Biodiversity and fear ecology: The cascading effects of species richness and nontrophic interactions

Shawn Alan Steffan

doi:10.7273/000005978

Food web complexity may arise as much from species richness as from behavioral interactions among species. An important yet underdeveloped area of food web ecology is the degree to which species richness and nontrophic interactions drive community dynamics. To examine these relationships, we isolated the consumptive and nonconsumptive effects of predator assemblages, as well as the effect of predator diversity per se. We found that species-rich predator assemblages evoked more frequent antipredator behaviors in their prey than less diverse predator assemblages, while similar numbers of prey were killed at both diversity levels. This effect cascaded to the host-plant of the prey, allowing greater plant biomass at the higher diversity level. Thus, we show that consumptive and nonconsumptive predator effects may operate independently and scale differently with predator diversity. There appeared to be two underlying mechanisms for these findings: several predator species visited the plants more often when among heterospecifics than conspecifics, and on a per-visit basis, these species evoked greater intimidation when within species-rich assemblages. These findings suggest that predator-predator intimidation may be reduced within species-rich assemblages. Given that cascading fear effects may significantly influence the basal resource of a given system, the density of prey transmitting the fear effect should be a significant factor. We investigated this hypothesis by subjecting varying prey densities to a nonlethal enemy. Nonlethal enemy effects increased linearly with prey density, indicating that cascading fear effects were a function of prey density. Not all nontrophic interactions in communities are predicated on cascading predator-prey interactions. We focused on the nontrophic impacts of a ubiquitous, important ecological phenomenon: insect folivory. In a simple collard system, we found that caterpillar folivory on collard plants dramatically altered leaf architecture, inducing predators to alter their foraging patterns, which ultimately facilitated increased aphid predation. Folivory also affected host plant quality such that aphids fed much less on damaged leaves. We quantified the per capita interaction strength of each link in these indirect interaction pathways to facilitate the parameterization of nontrophic effects in food web models.

Biodiversity and fear ecology: The cascading effects of species richness and nontrophic interactions

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