Genomic insights into fragmentation and translocation in European green toads

The European green toad (Bufotes viridis) is Sweden's most threatened amphibian. Its range has contracted over the past century, with many local extinctions; remaining populations are fragmented and often isolated. Since the 1990s, conservation has focused on translocations to existing breeding sites and new localities, but many efforts have had limited success. We detected lower genetic diversity in Scandinavian populations (southern Sweden and nearby Denmark) than in Poland, plus strong structure and differentiation among Scandinavian subpopulations, implying unexpectedly low gene flow despite translocations. Small, isolated populations are strongly affected by drift, and whole-genome analyses reveal inbreeding and high genetic load in some subpopulations. We recommend reassessing source populations for translocations: the stock used in captive breeding and most past releases shows intermediate diversity but also signs of divergent selection and putative local adaptation. Management should balance minimizing inbreeding depression against risks of outbreeding depression and erosion of local adaptation risks.