Evidence for source - sink dynamics in a regional population of arctic ground squirrels (Urocitellus parryii plesius)

Abstract

Context Variable demographic rates can manifest themselves between habitat types in the form of source–sink dynamics where populations in sink habitats would not exist without the addition of migrants from source habitats. Aims Arctic ground squirrels (Urocitellus parryii pleisus (Osgood, 1900)) occupy a large geographic area in northern Canada and live in a variety of habitat types, including boreal forest, low-elevation meadows and alpine meadows, providing an opportunity to investigate the possible existence of source–sink dynamics. Methods We hypothesised that arctic ground squirrels in the south-western Yukon exhibit demographic characteristics indicative of source–sink dynamics. Boreal forest habitat could be a sink in spite of previous high squirrel densities, whereas meadows could be a source. We investigated this by mark–recapture live-trapping and radio-telemetry. Key Results In the boreal forest in the Kluane region, we found reduced recruitment, reduced population growth rates (λ), and reduced survivorship for radio-collared individuals that moved from low-elevation meadows into the boreal forest. There was no evidence from radio-collared juveniles of dispersal from high-density ground squirrel populations in alpine meadows down into boreal forest. Conclusions Boreal forest is a sink habitat for arctic ground squirrels. Source–sink dynamics observed between low-elevation meadow and boreal forest habitats appear to result from increased predation pressure in the boreal forest. The result has been a near extirpation of boreal forest arctic ground squirrels in the Kluane region since 1998. Implications Because the source areas of low-elevation meadows occupy only 7–9% of the lowland habitat, recolonisation of boreal forest sites has been very slow. Whereas alpine populations remain high in 2011, boreal forest populations remain near zero. Alpine populations do not appear to be a source for the boreal forest.