Abstract
Top predators can exert strong influences on community structure and function, both via direct, consumptive effects, as well as through non-consumptive, fear-based effects (i.e. predation risk). However, these effects are challenging to quantify, particularly for mobile predators in marine ecosystems. To advance this field of research, here we used baited remote underwater video stations (BRUVs) to assess how the behavior of mobile fish species off Cape Cod, Massachusetts, was affected by exposure to large sharks. We categorized sites into three levels of differential shark predation exposure (white sharks, Carcharodon carcharias) and quantified the relative abundance and arrival times (elapsed time before appearing on screen) for six mobile fish prey groups to the BRUV stations. Increased large shark exposure was associated with a decrease in overall prey abundance, but the overall response was prey group-specific. Foraging of smooth dogfish, a likely important prey item for large sharks in the system, was significantly reduced in areas frequented by white sharks. Specifically, the predicted probabilities of smooth dogfish bait contacts or bite attempts occurring were reduced by factors of 5.7 and 8.4, respectively, in areas of high exposure as compared to low exposure. These modifications were underscored by a decrease in smooth dogfish abundance in areas of high exposure as well. Our results suggest that populations of large, roving sharks may induce food-related costs in prey. We discuss the implications of this work within the context of the control of risk (COR) hypothesis, for the purposes of advancing our understanding of the ecological role and effects of large sharks on coastal marine ecosystems.
Highlights
Top predators can exert strong influences on community structure and function, both via direct, consumptive effects as well as through non-consumptive, fearbased effects[1]
Summary results of a negative binomial Generalized Linear Models (GLMs) assessing the effect of shark exposure and temperature on the abundance (MaxN) of six prey groups, and summary of Likelihood Ratio Tests (LRTs)
The rates of the quantified foraging behaviors in smooth dogfish were markedly affected by shark exposure
Summary
Top predators can exert strong influences on community structure and function, both via direct, consumptive effects (i.e., killing/predation) as well as through non-consumptive, fearbased effects (risk effects)[1]. Controlled field experiments have demonstrated that behavioral responses of prey species under exposure to predators can affect foraging, habitat selection and reproductive fitness[8,9], and play an important role in overall community structure[10]. Applying these lessons and concepts to ecological studies of free-ranging large predators and prey (e.g., large mammals and fishes) remains challenging, due to the difficulties in monitoring large animals and the high rates of movement of top predators[11]. Recent studies have suggested that context is important when evaluating the effects of mobile sharks on prey, as the strength of predator effects can vary due to numerous variables, including predator hunting behavior and/or functional attributes, environmental conditions, landscape features, and the predictability of risk itself [11,21,31,32,33]
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