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).