Inconsistent effects of P enrichment and predator presence on food-web dynamics in a bromeliad system

Food webs consist of trophic interactions of variable strength. However, it remains challenging to predict how changes in environmental conditions, such as nutrient enrichment, can affect these interactions. In this paper, we propose to integrate food web theory with ecological stoichiometry, which predicts how elemental ratios affect trophic interactions. Because organisms prefer resources that have a stoichiometry more similar to their own, we predict that increasing the phosphorus (P) content of a resource should benefit more organisms with a higher P content, while these same high-P organisms should be targeted preferentially by predators. We test these predictions through an experiment replicated in two sites, separated by more than 4,000 km, where we manipulated the P stoichiometry of a basal resource and the presence of a top predator in experimental bromeliad communities. We found that our treatments had interactive effects on ecosystem variables, such as mass loss of experimental leaf litter and macroinvertebrate decomposition, but little effect on the organisms within our experimental communities. The results on ecosystem variables were congruent with increased recycling by predators following increases in nutrient supply. The lack of significant effects on the organisms themselves suggests that higher P input was offset by nitrogen (N) limitation or N-P co-limitation, and by physiological processes different than growth and development, such as the non-consumptive effects of predation. In conclusion, our study showed interactions between top-down and bottom-up processes, but these did not result in predictable shifts of interaction strength.