Indirect effects of predator interactions and alternative prey in food webs

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Interactions between predators and their prey strongly affect ecosystem function, but the impacts of predators on their prey are also affected by interactions with other predators and prey. The proposed research will investigate such indirect interactions in two ecosystems with slightly different food web configurations. Both involve predators with alternative prey, but in one system, one prey species also consumes another (i.e., intraguild predation). Raccoons and ghost crabs are the two most important sea turtle nest predators on high nest density beaches in Florida. Raccoons also consume ghost crabs, reducing predation risk to turtle eggs, but also use their burrows to locate nests, which enhances risk to prey. By manipulating ghost crab densities and determining the effects on egg survival we will determine the direct effect of each predator and interactive effects on turtle reproduction. As three of Canada's threatened marine turtles nest in Florida, this research has conservation implications for Canada. We will also investigate how prey of arctic fox are affected by changing predation pressure associated with population changes of alternative prey. Lemming population cycles are collapsing throughout the Arctic, and climate change is the most commonly invoked explanation. Yet the exponential increase in goose populations may benefit lemming predators, increasing predation on lemmings and preventing their recovery from cyclic lows. To test this hypothesis we will collect willow scars, which reflect lemming population trends, in areas with different histories of goose colony establishment and compare rates of scar-age frequency decline. We will also examine other influences on arctic fox population dynamics, including stress response and parasite loads related to changes in prey availability and diet. Understanding factors affecting indirect interactions in food webs and the mechanisms involved is important for predicting effects of future species loss or environmental perturbation.