Multiple Predator Interactions and Food-Web Connectance: Implications for Biological Control

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Abstract

The use of single versus multiple natural enemies in biological control remains controversial, largely due to the possibility for antagonistic interactions among predators (e.g., intraguild predation and cannibalism) that can reduce the potential for the top-down control of pest herbivores. Using a natural system, Spartina cordgrass and its associated community of arthropods (herbivores, strict predators and intraguild predators), we created 29 different treatment combinations of predators that varied in richness (number of predator species) and trophic composition (proportion of strict to intraguild predators) and measured the ability of each to reduce the density of a key herbivore (the planthopper Prokelisia dolus) in the system. We then calculated food-web connectance (the fraction of all possible directed feeding links that are realized in a food web) for each of the experimental food webs. Notably, food-web connectance is enhanced by predator-predator interactions such as intraguild predation and cannibalism. We found a significant negative relationship between food-web connectance and the ability of the predator complex to reduce prey populations. Specifically, well-connected food webs comprised of mostly intraguild predator species were far less effective at suppressing herbivores than webs consisting largely of strict predators. Importantly, trophic composition of the food web was more influential than predator richness in affecting top-down control. We also discovered that a food web comprised of multiple predators was more effective in suppressing herbivores when the structural complexity of the habitat was increased, a result that was attributable to spatial refuges for intraguild prey and relaxed intraguild predation. Thus, in this system, habitat structure has the potential to transform a well-connected food web into a less-connected one by reducing feeding links resulting from intraguild predation and cannibalism. Because of the remarkable similarity of the Spartina system to tropical Asian rice, this finding provides encouragement that the effectiveness of the predator complex can be enhanced by management practices that increase the structural complexity of the habitat and thereby dampen intraguild predation. Last, we discuss how food-web analyses might be used to evaluate particular combinations of predators for more effective biological control.