We must identify potential constraints on geographic range shifts under climate change to conserve biodiversity and the functions and services that it provides. Many current models of species’ range change assume that species are essentially uniform across space with respect to climate-related fitness. These models therefore predict that poleward populations will increase as peripheral conditions become more like the central characteristics of a species’ distribution. This study examines this assumption of ecotypic uniformity with respect to temperature and host plant useage across the geographic ranges of two butterfly species that co-occur and have contrasting degrees of host specialization and dispersal ability. To test for differential responsiveness of populations within a species’ range, we performed a reciprocal translocation experiment of peripheral and core populations in the field and a fully-crossed experiment of regional source population, temperature, and host plant in controlled environmental chambers for both species.

Results/Conclusions

We did not find evidence for regional adaptation in either species, but we did find evidence for differing responsiveness to varying conditions in these two contrasting taxa. Specifically, we found that the performance of both core and peripheral populations of the dispersal-limited specialist species (Erynnis propertius) is consistently enhanced in the range core in both field and lab experiments.  In the more generalized vagile species (Papilio zelicaon), we found that the fitness of core and peripheral populations is enhanced by core-like temperatures, depending on the interaction of temperature and host plant species. Furthermore, performance in this species was significantly reduced at the range core in field studies due to substantial mortality occurring during a heat wave. In the specialist species, peripheral populations may benefit from warming but a lack of available host plants will preclude poleward range expansion. In the generalist species, host plant species may mediate the effect of changing conditions, and extreme heat events may reduce core populations. Thus, predictions of range expansion from the simple assumption of enhancement in peripheral populations under warming are inappropriate for many species.