Estimating the extent of dispersal post-habitat fragmentation required to maintain/restore genetic connectivity and genetic diversity; are fauna crossing structures sufficient.
Thus far, we have shown that the immediate and modelled long-term genetic consequences of habitat fragmentation could carry a significant cost to the impacted koala population: the observed decrease in effective population size, higher risk of inbreeding and increased genetic drift threaten the long‐term persistence of the impacted koala population. The implementation of mitigation measures to minimize impact on koalas and maintain/restore habitat connectivity was integral to the design of the rail line transport infrastructure project. These included, but were not limited to, the installation of fences and culverts (n = 18) along the 12.6 kilometres of rail line construction, to allow for dispersal across the fenced rail line (Hanger et al.2017). In addition, targeted abatement of measured threats (chlamydial disease and wild dog predation) significantly reduced mortality and improved reproductive rates over the duration of the intensive management program (Beyer et al 2018). However, research has shown that landscape fragmentation often can reduce individual movement (Cote et al. 2017) and direct estimates of dispersal rate across linear transport infrastructure do not necessarily translate into genetic connectivity (Riley et al. 2006). This is because animals which cross must be successfully mating to translate into gene flow (Sawaya et al. 2014; Soanes et al. 2018).
Our results suggest that the mitigation measures put in place during construction have paid dividends. Even though koalas reduced their number of crossing events by more than 79% post-construction (Dexteret al. 2017), more than 18 koalas continued to cross the linear transport infrastructure more than 18 months following the establishment of the linear transport infrastructure fence as a barrier (Dexteret al. 2017). Based on our forward dispersal simulations, this amount of dispersal should be sufficient to maintain gene connectivity close to zero. This obviously relies on the assumption that koalas who do cross, are of an age to reproduce and that 30% of males and 90% of females who do disperse reproduce and do so successfully (Riley et al. 2006). The story is, however, quite different when it comes to maintaining genetic diversity, in particular for the population of koalas located above the linear transport infrastructure because of its small population size. In this case, a more cautionary number of 16 dispersing koalas per side per generation (32 koalas in total) would ensure that genetic connectivity and diversity would each remain unchanged. While this number may be much higher than the 18 koalas reported to have continued crossing the linear transport infrastructure 18 months post-construction, the reported increase in higher koala use of culverts toward the end of the construction (Dexter et al 2017) suggest that the total number of crossing koalas is likely to increase over the next 4.5 years which may mitigate against any additional loss of genetic diversity for the koala population located above the linear transport infrastructure.