Harnessing historical sampling to substantiate range shifts: southward movement of North American least weasels (Mustela nivalis)

Contemporary climate change is rapidly affecting species’ ranges and distributions. While there is a general trend of poleward movement, there are exceptions. For example, despite the North American least weasel’s ( Mustela nivalis ) elusive nature, mammalogists in the mid-1900s noted the potential southward extension of its range into the central US, opposite the general poleward paradigm. Historically, a record of a species in a new location was sufficient for documenting a range expansion; yet, such observations can be biased by the extent of regional sampling. To investigate this unusual southward range shift, we use historical specimen records to statistically assess the absence of least weasels in the central United States before the 1960s. We include records of similarly trapped mammals as a measure of sampling effort, to better distinguish between perceived absence and geographic sampling bias. We then use ecological niche modelling to measure the association between changing climate and least weasel range dynamics. Our results provide evidence of a significant expansion at the southern periphery of their range, consistent with the 1960s timeline of hypothesised dispersal in historical field notes. Comparison of historical and contemporary niche models shows a significant increase in suitability at the south-western leading edge during that same time frame. Our findings underscore the importance of natural history specimen records for understanding species’ responses to climate change and provide methods for a more robust validation of suspected changes in range dynamics, particularly for rare or elusive species. Species ranges and distributions are changing rapidly under contemporary environmental conditions, with a tendency to shift polewards. Observation of a species in a new location is insufficient evidence of range expansion and accounting for historical sampling provides a more robust test of changes in species distributions over time. We find evidence of a significant south-western range expansion by North American least weasels ( Mustela nivalis ), validating historical observations. We document a significant increase in suitable area for North American least weasels along the southern edge of their range, correlating with an increase in water availability. Consideration of historical surveys and collections can provide valuable leverage for understanding range dynamics and life histories of rare or elusive species.