Trophic omnivory and the structure, strength, and nonlinear nature of species interactions across a productivity gradient

Abstract

Two elements of food webs add significantly to their complexity: The presence of trophic omnivores and the nonlinear nature of predator-prey interactions. I introduce a new observational method for estimating the strengths of species interactions that accounts for the indeterminacy of omnivorous indirect effects and the saturating functional responses that predators exhibit. I present an empirical validation of the method’s accuracy by applying it to two populations of the predatory whelk, Haustrum (= Lepsiella) scobina, that is common to the rocky intertidal shores of New Zealand, and comparing these interaction strength estimates with those derived from concurrently performed experimental manipulations of H. scobina’s populations.

I then test two key predictions of intraguild predation theory by investigating how species abundances, food web structure and species interactions strengths change across six omnivorous food webs along a gradient of productivity present around New Zealand’s coastline. I find that the intermediate predator, H. scobina, is the superior competitor for shared prey species, as predicted by theory. Counter to theory, however, I show that it is the omnivorous whelk, H. haustorium, that is the superior competitor when all prey are considered, and that H. scobina’s abundance increases with increasing productivity. My analyses nevertheless reveal clear and regular cross-gradient shifts in interactions that can be incorporated into future modeling efforts.

Finally, I ask to what degree whelk feeding rates are saturated with respect to prey densities and, by extending and parameterizing the classic Rosenzweig-MacArthur model, ask whether empirical interactions are nonlinear enough to affect the stability of whelk-prey dynamics. Results indicate that whelk feeding rates are not strongly saturated and that increasing diet richness has a non-additive effect on a predator’s saturation such that alternative prey have a stabilizing effect on whelk-prey dynamics. I thereby offer a new mechanism by which generalist predators stabilize the dynamics of their species-rich food webs that does not invoke prey switching.

My dissertation brings empirical data to bear on the importance of omnivory and the nonlinear nature of trophic interactions. Furthering our understanding of these food web features can contribute much to both the conceptual and applied goals of ecology.