Biogeographic consequences of shifting climate for the western massasauga (Sistrurus tergeminus)

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.