COS 133-4
The contracting southern range boundary of a northern-adapted mammal tracks shifting forces of anthropogenic disturbance
Habitat loss and climate change are now dominate drivers of changes in global biodiversity. However, which of these forces is more influential in structuring species distributions is not well known for many species. Range boundary populations offer a unique opportunity to study the effects of both these stressors, as they are highly sensitive to climatic constraints and habitat is often patchily distributed. Snowshoe hares (Lepus americanus) reach their southern range limit in the northern Great Lakes Region, historically coinciding with a vegetative tension zone. Although detailed historical data exist on the location of this range boundary, the environmental drivers shaping the distributional limit and how they have changed over time are not well quantified. We revisited 148 historical survey sites for the species (circa 1980) at the southern edge of their distribution in the state of Wisconsin. We analyzed these data with a dynamic occupancy model to test the hypothesis that this species has experienced a two-phase range shift, first driven by rapid land use change post European settlement, and later by changing winter snow conditions.
Results/Conclusions
We detected snowshoe hares at 29 historical sites, 22 additional sites, and observed an average shift north of 29.5 km since 1980. We found that historical snowshoe hare presence was best explained by the distribution of forest cover (β = 1.41), while snow cover duration was less important (β = 1.14). Conversely, patterns of extinction between time periods were primarily driven by snow cover duration (β = -1.41), while the importance of forest cover declined (β = -0.84). Models containing contemporary snow cover as an extinction covariate predicted current occupancy better (AUC=0.78) than models with the change in snow cover between time periods (AUC=0.72), indicating the modern range boundary has a novel, and reduced snow regime.
The distribution of snowshoe hares has tracked the shifting paradigm of conservation research: the historical range boundary was primarily constrained by habitat when habitat loss was the most recognized biodiversity threat by conservation ecologists. More recently, as climate change has become a major research focus in ecology, the range of snowshoe hares has become increasingly limited by snow cover. Projections of their future range show that snow cover loss will likely continue to drive this species’ northward range shift into the future.