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 longāterm 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.