Model-predicted values for the relationship between rat capture probability and distance from forest edge.

<p>Values were predicted separately for those patches that were grazed by livestock (grey lines) and those that were not (black lines). Dotted lines show 95% confidence intervals for predicted values. The truncated values for grazed patches reflect the reduced range over which edge distances were measured in these patches.</p

Final path model showing relationships between distance from forest edge, grazing, vegetation structure, and rat capture probability.

<p>Plus/minus symbols at each arrow head show a positive/negative effect, and asterisks denote significance of each path: ***<0.001; **<0.01; *<0.05; ns  =  not significant. Dotted arrows show non-significant paths, and black arrows show variables which alter the strength of the relationship they feed into (i.e. interaction effects). Note that path coefficients are not directly comparable because of different scales of measurement of predictor variables (distance square root transformed; other variables untransformed) and different link functions for response variables (logit link for rat capture probability; identity link for other variables) used in models.</p

Alternative models of causal relationships between edge distance, grazing, vegetation structure, and rat capture probability.

<p><b>A</b>: model hypothesised in this study, in which distance from forest edge and grazing indirectly influenced rat capture probability by driving changes in vegetation structure, and in which grazing altered the strength of the distance from edge effect; <b>B</b>: an alternative ‘direct edge effects’ model, where livestock grazing effects on rat capture probability were mediated by vegetation structure, but edge effects were not; <b>C</b>: an alternative ‘direct effects only’ model, where distance from forest edge, livestock grazing, and their interaction affected vegetation structure variables and rat capture probability, but where neither edge effects nor livestock grazing effects on rat capture probability were mediated by vegetation structure. Black arrows represent variables which alter the strength of the relationship they feed into (i.e. interaction effects).</p

Discriminating the Drivers of Edge Effects on Nest Predation: Forest Edges Reduce Capture Rates of Ship Rats (<i>Rattus rattus</i>), a Globally Invasive Nest Predator, by Altering Vegetation Structure

<div><p>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.</p></div