Consequences of omnivory and alternative food resources on the strength of trophic cascades

Abstract

Omnivorous predators that feed on prey and plant resources are recognized as an important component of food webs but their impact on herbivore populations and trophic dynamics is unpredictable. Feeding on food items from multiple trophic levels increases the reticulate nature of food webs and the labile role of omnivores in promoting trophic cascades. Using carabid beetles in a corn agroecosystem, this research explored the interactive effects of predator guild (omnivore or carnivore) and the trophic origin of alternative food resources (seeds or fly pupae) on the control of herbivores (black cutworms) and plant survival. I demonstrated that the trophic guild and feeding performance of carabids can be predicted from their mandibular morphology. Carnivorous carabids, using mandibles with sharp points and a long shearing edge, kill and consume caterpillars more efficiently than omnivores that have mandibles with wide molar areas adapted for consuming prey and seeds. Omnivore preference for seeds and pupae further reduced their consumption of cutworms, which resulted in increased plant damage, ultimately dampening trophic cascades.

In open field plots the abundance of omnivorous carabids and ants increased in response to seed but not pupae whereas neither subsidy affected the abundance of carnivorous predators. Pupae subsidies reduced predation of cutworms by carnivores and omnivores, consequently reducing seedling survival. However, in seed subsidized plots omnivorous predators switched from seeds to higher quality cutworm prey. Thus, predation of cutworms increased with cascading positive effects for seedlings.

This research demonstrated that omnivorous carabids interacted more strongly with alternative food resources, particularly seeds, than carnivores. In addition, this difference can be linked to morphological differences that reduced omnivore efficiency as predators suggesting omnivores may be less effective agents of biological control. However, increased tenure time and aggregation to plant resources by omnivores helped restore trophic cascades, and should enhance biological control. Understanding the predacious behavior of omnivores in resource diverse environments is essential to predicting their role in trophic dynamics. I provide evidence that the trophic origin of alternative food drives the strength of this interaction and the extent to which omnivores promote trophic cascades.