Mass action predicts that contact rate among species will be proportional to the product of their local densities ( Cosner et al. If predation is proportional to contact rate between predators and prey, the principle of mass action provides a simple prediction of how edge responses will affect predation risk and total prey mortality. In other cases, attraction to edges may lead to convergence of predator and prey abundances, resulting in heavy mortality near the edge and creating an ‘ecological trap’ for prey ( Gates & Gysel 1978 Ries & Fagan 2003). In some cases, prey may actively avoid the locations of the highest risk (e.g. The spatial distribution of prey mortality will not necessarily match that of predation risk in fragmented landscapes. Total prey mortality determines the energy flow from prey to predators, altering the dynamics of both populations ( Fagan et al. The spatial distribution of predation risk affects the relative availability of suitable ‘core’ habitat for prey ( Robinson et al. Where predators and prey interact in fragmented landscapes, edge responses may influence the spatial distribution of both predation risk, the probability of predation for an individual prey in a given time interval, and total prey mortality, the total number of prey killed by predators in a given time interval. However, where species occupy distinct habitats, the edge provides a unique context for their interactions and edge responses may have a strong effect on where and how often interactions take place. Where two interacting species both occur on either side of an edge, such edge responses may simply amplify or reduce the frequency of normal interactions. In fragmented landscapes, animals often respond to habitat structure by avoiding or aggregating near edges, resulting in strong gradients of species abundance ( Lidicker 1999 Ewers & Didham 2006 a). Habitat fragmentation can affect communities by altering interactions among species ( Fagan et al. This is the first study to predict patterns of predation directly from continuous edge response functions of interacting species, establishing a link between models of edge response and species interactions. Hawksbill preference for vegetation edge becomes an ecological trap in the presence of mongooses. The product of predator and prey edge response functions accurately described the observed pattern of total prey mortality. Predation risk was strongly related to mongoose abundance but was not affected by nest density or habitat type. Using both artificial nests and hawksbill nesting data, we examined how the edge responses of these species predict the spatial patterns of nest mortality. Turtles nest in both open sand and vegetation patches, with a peak in nest abundance near the boundary between the two microhabitats mongooses rarely leave vegetation. In the Caribbean, mongooses ( Herpestes javanicus) prey on hawksbill sea turtle ( Eretmochelys imbricata) eggs. Because species respond differently to habitat boundaries and spatial overlap affects encounter rates, edge responses should be strong determinants of spatial patterns of species interactions.
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