Immediate and long-term genetic consequences of linear transport infrastructure: Can fauna crossing mitigate its cost?

The genetic consequences of the subdivision of populations are regarded as
significant to long-term evolution, and research has shown that the scale
and speed at which this is now occurring is critically reducing the
adaptive potential of most species which inhabit human-impacted
landscapes. Here, we provide a rare, and to our knowledge, the first
analysis of this process while it is happening and demonstrate a method of
evaluating the effect of mitigation measures such as fauna crossings. We
did this by using an extensive genetic dataset collected from a koala
population which was intensely monitored during the construction of linear
transport infrastructure which resulted in the subdivision of their
population. First, we found that both allelic richness and effective
population size decreased through the process of population subdivision.
Second, we predicted the extent to which genetic drift could impact
genetic diversity over time and showed that after only 10 generations the
resulting two subdivided populations could experience between 12–69% loss
in genetic diversity. Lastly, using forward simulations we estimated that
a minimum of 8 koalas would need to disperse from each side of the
subdivision per generation to maintain genetic connectivity close to zero
but that 16 koalas would ensure that both genetic connectivity and
diversity remained unchanged. These results have important consequences
for the genetic management of species in human-impacted landscapes by
showing which genetic metrics are best to identify immediate loss in
genetic diversity and how to evaluate the effectiveness of any mitigation
measures.