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.