DOI: 10.5061/dryad.m37pvmd3t
title: Biogeographic consequences of shifting climate for the western massasauga (Sistrurus tergeminus)

description: 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.
description: 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
 <i>S. tergeminus</i> 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
 “<i>Sistrurus</i> <i>catenatus</i>”, iNaturalist
 using “<i>Sistrurus</i> <i>tergeminus</i>”, and
 the PBDB using “<i>Sistrurus</i>”. We
 removed records east of the Mississippi River to remove <i>S.
 catenatus</i>, <i>sensu stricto</i>. 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 <i>S.
 tergeminus.</i>