Modelling seasonal habitat suitability and connectivity for feral pigs in northern Australia: towards risk-based management of infectious animal diseases with wildlife hosts

Infectious animal diseases are a major biosecurity threat in an increasingly connected world. Wildlife hosts are a well-recognized risk factor for disease introduction, establishment and spread. Northern Australia is vulnerable to disease incursions from neighbouring countries, and widespread invasive feral pigs (Sus scrofa) can seriously complicate post- order disease management. The aim of this thesis was to generate new regional-scale spatial knowledge of feral pig populations in northern Australia to inform risk-based management of directly transmitted infectious animal diseases for which feral pigs are a host.Due to environmental variability and empirical knowledge gaps across this vast region, I adopted a resource-based modelling approach, based on expert knowledge but rooted in landscape ecological theory, to answer three research questions at multiple levels of biological organisation. Specifically, I conceptualized feral pigs in northern Australia as a metapopulation and the landscape as displaying a patch-corridor-matrix structure. At the level of individual feral pig breeding herds, I explored the selection of supplementary and complementary resources within home ranges. At the level of local subpopulations, consisting of several herds with adjacent or overlapping home ranges, I used a habitat suitability modelling approach to investigate the distribution of potential patches of breeding habitat emerging from the interactions between resources and home range movements. At the metapopulation level, I examined potential dispersal pathways between many such patches using a habitat connectivity modelling approach. As feral pig movements and distributional patterns vary with conditions, I applied models to two seasonal scenarios (wet and dry) corresponding to northern Australia’s annual rainfall cycle.This thesis contributed methodological advances and new ecological insights. I developed a novel combined methodology, spatial pattern suitability analysis, for capturing feral pigs’ resourceseeking home range movements based on expert-elicited response-to-pattern curves and spatial moving window analysis. Based on landscape ecological principles, this methodology improves the application of resource-based Bayesian networks models to mobile animals. I found that habitat suitability for persistent feral pig breeding in northern Australia is dependent on spatial interactions between four key habitat requirements: water and food resources as well as protection from heat and from disturbance. Through scenario analysis and empirical validation I showed that habitat suitability at the regional scale is most reliably modelled as a function of distance to supplementary and complementary resource patches. When applied to a wet season and a dry season scenario, mapped model results indicated that the spatial distribution of feral pig habitat patches varied markedly. Importantly, empirically validated findings suggest that dry season conditions restrict overall habitat suitability for feral pig breeding in northern Australia more than previously thought. This thesis also provides the first attempt to describe seasonal habitat connectivity in the entire northern Australian feral pig population. By linking model assumptions to gender-specific differences in dispersal ability, I showed that dry season connectivity between habitat patches was limited for breeding herds, but less constrained for solitary males. Due to greater resource abundance, wet season connectivity was greater irrespective of dispersal ability. Three broad types of habitat patches were identified: some were always isolated; some were highly connected to form large habitat components; and some were mostly isolated but became connected to large components during the wet season or for wide- ranging males.By linking the landscape ecological research perspective on feral pigs to a landscape epidemiological perspective on directly transmitted infectious diseases, risk areas for the establishment, spread and persistence of disease in wildlife hosts could be identified. I used the example of classical swine fever to illustrate these links. Following introduction, successful establishment of such a disease is contingent on locally dense host populations in patches of breeding habitat; subsequent disease spread requires host dispersal between infected and susceptible subpopulations; and long-term disease persistence depends on a lasting supply of susceptible individuals within a regionally connected host metapopulation. Effective post-border disease management should capitalize on these links. For example, early detection surveillance activities could be targeted in habitat patches and designed so that each connected habitat component is sampled. In the event of an incursion, patch connectivity may be used to establish containment zones, focus population control and support delineation of disease-free compartments. Moreover, host risk could be combined with disease-specific introduction pathways, transmission rates and other parameters to generate deeper, dynamic insights into disease-host interactions for better incursion preparedness.In conclusion, the research contained in this thesis provides, for the first time, a complete and coherent, spatially-explicit, seasonally-specific and regional-scale picture of areas most at risk of disease establishment (via host habitat suitability) and spread (via host habitat connectivity) in feral pigs in northern Australia. The resource-based modelling approach is transparent and flexible, and could be applied to other invasive species and wildlife hosts of infectious animal diseases, especially in data-constrained situations and for wide-ranging species.