Immediate genetic consequences of linear transport
infrastructure.
We know that anthropogenic activities create a patchwork of fragmented
habitats that threatens the long‐term persistence of populations (Crookset al. 2017; Toczydlowski & Waller 2019). We also know from
theoretical and empirical research (Keyghobadi et al. 2005;
Frankham et al. 2017; Kleinhans & Willows-Munro 2019) that
habitat fragmentation through the subdivision of populations can reduce
the effective population size of impacted populations subsequently
increasing the rate at which genetic variation is lost through drift.
For instance, a recent meta-analysis has shown that, when compared to
rural counterparts, mammalian populations in fragmented landscapes show
decreased effective population size and genetic diversity (Schmidtet al. 2020). By using a rare and extensive genetic dataset which
follows a large koala population while their habitat was being
fragmented by linear infrastructure, we show the intricacies of
estimating the immediate genetic consequences of such environmental
disturbances on species. In particular, we show how the choice of
metrics influenced our ability to detect immediate post-disturbance
changes in patterns of genetic diversity for both the impact of death
from natural processes and the impact of the linear transport
infrastructure. For instance, we did not find any significant changes in
Shannon’s information index (\({}^{1}D\)) and heterozygosity (Ho, He
and \({}^{2}D\)).
In
contrast, we identified the following immediate changes for each
disturbance separately: (1) a decrease in Ne, (2) a
decrease in allelic richness and (3) an increase in the percentage of
homozygous loci (Table 1). While decreases in allelic richness were
overall small (0.8% [natural processes] up to 2.7% [linear
infrastructure project]), research has shown that when such effects
accumulate over many generations, they can result in considerable loss
of genetic diversity (e.g.(Dures et al. 2019; Hohwieler et
al. 2022)). Moreover, the sharp decrease in effective population size
accompanied with a two-to-three-fold increase in the percentage of
homozygous loci are additional cause for concern, because they may
influence the accumulation of deleterious variants, inbreeding
depression and adaptability (Hohenlohe et al. 2021). Together,
our results underline how important the choice of metrics is to our
ability to detect signals of immediate genetic consequences of sharp
population declines caused either by ‘natural’ processes or habitat
fragmentation by linear transport infrastructure (Lopez et al.2009; Schlaepfer et al. 2018).