Short-term impact of a newly built bypass highway and associated watercourse restoration on population genetic structure and recolonisation processes in the southern damselfly (Coenagrion mercuriale).

Highway construction often leads to habitat fragmentation, altering the genetic features of wild populations. While compensatory measures are commonly implemented to mitigate their negative demographic and genetic impacts, their effectiveness remains species-dependent and difficult to assess. We analysed the short-term impact of a newly built highway in eastern France and the associated restoration measures on population genetic structure in the protected southern damselfly (Coenagrion mercuriale). We genotyped 46 populations collected after the highway construction using microsatellite and SNP markers over two successive years to detect potential genetic discontinuities across the highway. We also evaluated the effectiveness of watercourse restoration and identified the likely origin of recolonising individuals. We found no significant barrier effect of the highway construction on gene flow. However, these encouraging results should be interpreted with caution, considering the likely time-lag in populations' responses to genetic drift effects. We observed a gradual recolonisation of restored watercourses that had undergone compensatory measures, suggesting positive effects of reproductive habitat recovery. Recolonisation involved nearby populations but also long-distance dispersal events occurring overland. Detected migrant individuals may come from geographically distant populations belonging to a different watercourse. This mirrors a migrant-pool model of colonisation that leaves high levels of genetic diversity and no evident spatial patterns in populations located close to the highway. Altogether, our study highlights both the potential and limitations of genetic tools to assess the impacts of linear infrastructure on gene flow and for understanding watercourse recolonisation processes after ecological management restoration.