Temporal dynamics of edge effects on nest predation risk for the marbled murrelet

Abstract

Habitat fragmentation can cause population declines greater than those expected from habitat loss alone. This can result from detrimental “edge effects”, which occur when predation rates are higher at habitat edges relative to interiors. The marbled murrelet ( Brachyramphus marmoratus) is a threatened seabird which nests in old-growth forests, a habitat that is being fragmented by ongoing harvest. There is little consensus on the magnitude of edge effects on marbled murrelets, or how they might vary by edge-type. We compared the fates of experimental murrelet nests at paired edge and interior locations at 52 sites in two regions of south-western British Columbia, Canada. Sites were chosen at “hard” edges (recent clearcuts), “soft” edges (regenerating forest), and natural edges (i.e., riparian areas). We used nest cameras to distinguish disturbances caused by known predators of real nests. Accounting for landscape-scale fragmentation, disturbances by avian predators were significantly more frequent at hard edges relative to interiors, but less frequent at soft edges. There were no edge effects at natural-edged sites. These results imply that detrimental edge effects adjacent to recent clearcuts may decline with time due to successional processes. Survey data suggest that this pattern was caused by Steller’s jays ( Cyanocitta stelleri), who were observed more often at hard edges than soft edges in one region. Where corvids are important predators, we recommend that managers maintain reserves that lessen the amount of hard edge per patch area. Harvest adjacent to reserves should proceed in stages to limit hard edge effects at any given time.