Alternative trophic cascades: cascading effects of predator diversity and intraguild predation on functioning and stability at the base of the food web

Chase J. Rakowski and Caroline Farrior, Department of Integrative Biology, University of Texas at Austin, Austin, TX, Mathew A. Leibold, Department of Biology, University of Florida, Gainesville, FL

Background/Question/Methods
A recent review by trophic cascade experts defined trophic cascades broadly as indirect effects stemming from predators and propagating downwards through food webs. Included in this definition are many potential indirect trophic interactions relevant to global change and management which are little studied compared to the classic trophic cascade where a change in predator biomass indirectly alters autotroph biomass. For example, changes in predator diversity or intraguild predation can modulate the suppression of herbivore prey, which in turn could affect the stability of autotroph biomass.
We performed a field experiment and model simulations to test for such alternative trophic cascades in pond food webs. In tritrophic pond mesocosms we manipulated the presence of two insect predators in a 2×2 factorial design. We sampled repeatedly to assess the effect of predator diversity on the biomass of the lower trophic levels as well as the stability of autotroph (phytoplankton) biomass. Then we developed a model to help explain any effects and explore the potential for other types of trophic cascades. Simulating the dynamics of a simple pelagic food web, we tested for indirect effects of predator diversity and intraguild predation strength on the average and stability of autotroph biomass and nutrient uptake.
Results/Conclusions
In the field experiment, the insect predator species partitioned their zooplankton prey, leading indirectly to a synergistic stabilization of phytoplankton biomass when the predators were together (a reduction of temporal CV by 52%). However, average phytoplankton biomass was not affected by predator treatment. The model simulations agree that cascading effects of predator diversity tend to affect the stability of autotroph biomass more strongly than they affect average autotroph biomass, as long as intraguild predation is not high. Similarly, the model simulations show that intraguild predation strength has cascading effects which affect the stability of autotroph biomass more strongly than average autotroph biomass. Specifically, reducing intraguild predation stabilizes autotroph biomass. Additionally, nutrient uptake and its stability follow the same pattern as autotroph biomass. These results demonstrate that understudied types of trophic cascades, especially those resulting in a change in stability, can exert an important regulating influence on food web functioning. Such alternative trophic cascades are relevant to global changes in predator diversity and composition, and also suggest methods for optimizing crop yield stability via management of biological control agent communities.