Variation in local-scale edge effects: Mechanisms and landscape context

Ecological processes near habitat edges often differ from processes away from edges. Yet, the generality of ''edge effects'' has been hotly debated because results vary tremendously. To understand the factors responsible for this variation, we described nest predation and cowbird distribution patterns in forest edge and forest core habitats on 36 randomly selected plots in three states in the midwestern United States. We tested four hypotheses that may explain the variation and mechanisms responsible for edge effects among the 36 plots: (1) the landscape context, (2) the local predator community, (3) the local bird (host-prey) community, and (4) the nest site microhabitat structure. We used artificial nests baited with quail and clay eggs to determine nest predation patterns and predators and used point count surveys to determine cowbird and host abundance in forest edge and forest core habitats. Raccoons, opossums, canids, and birds accounted for most predation of artificial nests. Neither local host abundance nor mean nest concealment of artificial nests significantly influenced nest predation rates in habitat edge or in habitat core. Nest predation was significantly greater in highly fragmented landscapes than in unfragmented landscapes and was significantly higher in edge habitats than in core habitats. However, detection of edge effects varied, depending upon landscape type. Higher predation rates in edge habitats were detected in highly and moderately fragmented landscapes, but not in unfragmented landscapes. Both mammalian and avian predator groups contributed to higher predation rates along edges in highly and moderately fragmented landscapes. Cowbird abundance was significantly related to host abundance, but the effect of hosts varied depending upon habitat type. In edge habitats, cowbird abundance was negatively associated with host abundance in all three landscapes studied. By contrast, cowbird abundance was positively associated with host abundance in core habitats. Once the effects of host abundance were removed, cowbird abundance in core habitat was greater in highly fragmented landscapes than in moderately and unfragmented landscapes, but did not differ between the latter two. In edge habitat, cowbird abundance did not differ between landscapes, but abundance in edges tended to be highest in the highly fragmented landscape and lowest in the unfragmented landscape. Cowbird abundance did not vary between edge and core habitat in any of the landscapes studied. We suggest that the first approximation to predicting the impact of agricultural or permanently managed edges on forest songbird reproductive success is to assess habitat characteristics at the landscape scale. Given geographic location, local factors such as host abundance and predator composition should be assessed.