Global origins of invasive brown rats (Rattus norvegicus) in the Haida Gwaii archipelago.
Brown rats (Rattus norvegicus) have commensally spread from northern China and Mongolia to become among the most invasive species on the planet. Understanding the proximate source(s) of invasion can inform biosecurity plans and eradication strategies for preventing or mitigating impacts to native biodiversity. The Haida Gwaii archipelago, located off the coast of British Columbia, Canada, is a significant nesting site for 1.5 million seabirds across 12 species, half of which are now threatened by brown rats. Local knowledge points to a European origin in the late 1800's to early 1900's, though the true source(s) and firm date(s) of invasion remain unknown. To fill these knowledge gaps, we analyzed genotypic data (16,598 SNPs) for 280 brown rats sampled throughout Haida Gwaii relative to a published global database of potential source populations. Principle component analysis and population assignment tests supported multiple potential invasion sources from Europe and North America. Likewise, demographic modelling best supported two invasions into the islands. The first invasion likely occurred in the early 1900's into the south-central archipelago from Western Europe followed by a more recent invasion in the early 2000's from Vancouver, British Columbia, into northern Haida Gwaii. The northern invasion of Haida Gwaii could also be indicative of contemporary gene flow between Haida Gwaii and the mainland, representing a significant biosecurity risk. Our results will inform management strategies for invasive rats in Haida Gwaii and serve as a guide for studies in other isolated systems worldwide.