Environmental temperature gradients contribute to range limits for high-elevation butterflies

Preserving biodiversity in naturally or anthropogenically fragmented habitats, especially those sensitive to climate change, depends on a thorough understanding of how organisms are distributed in space and time. Recent studies suggest integrating historical biogeography and ecology will  improve our understanding of distribution patterns. We are studying two closely related butterfly species, the pine butterfly, Neophasia menapia, and the Mexican pine butterfly, N. terlooii (Lepidoptera: Pieridae), because both are restricted to naturally fragmented pine forests and, despite having large latitudinal ranges and similar life histories, are geographically isolated by only 120 km. Because freezing temperatures often limit the geographic ranges of insects, we sought to quantify the cold hardiness of each species during their winter diapause. We exposed field-collected eggs to steadily decreasing temperatures in a chill bath until they reached their supercooling points. Additional eggs were exposed to sub-supercooling temperatures for varying amounts of time to determine their lower lethal temperatures. Neophasia menapia eggs (n=23) supercooled at -29.00 ± 0.60°C, and N. terlooii eggs (n=18) at -21.79 ± 0.65°C. Preliminary lower lethal temperatures appear to reflect these differences, with fewer N. terlooii surviving at lower temperatures. Colder winters in northern Arizona likely contribute to the northern range limit for N. terlooii. When considering the historical biogeography of Neophasia, future movements of range boundaries could reflect an impact of climate change on the forest ecosystem. We hope the long-term outcome of this research and other studies involving Neophasia will demonstrate the utility of integrative biogeography to applied conservation science.