Keystone Aliens? The Multiple Impacts of Brushtail Possums

Abstract

The Australian brushtail possum (Trichosurus vulpecula) was introduced to New Zealand from eastern Australia and Tasmania in the mid-19th century as a furbearer (Clout and Ericksen 2000). It is a solitary, nocturnal, arboreal phalangerid marsupial weighing 2–4 kg. In Australia, these animals have a wide distribution, occupy a range of forested or wooded habitats, and are one of a suite of arboreal marsupials. In New Zealand, they are the sole animal of this type, and are one of the most ecologically disruptive and economically damaging alien mammals. From the 1860s onward (peaking in the 1890s), there were many importations of brushtail possums to New Zealand to establish a fur industry. These introductions, and subsequent liberations, were carried out by private individuals, acclimatization societies and government agencies. Multiple liberations of New Zealand-bred possums, which continued illegally up to the 1970s, helped to establish the species and accelerated its spread. Possums are now present throughout the main islands of New Zealand, from coast to treeline (Clout and Ericksen 2000). In some lowland forests, they achieve densities of more than 10 ha–1, an order of magnitude greater than typical densities of <1 ha–1 in their native Australia. The success of the brushtail possum is attributable to its generalist habits, the lack of competitors, parasites or predators, and the relative abundance of palatable, nutritious vegetation. Since 1947, brushtail possums have been officially recognized as a pest in New Zealand. It is now widely acknowledged that they not only damage crops and native forests, but also carry bovine tuberculosis and prey on nests of native birds. Considerable and growing resources have been directed toward possum control over the past few decades, but the species remains a serious threat to the livestock industry and to native biodiversity in many parts of the country. This chapter reviews the multiple impacts of brushtail possums in Ecological Studies,Vol. 186 R.B.Allen and W.G. Lee (Eds.) Biological Invasions in New Zealand © Springer-Verlag Berlin Heidelberg 2006 New Zealand ecosystems, concentrating on their impacts on native biodiversity and concluding with a consideration of their role as “keystone aliens”.