Longer-term genetic consequences of linear transport
infrastructure.
In addition to the immediate genetic consequences of habitat
fragmentation by linear transport infrastructure on our koala
population, we further show how the progressive decrease in population
size through (1) existing ‘natural’ threatening processes and (2) the
additional translocations and population subdivision resulting from the
rail line construction project (above and below) could influence
patterns of genetic erosion over time (Figure 1). While treating the two
subdivided populations as isolated ‘islands’ (no gene flow) is modelling
the worst-case, and most unlikely, scenario, the results nonetheless
emphasized the necessity to ensure genetic connectivity of these
subdivided populations given the predicted rate of genetic loss over the
next generations, in particular for the koala population which now
resides above of the linear transport infrastructure.
For instance, in the unlikely event that the linear transport
infrastructure presented a complete barrier to dispersal, our modelling
showed that after only 5 generations (~30 years)
post-construction, the levels of genetic diversity could decline by as
much as 49.6% (above, n = 27) to as little as 6.65% (below, n = 75).
This genetic loss then increases to 74.16% (above) and 13.3% (below)
after 10 generations (~60 years). Such predicted genetic
erosion trends would be significant for population resilience and
long-term persistence (Hohenloheet al. 2021). Firstly, the predicted magnitude of predicted
genetic diversity loss was extensive and far exceeded the recently
estimated 5.4-6.5% overall vertebrate decline in genetic diversity
since the industrial revolution (Leigh et al. 2018). Secondly, a
decrease in genetic diversity in koalas is known to carry negative
fitness consequences including body condition, reproductive success and
sperm quality (Schultz 2019). Lastly, research has shown that genetic
erosion, combined with small population size, are significant
contributors to population extinction risk (Templeton et al.2001; Hohenlohe et al. 2021). While we acknowledge that
simulations make simplified evolutionary dynamics assumptions (Hindersinet al. 2019) (e.g. models finite populations, etc), our results,
nonetheless, have important conservation implications for mitigation of
genetic impacts on future linear infrastructure development,
particularly with respect to longer-term impacts.