Determining the spatial extent of the biodiversity outcomes of mammalian predator pest management

Abstract

Introduced mammalian predators have had significant impacts on New Zealand’s native flora and fauna, through both direct predation and competition for resources such as food. As a result, management action is often undertaken in areas of high ecological significance to control mammalian pest densities to a low level. The aim of this study is to determine the spatial relationship between management of mammalian predators and its outcomes. The outcomes consist of changes in the abundance and diversity of taxa impacted by these predator pests. Predicted outcomes include reduced levels of benefit to native biodiversity inside but close to the border of the pest management area (edge effect) and some level of benefit to native biodiversity in the area immediately surrounding the pest management area (spill-over effect). The biodiversity indices were measured across a 1200 m distance scale with 600 m either side of the pest management border of Ark in the Park in the Waitakere Ranges. Biodiversity indices measured were diversity and relative abundance of native birds, lizards and invertebrates and density of seedlings. Additionally, relative abundances of the pests themselves were measured, and forest structure and composition assessed in order to relate these factors to the biodiversity indices. Forest structure and composition was not significantly different inside and outside the pest management area. Stoat and rat densities were found to decrease linearly from the point furthest outside to the point furthest inside the pest management area. Lizards were never observed. For all other biodiversity indices, at least some taxa were observed with increasing frequency from the point furthest outside to the point furthest inside the pest management area. These results suggest the occurence of both edge and spill-over type effects occuring at the borders of the pest management area, and that these effects are a consequence of predation by or competition with mammalian pests. This information will be of use to conservation managers and practitioners to improve cost-effectiveness or ensure adequate protection is provided to an area to meet its conservation goals. Evidence of spill-over benefits also indicate that localised pest management may improve biodiversity outcomes over a larger landscape level by promoting connectivity of otherwise isolated pest management areas.