Past, present and two potential futures for managing New Zealand’s mammalian pests

In 2003, a review of how introduced mammals were managed as pests in New Zealand was published. Since then trends for the control of these mammals include moves from pest-by-pest prioritisation towards site-based and multiple-pest management, extension of large-scale aerial control of predators to include beech forests, increasing intensive management of sites by private and non-government agencies, and increasing effort by regional councils and managers of vectors of bovine tuberculosis. The current deployment is the sum of largely independent decisions made by different agencies and individuals with different priorities. National agencies might optimise their own decisions, but overall deployment will always be nationally suboptimal when objectives are set at regional or local scales. This system includes both small areas where most of the mammals present are managed intensively and large areas where fewer species are managed. We discuss how this deployment might evolve as a network of smaller core areas aiming to achieve zero or low densities of all or most pests, with surrounding halos with lesser control effort against fewer species, enough to allow for at least ‘safe passage’ of native animals between the cores. This system might work best along the more-or-less contiguous native habitats along the axial ranges of the North and South Islands, especially if agencies with a national mandate are prepared to act in the extensive areas between core sites. Some have asked whether the successful eradication of mammals on small islands can be scaled up to the main islands of New Zealand. The current version of this predator-free model aims at the ubiquitous ship rat, brushtail possum and stoat. A key need for the predator-free model is a tool that kills 100% of each of the three main target species, ideally with one large-scale, socially-acceptable application. Without this tool the cost to find and kill survivors would raise the costs of national eradication to about $32 billion, assuming current costs of multiple-species eradications on islands. For sustained control options, research is needed to inform the frequency and intensity of control, and thus the best control tools. The ability to detect target pests at low densities and management of reinvasion are essential for achieving zero or low pest density goals. We suggest that the core/halo model of pest management, within a network of assets to be protected, will best protect New Zealand’s biodiversity for the foreseeable future. The national scale pest- or predator-free aspiration is not currently (and may never be) feasible and risks diverting resources from more optimal solutions, as occurred with the ‘last rabbit’ and ‘last deer’ programmes promoted last century.