Habitat size and complexity mediate predator effects on nitrogen cycling

Background/Question/Methods

Studies of trophic cascades show that predators can impact the biomass and species composition of trophic levels below them, as well as the ecosystem processes carried out by these organisms. Habitat characteristics, however, can be critical in determining the magnitude of predator impacts on their prey. In particular, habitat size and complexity impact the abundance and composition of populations, as well as the nature of predator-prey interactions. We investigate the relationship between predation and N cycling in the context of changing habitat complexity and volume using a detritus-based bromeliad-insect ecosystem. We ask: (1) What is the relationship between N flow from litter to bromeliads and the changing insect community along a habitat size gradient?; and (2) How does habitat complexity affect this relationship?  In a field experiment, we used detritus labeled with stable isotopes to track N movement from leaf litter to bromeliads and surveyed the insect community along a natural gradient in bromeliad size.

Results/Conclusions

We find that increasing habitat size results in changes to the composition of the insect community to favor greater predator abundance relative to detritivores. Higher nitrogen flow from detritus to the bromeliad leaves, as indicated by the incorporation of ¹⁵N from labeled litter in plant tissue, was associated with higher predator densities relative to detritivores. This is contrary to the predictions based on trophic cascade theory, but can be explained by the reduction in insect emergence under increased predation. The effect of predators on prey dispersal is itself modified due to increased foraging efficiency for both predators and detritivores, and greater vulnerability to intraguild predation among the predators in less complex habitats. Our results emphasize the importance of considering the spatial scale of organism dispersal when examining nutrient flux, as well as the habitat context of these interactions.