Impacts of an invasive herbivore on indigenous forests

Invasive herbivores can have large negative impacts on natural ecosystems. Management of invasive populations often requires frequent, broadscale, expensive control, which must be justified by demonstrating progress in achieving conservation objectives. This study evaluates benefits of regular extensive control of an invasive herbivore and develops an alternative strategy based on damage thresholds. We carried out replicated experimental management of brushtail possums, Trichosurus vulpecula, in three areas of native forest in New Zealand. Each area included a site that had extensive possum control for 10 years, prior to and during the 5-year study, and a paired site with no control. We measured indices of possum browse on c.2400 possum preferred and c.1200 non-preferred trees, and an index of possum abundance, at the beginning and end of the experiment. Extensive control was effective in reducing possum browse on preferred tree species. Reductions in browse led to increased foliage cover and decreased probability of tree mortality. The probability of browse on an individual tree decreased with increasing amounts of possum-preferred foliage on nearby trees but increased with tree size and with increasing levels of browse on nearby trees. At one site where possum control ceased prematurely, foliage cover decreased, reducing benefits from earlier control. Synthesis and applications. Our study provides evidence that sustained, extensive control of invasive herbivores can result in significant conservation benefits to susceptible tree species, and that both impacts and benefits can be measured using data typically collected in herbivore impact studies. Furthermore, it shows how local factors such as forest composition can influence the impact of herbivory, how this can be included in large-scale assessments of the benefits of pest control and how site- and species-specific damage thresholds can be derived for improving pest management.