From critically endangered to a fragile conservation success: Population genomics and historical demography of Aeshna viridis (Odonata: Aeshnidae) in Denmark

Abstract The green hawker ( Aeshna viridis Eversmann), considered a species at risk in most of its Western Palearctic range, has experienced a significant range expansion in Denmark in recent decades after more than half a century of decline. This provides an excellent opportunity to apply conservation genomics methods to study the underlying genomic consequences of both a well‐documented decline and a subsequent strong increase. We therefore use the species as a model organism in the first whole genome sequencing conservation genomics study of an Odonata species based on 54 specimens sampled across the species' Danish range. The analyses of the historical trends and demography show a strong correlation with known climatic changes until the onset of the industrial revolution ca. 200 years ago followed by a strong decline in the second half of the 20th century. The recent recovery is not yet reflected in the genetic diversity of Danish A. viridis . The present‐day genomic diversity shows that Danish A. viridis are divided into a relict Zealand population and a Jutland population assemblage that has likely immigrated from continental Europe in several recent waves. The two population assemblages likely mix in eastern Jutland. The island of Bornholm represents another admixture zone between Zealand and continental Europe. A separate analysis of mitochondrial genomes shows that the observed results cannot be explained as an underlying phylogeographical pattern. Despite the recent influx, all sampled Danish populations show high to very high levels of inbreeding, raising further conservation concerns for the species in Denmark. However, improved habitat quality and connectivity may facilitate natural long‐distance dispersals that will improve the long‐term conservation prospects for A. viridis , not only in Denmark, but across its range.