Spillover and edge effects in pest control

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Clearly, native biodiversity benefits when invasive predators are controlled. However, the spatial extent of these benefits is poorly understood. For any given area under pest control, both edge effects and spillover effects are likely. Edge effects occur when the control area closest to the boundary (the edge) receives a lower level of benefit than areas closer to the centre of the control area (the core) (Fig. 1). Edge effects may occur if pests from the surrounding uncontrolled area reinvade the edges of the management area and adversely affect the biodiversity there. Conversely, spillover effects may occur when the area directly outside the boundary of the control area receives some level of biodiversity benefit due to the proximity of pest control. Spillover may occur if native plants and animals that benefit from the pest control are also present in the surrounding area, or if linked to the surrounding area via processes such as seed dispersal. Edge effect and spillover effect are not mutually exclusive concepts and may or may not be observed at individual pest control areas.

Knowing the spatial extent of edge and spillover effects in regard to pest management goals and conservation outcomes is important but the effects are not well understood. Edge effects could reduce the actual area receiving biodiversity benefits to only part of the entire area being managed, In such situations, a ‘buffer zone’ of pest control surrounding the managed area may be necessary to protect the whole area as intended. On the other hand, if significant biodiversity spillover effects occur outside the managed area, it may not be necessary to actively manage pests over the entire area in order to achieve beneficial outcomes.

Mustelids and rodents are two groups of predators that have had particularly severe impacts on, and continue to devastate, New Zealand’s native animals, plants and ecosystems. As a result, these pests are targeted in many areas of high conservation value around the country. One such area is the Ark in the Park Open Sanctuary Project, a 2300-ha area in the forested Waitakere Ranges Regional Park west of Auckland City. There, rodents and possums are poisoned and mustelids are trapped.

In the Ark in the Park area, Eru Nathan of the University of Auckland has established transect lines 1200 m long, 600 m within the pest control boundary and 600 m beyond in untreated parkland. At 200-m intervals along these lines, Eru records several easy-to-measure biodiversity indicators that are representative of the range of local biodiversity. These indicators are bird counts, occupancy rates of gecko and skink artificial refuges and of weta ‘motels’, counts of ground invertebrates, seedlings on seedling density plots, and counts of rodent and mustelid tracks in tracking tunnels.

Although the fieldwork is continuing, preliminary analyses of the tracking tunnel data suggest that both edge and spillover effects are occurring at this site (Fig. 2). At 200–400 m outside of the control area, pest numbers are lower and weta numbers higher than at 600 m beyond the boundary, suggesting that some level of spillover benefit is occurring outside the management area. Conversely, from the control boundary to 200 m inside the management area, pest numbers are higher and weta numbers lower than at the core of the area, suggesting that an edge effect is also occurring at this site.

Given the heavy use of pest control for conservation benefits in New Zealand, better knowledge of the spatial extent of control benefits could potentially be used to make pest management more time- and cost-efficient, and allow for better allocation of conservation resources overall.

This work is funded by the University of Auckland and Landcare Research and is supervised by Margaret Stanley and Al Glen.

Eru Nathan (MSc student, Joint Graduate School in Biodiversity and Biosecurity, School of Biological Sciences, University of Auckland)

Contact Eru: enat006@aucklanduni.ac.nz