Contrast in edge vegetation structure modifies the predation risk of natural ground nests in an agricultural landscape.

Nicole A Schneider,Debora Arlt,Matthew Low,Tomas Pärt,Darren Mark Evans

doi:10.1371/journal.pone.0031517

Nicole A Schneider, Debora Arlt + Show 3 more

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https://doi.org/10.1371/journal.pone.0031517

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Abstract

Nest predation risk generally increases nearer forest-field edges in agricultural landscapes. However, few studies test whether differences in edge contrast (i.e. hard versus soft edges based on vegetation structure and height) affect edge-related predation patterns and if such patterns are related to changes in nest conspicuousness between incubation and nestling feeding. Using data on 923 nesting attempts we analyse factors influencing nest predation risk at different edge types in an agricultural landscape of a ground-cavity breeding bird species, the Northern Wheatear (Oenanthe oenanthe). As for many other bird species, nest predation is a major determinant of reproductive success in this migratory passerine. Nest predation risk was higher closer to woodland and crop field edges, but only when these were hard edges in terms of ground vegetation structure (clear contrast between tall vs short ground vegetation). No such edge effect was observed at soft edges where adjacent habitats had tall ground vegetation (crop, ungrazed grassland). This edge effect on nest predation risk was evident during the incubation stage but not the nestling feeding stage. Since wheatear nests are depredated by ground-living animals our results demonstrate: (i) that edge effects depend on edge contrast, (ii) that edge-related nest predation patterns vary across the breeding period probably resulting from changes in parental activity at the nest between the incubation and nestling feeding stage. Edge effects should be put in the context of the nest predator community as illustrated by the elevated nest predation risk at hard but not soft habitat edges when an edge is defined in terms of ground vegetation. These results thus can potentially explain previously observed variations in edge-related nest predation risk.

Highlights

In many landscapes habitat fragmentation has increased the amount of edges relative to habitat area [1,2,3]
We found a similar pattern for distance to mature crop field edges
Nests within short field layers had a higher predation risk the closer they were located from crop field edges, while nest predation risk in tall field layer habitats showed no such distance relationship (Figure 1b)

Summary

Introduction

In many landscapes habitat fragmentation has increased the amount of edges relative to habitat area [1,2,3]. Predator-prey interactions have been shown to change substantially at habitat interfaces [1,6], which in the case of bird communities may result in an increased nest predation risk near edges [7,8,9]. Farmland birds have declined dramatically during the last century [10], and agricultural landscapes are often highly fragmented with a high proportion of edges. In agricultural landscapes studies on edge effects have largely focused on forest interfaces and nest predation caused by avian predators e.g. A forest bordering on a mature crop for example may function as a hard edge for avian predators, but as a soft edge for mammalian predators, as there is continuous ground vegetation cover which tends to promote the general diffusion of small mammals from one habitat type to another [17,18]

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