RESOURCE ESTIMATOR

This study was un-replicated and did not include an experimental control that was monitored concurrently at a site of the same habitat and population density. Therefore, it is possible that the observed spatial perturbation at the high-density manipulated site was not due to density reduction, but some other factor, such as breeding. To more definitively test whether density is a driver of space use in possums, further research should be conducted that includes replication in sites with similar population densities and habitat, and experimental controls. Monitoring should also be long- term, to determine whether spatial perturbation is maintained, or is temporary in nature, and to investigate seasonal or annual variation in responses.

This research could also be extended to investigate the mechanisms of spatial perturbation in possums and other species. As discussed in Chapter 3, there are likely to be thresholds for territoriality. When productivity is low, the cost of defending a territory outweighs the benefits, and when productivity is high, there is enough of the resource that territoriality is not necessary (Carpenter and Macmillen, 1976). It is hypothesised that there is also a threshold for the occurrence of spatial perturbation due to density reduction, related to the scale of density reduction and the starting density of the population. This might explain why density reduction at the second low- density site did not result in spatial perturbation. Furthermore, it is hypothesised that varying levels of density reduction will result in differences in the magnitude of perturbation; this relationship is expected to be non-linear due to the predicted threshold of perturbation. For example, increases in home range size may occur to a greater degree, the greater the level of density reduction. Although studies on badgers have investigated perturbation in populations subjected to various levels of culling (e.g., Woodroffe et al., 2006a), there does not appear to be any published study in any species that has specifically quantified the magnitude of spatial perturbation based on the level of density reduction. Furthermore, research that empirically tests the predictions of the Perturbation Model (Appendix A) would also provide more information on the mechanisms of spatial perturbation and the predictive value of this model. This could be achieved by observing how populations of a range of known distribution patterns respond to various types of disturbance events, including density reduction. The difficulty in this would be that prior research would be required to determine the baseline distribution patterns of the populations being investigated.

Manipulating densities whilst resources levels remain constant is one way to test if density is a driver of home range characteristics and not food availability. Conversely, it is also possible to manipulate food supply whilst keeping density constant, to test if food availability is a driver (as discussed in Chapter 3). Incorporation of these experimental food supplementation studies into density manipulation research would help tease out the drivers of home range characteristics in species. In possums specifically, it is possible that both density and food availability drive home range characteristics (see Chapter 3). Populations that are naturally low in density (i.e., they are not low in

density due to control) are predicted to be due to resource availability in these areas being low. Therefore, it is hypothesised that food supplementation of these populations would result in individuals contracting their home range sizes, due to their energy demands then being met. In contrast, high-density populations are predicted to be due to high levels of resource availability in these areas. It is therefore hypothesised that the home ranges of these possums will not change in response to increases in food availability, due to their energy demands already being met.

Home range overlap is often used to infer direct and indirect contact (through environmental contamination from bTB excretory products, such as urine and faeces) between individuals and therefore disease transmission risk (Hamede et al., 2009), as was the case in this study. However, directly recording contact rates between individuals through the use of proximity collars may be a more robust way of measuring transmission risk (Bohm et al., 2009). Proximity collar studies have been undertaken in a number of species that transmit diseases, such as the Tasmanian devil (Sarcophilus harrisii), which are affected by devil facial tumour disease (Hamede et al., 2009). Other studies have also used proximity collars to determine contact rates amongst badgers, and between badgers and livestock, to quantify bTB transmission risk (Bohm et al., 2009; Drewe et al., 2013). It is important to understand whether density reduction results in increases in contact rates and therefore transmission risk, to confirm that the appropriate control strategies are being used. Furthermore, an understanding of changes in contact rates due to density reduction is important for models that predict disease transmission; for example, the possum-bTB model in New Zealand is sensitive to contact rates, for which there are few data (Ramsey et al., 2002). A study by Ramsey et al. (2002) has investigated changes in contact rates following density reduction in possums, but only using VHF radio-tracking techniques that could not definitively determine whether individuals were close enough to make direct contact (no change in contact rates was recorded). Another study has also investigated contact rates between possums using proximity loggers, at three sites of varying density and habitat (Ji et al., 2005). This study recorded no difference in the frequency or duration of contacts between sites, suggesting that contact rates are not density-dependent (Ji et al., 2005). However, these results may have been confounded by differences in behaviour between sites, due to variations in habitat. It appears that no study has definitively quantified changes in contact rates following density reduction in any species, including possums. Although it is noted that the relationship between contact behaviour and disease transmission is unknown (Ji et al., 2005), it would still be beneficial to undertake this research. It is hypothesised that contact rates of possums do not increase following density reduction, based on the lack of density-dependence in contact rates recorded in the Ji et al. (2005) study.

The observed variations in home range size between the low and high-density sites in this study were used to validate one of the parameters of the Animal Health Board Spatial Possum Model (Chapter 3). As a number of additional parameters in this model are estimated or based on limited

field data (Ramsey and Efford, 2010; Ramsey and Efford, 2005), it is possible that the results from this research could also be used to update other key parameters in this model. For example, the results of this study could be used to assess the current assumption that possum home ranges are randomly distributed in space, rather than aggregated or uniformly distributed. Additional research could also be undertaken to field validate other parameters in the model, such as contact rates and therefore transmission rates.

Photos of some of the possums studied during this research

Photo courtesy of Millie Coleing Photo courtesy of Millie Coleing

Appendix A:

Perturbation Model