Nest Predators and Fragmentation: a Review and Meta-Analysis

Population declines of many avian species are often attributed to increased rates of nest predation in fragmented landscapes, yet mechanisms underlying these effects have rarely been examined. We reviewed the literature to determine the extent to which hypotheses about nest predators and fragmentation have been invoked and compared this to the number of direct tests of predators with respect to habitat edge, patch size, or landscape type. We also conducted a meta-analysis of tested predator effects to evaluate whether predator responses-numerical, functional, or species richness-to fragmentation depend on spatial scale (edge, patch, or landscape), landscape type, geographic region, or predator taxa. We found 120 papers containing hypotheses about nest predators and fragmentation, but only 31 with hypothesis tests. Most tests were of a single predator species or guild, whereas most cited hypotheses generalized across broader taxonomic groups. Results of predator tests were variable, but some general patterns were evident. Predator effects, including increased abundance, activity, or species richness in edges, small patches, or certain landscapes, were more prevalent (1) in tests conducted at the landscape scale than at the local scale, (2) in agricultural landscapes than in predominantly forested landscapes, (3) in certain biogeographic regions, and (4) for avian predators than for mammalian predators. Local-scale (edge and patch) effects were most common when the land surrounding patches was agricultural and when tests were conducted within agricultural landscapes. The response of nest predators to fragmentation is complex, taxon-specific, and context-dependent. Conservation efforts for declining avian species may therefore need to be customized according to the nest-predator species primarily responsible for local nest mortality and the nature of the landscape mosaic.