increasingly recognized. However, trophic cascades vary in magnitude and can be difficult to predict. Recent theoretical and modeling research suggests that intraspecific variation can play a role in modulating the effects of a trophic cascade. In particular, variation related to ontogeny, which includes changes in diet and body sizes, can play an important role in community dynamics. Exploring the influence of interspecific variation can further our understanding of these complex ecological interactions.
In this project, we investigated the impact of body size variation in a top predator. Specifically, we used mole salamanders (Ambystoma talpoideum), which are aggressive predators in fishless ponds. Pond-dwelling A. talpoideum exhibit a wide range of body sizes due to their unique paedomorphic life history. We hypothesized that populations of similarly sized A. talpoideum (with smallest variation in body sizes) would exhibit strongest trophic cascades due to predation on similar prey items. Trophic cascade strength was predicted to weaken as body size variation increased, due to cannibalism and more diffuse predation pressure. To test these questions, we constructed 54 1000-L mesocosms which were populated with one of three salamander size-structure treatments. We monitored changes in the ecosystems through a suite of abiotic and biotic parameters over a period of six months.
Results/Conclusions
Presence of salamanders significantly increased chlorophyll-a concentrations, suggestive of a trophic cascade. Preliminary results suggest that the cascade is not mediated by a traditional pathway through zooplankton, but instead by tadpoles which act as strong herbivorous interactors and limit concentrations of primary producers. These tadpoles were opportunistically introduced to the mesocosms by ovipositing tree frogs, which have been shown to selectively oviposit in ponds that do not contain A. talpoideum. This suggests that A. talpoideum could mediate a trophic cascade via indirect fear-based interactions. Further analysis of the macroinvertebrate community is currently in progress to better understand the linkages within this food web.