The Effectiveness of Wildlife Corridors in Facilitating Connectivity: Assessment of a Model System from the Australian Wet Tropics

Wildlife corridors have become a widely adopted management strategy for the conservation of species in fragmented habitats. Fragmentation reduces the size of habitat patches and increases the isolation of the populations within them, potentially resulting in extinction due to stochastic processes. The provision of a corridor between habitat patches is believed to increase the level of connectivity through the integration of populations into a single demographic unit, thus increasing the probability of survival. This assumption remains largely untested due to both a lack of investigation, and design limitations in some of the few studies performed. Connectivity is often assumed to occur simply from the presence of individuals within the corridor. Design criteria essential for the rigorous assessment of connectivity were identified and a landscape meeting these criteria selected. The vegetation within the corridor was found to be comparable in both structure and species composition to that of the patches that it connected. Two target species (Melomys cervinipes and Uromys caudimaculatus) were shown to occur along the corridor but not within the surrounding matrix. The combination of these factors indicated that the corridor was suitable for use as a model system and ensured that any subsequent results truly reflected the capacity of the corridor to function in the desired manner. The population structure was similar within the corridor and the connected patches for both species. Weights of individuals, sex ratios and the percentage of juveniles were consistent between the two system components, suggesting that the corridor contained breeding populations. Connectivity was therefore possible via generational gene flow for both species, while long distance movement events for U. caudimaculatus also indicated that direct movement between habitat patches may be possible for this larger species. Despite all ecological parameters indicating that connectivity was likely, genetic markers (mtDNA and nDNA) revealed significant population differentiation between the connected patches for both species. Populations linked by the corridor and those in isolated habitats were found to show the same level of genetic differentiation. Sampling at a finer spatial scale within connected patches and a continuous control habitat showed that population differentiation was common for M. cervinipes. Given the continuity of suitable vegetation, and the presence of individuals of breeding age along the corridor system, this was attributed to social structuring. U. caudimaculatus populations also showed evidence of genetic differentiation within a connected patch and along the corridor, despite panmixia within the continuous habitat. Having investigated a model system, the data from this study has implications for other wildlife corridor studies and for landscape managers. Firstly, the advantages of using an integrated ecological and genetic approach have been demonstrated. While genetic data determined the level of connectivity, the ecological data provided an understanding of the processes operating within the system. Secondly, the level of scale at which wildlife corridor studies are conducted may need addressing. Most studies currently treat a fragmented landscape in a binary manner and consider the connected patch to be the finest "grain". However, the processes responsible for the lack of connectivity were found to operate at the much finer within-patch scale. Finally, this study clearly indicated that not all wildlife corridors will provide connectivity between the connected populations and that connectivity cannot be inferred from the presence of individuals within the corridor. Given that social behaviour such as territorial defence and philopatry are common in many species, especially small mammals, a lack of connectivity via a wildlife corridor may be more common than currently assumed. The successful use of wildlife corridors as a management strategy, and the accurate assessment of their effectiveness therefore requires careful consideration of not only structural attributes of the corridor, but also behavioural, demographic and genetic parameters of the target species