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There is now unequivocal evidence for global climate change; however, its potential impacts on evolutionary processes remain unclear. Many species have responded to contemporary climate change through shifts in their geographic range. This could lead to increased sympatry between recently diverged species; likely increasing the potential for hybridization. Recently, following a series of warm winters, southern flying squirrels (Glaucomys volans) in Ontario, Canada rapidly expanded their northern range limit resulting in increased sympatry with the closely related northern flying squirrel (Glaucomys sabrinus). This provided the opportunity to test the prediction that contemporary climate change can act as a catalyst creating conditions for the formation of hybrid zones. Following extensive sampling and molecular analyses (nuclear and mitochondrial DNA), we identified the occurrence of hybridization between sympatric G. sabrinus and G. volans. There was evidence of backcrossing but not of extensive introgession, consistent with the hypothesis of recent rather than historic hybridization. To our knowledge, this is the first report of hybrid zone formation following a range expansion induced by contemporary climate change. This is also the first report of hybridization between North American flying squirrel species.

We undertook a large-scale survey of the distribution of northern, Glaucomys sabrinus (Shaw, 1801), and southern, Glaucomys volans (L., 1758), flying squirrels in Ontario, Canada. Livetrapping was conducted along a northsouth transect spanning about 500 km, from 42.5°N to 47.2°N. During 2002-2004, we conducted 42 971 trap-nights at 26 sites and captured 232 northern and 538 southern flying squirrels. During 2002 and 2003, southern flying squirrels occurred >200 km farther north than we expected. However, the range of this species appeared to contract to the south by about 240 km after the winter of 2004. Weather and food data suggested that cold temperatures during January and February 2004 combined with a failed mast crop in the autumn of 2003 resulted in an energetic bottleneck and subsequent population crash. We speculate that prior to 2004 southern flying squirrels had expanded their geographic range in response to recent climate warming. In particular, the nine winters between 1994 and 2004 were relatively warm. By measuring the range expansion over this warm interval, we were able to estimate a rate of spread of 22 km per year, and a diffusion coefficient of 6.9 × 10 7 m 2 per generation.

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Climate is an important factor limiting species distributions. Historic climate-change related range movements have modified the genetic diversity of species by the merging and splitting of gene pools and by the effects associated with recurrent founder events. These effects are often inferred, either from retrospective analyses of current genetic patterns or from simulations. Rarely has it been possible for the population genetic effects of range expansion to be examined with contemporaneous demographic data. We characterized the genetic signature of rapid range expansion by southern flying squirrels (Glaucomys volans) and compared these results to a stationary population of the closely related northern flying squirrel (Glaucomys sabrinus) in Ontario, Canada. Samples were taken during an approximately 200 km range expansion by G. volans (1994-2003) and genotyped at 6 (G. sabrinus) and 8 (G. volans) microsatellite loci. For G. volans, but not G. sabrinus, we found evidence of a latitudinal gradient in allele frequencies and a decrease in allelic richness along the axis of expansion. We found no evidence of isolation-by-distance in either species or of genetic bottlenecks in the area of G. volans expansion. These results suggest that serial founder events can cause an immediate reduction in genetic diversity following rapid range expansion with high levels of gene flow giving rise to heterogeneity within what would classically be termed panmixia. Given the pace of anthropogenic climate change, and the increasing incidence of range movements in response, this may be an important, immediate consequence of climate change.

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Although partial forest harvesting is practiced over large areas, managers know little about its impacts on sciurid rodents, particularly on northern (Glaucomys sabrinus) and southern flying squirrels (G. volans) in the northeastern United States and Canada. We examined habitat relationships of sciurid rodents (northern flying squirrels, southern flying squirrels, red squirrels [Tamiasciurus hudsonicus], and eastern chipmunks [Tamias striatus]) at 2 spatial scales in managed and unmanaged coniferous and hardwood forests of Algonquin Provincial Park, Ontario, Canada. We live‐trapped rodents in 26 northern hardwood stands and in 16 white pine (Pinus strobus) stands from 2002 to 2004. Northern flying squirrel and red squirrel densities were significantly lower in recently harvested (3–10 yr since harvest) shelterwood stands than in unmanaged stands. In contrast, southern flying squirrel densities were higher in selection‐harvested stands than in old‐forest areas. The densities of northern flying squirrels and red squirrels had a strong relationship with the density of large spruce (Picea sp.) and hardwood trees and snags in conifer sites. Southern flying squirrel numbers had a positive association with the density of mast trees at the landscape level but not at the stand level in hardwood forests. Eastern chipmunk density had a positive correlation with the volume of old downed woody debris and the stems per hectare of declining trees. We recommend forest managers retain more large spruce and hardwood trees to mitigate the impacts of shelterwood harvesting on northern flying squirrels and red squirrels, and that they maintain high mast availability at the landscape level to ensure the persistence of southern flying squirrels.

No abstract