Fear, fire, and behaviorally mediated trophic cascades in a frequently burned savanna

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Ecologists increasingly recognize the importance of trait-mediated indirect interactions and suggest behavioral traits aimed to reduce predation risk can induce trophic cascades. However, the application of theory developed in simple experimental systems to complex natural systems has spurred debate regarding whether predation risk, independent of direct killing, can structure ecosystems. We examined the effects of white-tailed deer (Odocoileus virginianus) foraging under the risk of predation by coyotes on oak (Quercus spp.) regeneration and the abundance of palatable understory species in a frequently burned longleaf pine savanna. Oaks represent a common but relatively low quality forage item for deer, and are an important component of this system because oak encroachment into pine-dominated uplands can reduce biodiversity. In frequently burned ecosystems oaks and other fire-impeding species often exist in a demographic fire-trap, where they sprout following fire but rarely escape into the midstory, because they are repeatedly top-killed by subsequent fires. Many factor including herbivory stress can influence the probability of fire survival for oak saplings. Other Cervids, including elk (Cervus canadensis) and moose (Alces alces) are known to increase use of woody browse with predation risk because of selection for brushy areas that offer concealment cover. We experimentally manipulated predation risk for 10years, by establishing 4 approximately 40-ha predator exclosures and 4 control plots to test the effects of predation risk on aspects of the plant community. Reduced predation risk resulted in increased oak recruitment and decreased abundance of palatable forage. A density-mediated trophic cascade does not explain the release of oaks and our results provide support the behaviorally-mediated trophic cascade hypothesis. We offer evidence that frequent disturbance can facilitate trophic cascades and that predator-sensitive foraging can enhance fire suppression of oaks and influence groundcover composition in longleaf pine savannas.