The western massasauga (Sistrurus tergeminus) is a small pit viper with an extensive geographic range, yet observations of this species are relatively rare. They persist in patchy and isolated populations, threatened by habitat destruction and fragmentation, mortality from vehicle collisions, and deliberate extermination. Changing climates may pose an additional stressor on the survival of isolated populations. Here, we evaluate historic, modern, and future geographic projections of suitable climate for S. tergeminus to outline shifts in their potential geographic distribution and inform current and future management. We used maximum entropy modeling to build multiple models of the potential geographic distribution of S. tergeminus. We evaluated the influence of five key decisions made during the modeling process on the resulting geographic projections of the potential distribution, allowing us to identify areas of model robustness and uncertainty. We evaluated models with the area under the receiver operating curve and true skill statistic. We retained 16 models to project both in the past and future multiple general circulation models. At the last glacial maximum, the potential geographic distribution associated with S. tergeminus occurrences had a stronghold in the southern part of its current range and extended further south into Mexico, but by the mid-Holocene, its modeled potential distribution was similar to its present-day potential distribution. Under future model projections, the potential distribution of S. tergeminus moves north, with the strongest northward trends predicted under a climate scenario increase of 8.5 W/m². Some southern populations of S. tergeminus have likely already been extirpated and will continue to be threatened by shifting availability of suitable climate, as they are already under threat from desertification of grasslands. Land use and habitat loss at the northern edge of the species range are likely to make it challenging for this species to track suitable climates northward over time.

We collected occurrence data from online databases, direct contact with collections, literature searches, and targeted field survey efforts. We queried the Global Biodiversity Information Facility (GBIF, accessed 20 June 2019) using package “rgbif” , iNaturalist (accessed 13 June 2019, 16 November 2017, and 27 June 2017), and gathered specimen records from natural history collections using VertNet (accessed 20 June 2019) or through directly contacting collections with significant S. tergeminus holdings. These observations were distributed between 1903 and 2019, with the majority of the data being collected after the 1990s. Occurrences associated with fossils were found from the paleobiology database, PBDB (accessed 20 June 2019) and literature searches. Because of the differences in taxonomy updates across these different platforms, we queried GBIF using “Sistrurus catenatus”, iNaturalist using “Sistrurus tergeminus”, and the PBDB using “Sistrurus”.

We removed records east of the Mississippi River to remove S. catenatus, sensu stricto. We also removed duplicate records and any records with recorded location uncertainty greater than 1 km. Occurrences were plotted and compared with the species’ known geographic range; any questionable outliers determined by a subject matter expert (TJH) were removed. This left us with a data set of 999 occurrences spanning the range of S. tergeminus.