Forest structure mediates occupancy and extinction of vertebrate prey species on the trailing edge of the boreal forest

Abstract

Climate change is expected to shift the distributions of global biomes. High latitude biomes, such as the boreal forest, are among the most vulnerable and serve as a bellwether for the effects of climate change on other ecosystems. We examined patterns of occupancy, local extinction, and abundance in a guild of forest-associated vertebrate prey species on the southern margin of the boreal forest in Minnesota (USA). We used multi-season, multi-state, and multi-species occupancy modeling approaches to determine the effects of land cover, forest structural characteristics, and climatic variation on two boreal-associated species, spruce grouse (Canachites canadensis) and snowshoe hare (Lepus americanus), and a forest generalist, ruffed grouse (Bonasa umbellus). Occupancy, abundance, and extinction of spruce grouse was strongly associated with landscape-scale characteristics. In particular, deciduous forest cover may govern their southern range limit. Broadly, forest understory structure played a significant role in occupancy, extinction, and abundance patterns of all three species, with ruffed and spruce grouse generally associated with vegetation density in the lower and mid-canopy layer (5.00–15.00 m) and snowshoe hare associated with density in the shrub layer (1.37–5.00 m) in winter. Co-occurrence varied annually but was greatest in mixed forests during an uncharacteristically warm and snow-sparse year. Climatic variables (winter temperature and snowfall) were associated with extinction probabilities for all three species, but the effect was generally weaker than that of forest structure. Our results suggest that forest management practices that promote dense understory structure may help provide climate refugia for vertebrate prey species in boreal forest ecosystems.