Home range characteristics of the Australian brushtail possum in New Zealand: Is density a driver?

Abstract

There is conflicting research regarding whether density is a driver of the home range characteristics of the Australian brushtail possum (Trichosurus vulpecula). This species transmits bovine tuberculosis (bTB) to livestock in New Zealand. Therefore, variations in movement patterns between populations of different densities, or changes in movement patterns due to density reduction (i.e., pest control), may result in variations or changes in bTB transmission risk. The first objective of this study was to investigate whether the home range characteristics of possums varied between two low-density populations within pine (Pinus radiata) habitat, and one high-density population within oak (Quercus robur) and sycamore (Acer pseudoplatanus) habitat, using GPS and VHF-tracking. Possum home range characteristics did not vary between the two low-density populations. However, these possums displayed considerably larger home range sizes and home range overlap with conspecifics than those in the high-density population. In addition, the low-density populations used more dens and changed these more often. Across all sites, there were generally no intersexual differences in home range characteristics. The second objective of this study was to test whether possums change their home range characteristics (i.e., exhibit spatial perturbation) in response to density reduction. Experimental density reductions were undertaken at the high-density site and one of the low-density sites. Comparisons of the home range characteristics of the same possums before and after reduction were then compared, using GPS and VHF-tracking. Density reduction was not carried out at the second low-density site, but movement patterns were still monitored. Possums displayed spatial perturbation at the high-density manipulated site by increasing their home range size and overlap following density reduction. Females at this site also increased the number of dens they used and changed their dens more often. There were no other intersexual differences in the occurrence or magnitude of perturbation at this site. In contrast, possums at the two low-density sites did not change their home range characteristics, as density at these sites remained constant between the pre-and-post monitoring periods, even though a number of individuals were removed at one of these sites. This thesis also presents a theoretical model that predicts whether species are likely to display perturbation after a disturbance event, such as density reduction, depending on the distribution pattern of the population. The predictive capacity of this model was assessed against a range of case studies and this model predicts that possums distribute themselves predominantly based on the availability of resources. Overall, this PhD research suggests that the home range characteristics of possums are density-dependent and that possums will exhibit spatial perturbation in response to density reduction. This might mean that bTB transmission risk is not uniform across populations and could increase in some cases following density reduction, due to increases in home overlap and consequently contact rates. Therefore, control should continue to include low-density possum populations that are infected with bTB or are in vector risk areas. Furthermore, efficient density reductions of bTB infected populations should be carried out that reduce populations to very low densities across the entirety of sites (i.e., avoiding patchiness in control effort), to prevent potential increases in bTB transmission.