Prey often modify their behavior in response to the threat of predation. Such anti-predator behavior can alter patterns of resource exploitation by prey and ultimately trigger changes to prey populations and community properties (i.e., risk effects). The implications of risk effects for particular communities, however, have proven difficult to anticipate in large part because anti-predator behavior is not generalized. Rather, prey responses to predation risk appear to be highly contingent, or context dependent. One of the main goals of my research program is to explore the sources of this contingency, asking whether and to what extent risk effects in particular cases hinge on predator hunting modes, prey escape tactics, and the setting of the predator-prey interaction (i.e., landscape context). My graduate students and I will address these questions by focusing on interactions between tiger sharks (Galeocerdo cuvier) and large marine consumers (e.g., dugongs, Dugong dugon) at my long-term field research site in Shark Bay, Western Australia and between a variety of carnivores and their prey in the Pacific Northwest of the United States (e.g., between gray wolves, Canis lupus, and cervids). Our goal is to help develop a general framework that uses the drivers of behavioral contingency to better understand and presage risk effects and their community consequences (see figure from Wirsing et al. 2010 below). The development of such a framework is important for conservation, for it will enable more reliable prediction of the ecological impacts of losing predators from, and repatriating predators to, ecosystems.

Ecologists working in aquatic and terrestrial environments rarely exchange information about their findings. Yet, where it has occured, communication acorss ecosystem boundaries has led to powerful new insights (e.g., the importance of spatial and temporal scale), popularized general processes (e.g., trophic cascades), and helped spread the use of experimental manipulation. Thus, there is great value in continued 'cross-pollination' of work in different environments. Accordingly, another goal of my research program is to demonstrate the benefits of an 'ecology without borders' approach that compares mechanisms underlying predator effects in different systems. To that end, one of my recent papers (Wirsing and Ripple 2011) reveals remarkable similarity in the forms of anti-predator behavior that are elicited by gray wolves in the greater Yellowstone ecosystem, USA and tiger sharks in the coastal seagrass ecosystem of Shark Bay, Australia (see figure).

Caption: Similar risk effects of tiger sharks on dugongs and gray wolves on elk (Cervus elaphus). (a) As relative shark abundance (catch rate per hour) increases, the proportion of dugongs foraging over shallow seagrass meadows falls farther and farther below that expected based on the proportion of food (seagrass) in the shallows (dashed black line), indicating a predator-induced shift into deeper water where encounters with sharks are less likely. (b) Wit rising shark abundance, the proportion of foraging dugongs along the edges of shallow seagrass meadows begins to exceed that expected based upon the proportion of food in edges (dashed black line), indicating a shift from interior portions of meadows to peripheral areas where the probability of escape into deep water is elevated. (c) The proportion of time dugongs devote to excavation, a foraging tactic that inhibits vigilance, declines as tiger shark abundance increases. (d) When wolves are present, elk often shift from open grasslands to conifer forests (i.e., probability of conifer occurence at locations of elk fitted with GPS increases), apparently to decrease encounters with wolves. (e) Elk browse more often at sites without downed logs (escape impediments), likely to enhance escape possibilities in case of wolf attack. (f) Elk vigilance is highest near escape impediments, likely in response to enhanced vulnerability to wolf predation.

The Ecology of Low-density Snowshoe Hare Populations