Is the conservation value of small urban remnants of eucalypt forest limited by increased levels of nest predation?

Abstract

Levels of predation on bird nests may be increased in small remnant patches compared with levels in extensive forests, owing to edge-related nest predation or mesopredator release. Previous work using artificial nests has demonstrated that increased rates of nest predation, owing mainly to avian predators, occur near edges of fragmented subtropical Australian eucalypt forests. This implies that levels of nest predation within small patches of remnant forest, which could effectively be all 'edge', may also be increased. We used artificial nests, deployed on the ground and 1-4 m above the ground in shrubs, to compare levels of nest predation and predator assemblages between interiors of small (10-20 ha) suburban remnant patches and extensive tracts (>400 ha) of subtropical Australian eucalypt forest. Most predation of nests in shrubs in both remnants and extensive forests was by birds, although there were some differences in the assemblages involved. Ground-nests were far more likely to be depredated by mammals or lizards in the remnants than in the extensive forests. However, despite the differences between their predator assemblages, overall levels of nest predation did not differ significantly between remnants and extensive forests, suggesting that any edge-related predation was of insufficient spatial extent to affect the interiors (>50 m from external edges) of remnants in this size-class. Eucalypt forest remnants of 10-20 ha often support bird assemblages typical of extensive forest tracts, but may act as population sinks for these species if levels of nest predation are excessive. Our results support the contention, however, that remnants in this size-class are useful to the conservation of forest bird assemblages.