Inverse edge effect on nest predation in a Kenyan forest fragment: an experimental case study

Fragmentation of continuous habitat blocks into small, isolated patches may Cause gradual replacement of habitat specialists by generalists and edge-dwelling species, thereby altering trophic interactions within communities. When fragmentation progressively increases the edge-to-interior ratio of forest remnants, forest birds are typically exposed to higher levels of nest predation, especially so near forest edges. However, the strength and direction of this 'edge effect on nest predation' may vary with predator communities, which in turn call be affected by habitat type, landscape structure and geographical location. We studied avian nest predation in two isolated forest fragments in the Taita Hills of southeast Kenya, where small mammals, rather than birds, dominate predator communities. The use of artificial ground nests allowed us to reduce variation in laying date and microhabitat use, which are known to affect predation on natural nest in the study area. Predation rates did not increase near the forest boundary, but instead, were higher in the forest interior of one of the fragments. Such inverse edge effects may be caused by forest-dependent small mammals or snakes that can remove bird eggs. Although inverse edge effects may be more common in East African tropical forests than previously thought, and should therefore be taken into account when drafting conservation plans, the underlying mechanisms remain unclear. Further study of the population dynamics of small mammals and snakes, and their relative impact on nest predation in fragmented habitats, is therefore required.