Abstract
Forest edges can strongly affect avian nest success by altering nest predation rates, but this relationship is inconsistent and context dependent. There is a need for researchers to improve the predictability of edge effects on nest predation rates by examining the mechanisms driving their occurrence and variability. In this study, we examined how the capture rates of ship rats, an invasive nest predator responsible for avian declines globally, varied with distance from the forest edge within forest fragments in a pastoral landscape in New Zealand. We hypothesised that forest edges would affect capture rates by altering vegetation structure within fragments, and that the strength of edge effects would depend on whether fragments were grazed by livestock. We measured vegetation structure and rat capture rates at 488 locations ranging from 0–212 m from the forest edge in 15 forest fragments, seven of which were grazed. Contrary to the vast majority of previous studies of edge effects on nest predation, ship rat capture rates increased with increasing distance from the forest edge. For grazed fragments, capture rates were estimated to be 78% lower at the forest edge than 118 m into the forest interior (the farthest distance for grazed fragments). This relationship was similar for ungrazed fragments, with capture rates estimated to be 51% lower at the forest edge than 118 m into the forest interior. A subsequent path analysis suggested that these ‘reverse’ edge effects were largely or entirely mediated by changes in vegetation structure, implying that edge effects on ship rats can be predicted from the response of vegetation structure to forest edges. We suggest the occurrence, strength, and direction of edge effects on nest predation rates may depend on edge-driven changes in local habitat when the dominant predator is primarily restricted to forest patches.
Highlights
The majority of the world’s forests are fragmented by human activities to some extent [1], and the species that occur within them are vulnerable to the impacts of forest fragmentation
Vegetation structure has been found to correlate with nest predation rates previously [55,56,57], we know of no studies that have identified change in vegetation structure as a driver of edge effects on nest predation
We found that the relationships between forest edges, vegetation structure, and ship rats gave rise to ‘reverse’ edge effects in our study system, in which rat capture rates were higher in the interior of forest fragments than at the forest edge
Summary
The majority of the world’s forests are fragmented by human activities to some extent [1], and the species that occur within them are vulnerable to the impacts of forest fragmentation. The extent of forest fragmentation is likely to increase dramatically into the foreseeable future as the human population and its resource requirements grow [2,3]. Nest predation is the major cause of nest failure in birds, and is believed to strongly influence the population dynamics of many species [7,8]. Causes of variability in nest predation rates are of substantial interest to ecologists and conservation managers [6], and there are many hundreds of studies describing the relationship between nest predation rates and distance from the forest edge
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