Application of underpasses to expand nature reserves: responses of a critically endangered marsupial, the woylie, Bettongia penicillata

Despite the conservation benefits that it yields, fencing for conservation presents management challenges. One major problem is that populations in fenced reserves can increase beyond the carrying capacity of the area. This was a concern for a population of woylies, Bettongia penicillata ogilbyi at Whiteman Park's fenced reserve in Western Australia. Two underpasses were constructed linking the original reserve to a larger, newly established fenced reserve to provide the resident woylies with opportunities for expansion. Underpasses were monitored with microchip readers and infrared cameras. Woylies were also tracked using GPS technology to determine if they would use the underpasses to disperse into the new area and if, in doing so, there would be a decrease in population density and associated expansion in home range size of woylies in the original reserve. The use of underpasses by woylies was clearly demonstrated with 1657 crossings by at least 51 individuals. Contrary to expectations most woylies used the underpasses to move between the two reserves, rather than permanently dispersing into the new area. Although there was an apparent decrease in population density from 3.4 +/- 0.8/ha (S.E.) to 1.36 +/- 0.08/ha (S.E.), only the core home range of males increased by 38% after the underpasses were opened. However, woylies using the underpass did shift their home ranges to incorporate the underpasses and parts of the second reserve. Findings from this study demonstrate that the use of underpasses to connect reserves separated by roads or other barriers is an effective method to manage populations limited in their expansion by natural or anthropogenic barriers.