Decomposer diversity increases a proportional contribution of detritus to the diets of consumers in agricultural ponds

Background/Question/Methods

Theory suggests that bottom-up effects of resource diversity to upper trophic levels increase ecosystem stability. In particular, energy flux from the detritus to an ecosystem stabilizes food web dynamics. To our knowledge, no empirical studies have examined how detritivore diversity alters the energy flux to upper trophic levels in a food web. Here we test the hypothesis that the species diversity of benthos within slow energy channels promotes energy flow between the resource and top predators in agricultural ponds. We measured stable carbon and nitrogen isotope ratios of consumers and also identified their stomach contents. Based on stomach content data we described binary food webs in which species and trophic links are either present or absent, but are not quantified. We used a Bayesian isotope-mixing model (IsoWeb) to estimate the proportional dietary contributions of each source based on all of the consumer species in the ponds. The proportional contribution of litter to the diets of consumers was determined by calculating the proportional dietary contribution of each source, multiplying them by their associated proportional contribution and, finally, summing these rates across all links.

Results/Conclusions

We found that the diversity of litter decomposers had a significant positive effect on the contribution of litter to the diets of consumers. In contrast, when we used the diversity of all benthic species instead of litter decomposers, only the abundance of benthos was significantly associated with the dependent variable in the model. These results support our hypothesis that litter decomposer diversity increases the proportional contribution of detritus to the diets of consumers through trophic levels. To our knowledge, our study is the first to show that benthos species diversity in the slow energy channel determines the amount of energy flow between the basal resource and top predators in a multi-trophic level food web. Because our study was primarily based on correlative relationships, experiments need to be conducted to verify the causal factors in these relationships. However, we did demonstrate that decomposer diversity had a significant effect even after considering confounding factors, such as Chl-a concentrations, in the model. This supports the conclusion that there is a causal link between decomposer diversity and the dependence of consumers on detritus-based energy flow.