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UBC Theses and Dissertations

Invasive rat colonization history and movement dynamics in the Haida Gwaii archipelago Sjodin, Bryson


Brown rats (Rattus norvegicus) and black rats (R. rattus) are among the most invasive animals on the planet with global distributions. These species are found on >80% of all oceanic islands with devastating impacts on native fauna. Nesting seabirds are particularly affected by rat invasions, as they have evolved ground-nesting strategies in the absence of a terrestrial predator. It is estimated that 60% of all seabird extinctions are due to invasive rats. On the archipelago of Haida Gwaii, BC, invasive rats are having strong detrimental effects on native seabird populations, causing several extirpations of large breeding colonies and population declines in six of the twelve seabird species present. To promote seabird recovery, Parks Canada began initiating whole-island rat eradications in 1997. Since then, there have been seven successful eradications; however, brown rats have reappeared on the Bischof Islands after two eradication attempts. To better inform future management, we investigated population history and movement dynamics of invasive rats in Haida Gwaii. 606 rats were sampled from 2008-2018, including pre- and post-eradication samples from the Bischof Islands. We used double-digest restriction-site associated DNA sequencing to genotype individuals at approximately 28 000 single nucleotide polymorphisms identified in each species. We used population genetic and spatially-explicit analyses to determine the source of re-established populations and to quantify the extent and direction of dispersal and infer levels of gene flow throughout the system. We showed that populations were largely structured based on island sampled, and we identified groups of islands that should be eradicated simultaneously to prevent reinvasion. We determined that the source of brown rats on the Bischofs was due to reinvasion from neighbouring Lyell Island and not from bait failure, and that recent invasions onto Faraday and Murchison Islands (with subsequent introduction to Hotspring Island) were also from Lyell Island. Using a previously compiled database, we identified a likely western European origin for brown rats in Haida Gwaii consistent with anecdotal evidence. These results will help facilitate future eradications and provide useful insights to prevent the further spread within the system.

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