Top down or bottom up? Comparing the impacts of introduced arboreal possums and ‘terrestrial’ ruminants on native forests in New Zealand

Invasive species and their impacts on agri-ecosystems: issues and solutions for restoring ecosystem processes

“…This is supported by evidence that microbes and pathogens that have entered as passengers of domestic animals can have major impacts on native wildlife (e.g. toxoplasmosis -causal agent Toxoplasma gondii -on marsupial reproduction, Canfield et al 1990; hydatidosis -causal agent Echinococcus granulosus -on brush-tailed rock-wallabies, Petrogale penicillata, Barnes et al 2007; and bovine tuberculosis -causal agent Mycobacterium tuberculosis -in New Zealand brush-tailed possums, Nugent et al 2001). The fifth hypothesis (Table 1) is an unlikely scenario for Australasia.…”

“…This is particularly so in Australasia, where Old World Laurasian and domesticated taxa have collided with ancient and geographically isolated ancient Gondwanan ecosystems, the fauna and flora of which Darwin (1859) noted, were utterly dissimilar in form but analogous in function and trophic position. Plants and animals have also been introduced to New Zealand from Australia, for example, red necked wallabies (Notamacropus rufogriseus) and brush-tailed possums (Trichosurus vulpecula), the latter having devastating impacts on native biota, cattle production and the economy (Nugent et al 2001).…”

Management of introduced mammals in New Zealand

“…However, the extent depends on the species and place (e.g., hunters do hold thar numbers at the low densities achieved by past commercial exploitation, but only in places where they have easy access; Forsyth 1999), and on the density of animals required to benefit conservation values. Recreational hunters rarely maintain densities of their prey low enough to protect particularly vulnerable species (Choquenot & Parkes 2001;Nugent et al 2001b). …”

“…Commercial harvesting of deer is biologically sustainable because the forests provide a refuge for deer that survive and remain difficult to access by the helicopter-based hunters. The conservation benefits of the venison harvest are very large in the open grassland habitats of the alpine zones once favoured by deer, but are limited in the forests (Nugent et al 2001b). Commercial harvesting of thar decimated the population, and provided significant conservation benefits (Parkes & Thomson 2001), but was not politically sustainable (Hughey & Parkes 1997).…”

“…It generally provides little explanation of how the pest-resource-management interactions work, and so its predictions are often site-and statespecific. Managers with some knowledge about how the system works can predict outcomes of management, and the best way to do this is to use some form of model, either conceptual, e.g., as used to describe the relative effects of red deer and possums in a forest (Nugent et al 2001b), or more formally in a deterministic model that predicts outcome without explaining mechanisms (e.g., Barlow 2000 for Tb epidemiology in possums), or a mechanistic model that purports to explain how the interactions work (e.g., Choquenot & Ruscoe (2000) for beech mast-mouse systems, or Bayliss & Choquenot (2002) for how food regulates possum densities). Choquenot & Parkes (2001) have reviewed three types of pest-resource models, damage functions, density-dependent predator-prey models, and interactive models, to see how useful they are to help managers set threshold densities for vertebrate pest control in New Zealand.…”