Nutritional ecology of common brushtail possums in New Zealand

Abstract

Disentangling the many bottom-up and top-down processes that regulate populations is fundamental, especially when investigating and predicting the impacts of invasive species. Plants form the basis of most terrestrial ecosystems as the primary source of energy and nutrients. However, the implications of changes in nutritional quality are difficult to predict due to complexity of most natural ecosystems. Common brushtail possums (Trichosurus vulpecula) are a primarily folivorous 2-3 Kg marsupial, native to Australia. It has long been suggested that introduced possums in New Zealand have experienced a release from both top-down and bottom-up regulatory processes. The dominance of nutritious, chemically undefended plant foliage has been touted as a factor in the notorious success of possums in New Zealand. In this thesis, I aimed to determine if the nutritional quality of foliage can explain the observed browsing damage inflicted by possums in New Zealand. To establish whether possums exhibit variation in their diet based upon nutritional quality, I performed two captive feeding experiments. Firstly, I examined the physiological capacity of possums in New Zealand to tolerate plant defence compounds that are common in foliage. Secondly, I assessed the response of New Zealand possums to the deactivation of tannins in their diets using polyethylene glycol (PEG). Thirdly, in order to facilitate landscape-scale assessments of nutritional quality in New Zealand forests, I developed calibrations using near infrared reflectance spectroscopy (NIRS) that could successfully predict nutritional parameters of New Zealand foliage, using a multi-species data set. Finally, I related the extent of browse damage inflicted upon trees in the Tararua Mountain Range to nutritional quality, to reveal if the browsing decisions of wild possums are being driven by the nutritional quality of the foliage. New Zealand possums demonstrated a greater tolerance to both jensenone (a PSM exclusively found in Eucalyptus) and salicin than Australian populations. In the second captive feeding experiment, possums fed foliage supplemented with polyethylene glycol (PEG) which increased the in vitro available nitrogen content of Weinmannia racemosa foliage resulted in possums consuming significantly more than when foliage was untreated, indicating that possums do respond to nutritional quality. However, the results of this experiment highlighted the need for further investigation in to the use of PEG as there was evidence that the intake response to PEG may not necessarily be completely due to an associated reduction in protein digestibility. There was significant spatiotemporal variation in nutritional quality across native New Zealand tree species in the Tararua Mountain Range. Most significantly, trees with higher available nitrogen concentrations (higher nutritional quality) were more severely browsed upon by possums. These findings indicate that nutritional quality is a key driver of the diet choice of possums in New Zealand, and may directly explain the observed impacts and patchy distributions of possums in New Zealand forests. Show more