To predict changes in species' distributions due to climate change we must understand populations at the poleward edge of species' ranges. Ecologists generally expect range shifts under climate change caused by the expansion of edge populations as peripheral conditions increasingly resemble the range core. We tested whether peripheral populations of two contrasting butterflies, a small-bodied specialist (Erynnis propertius) and a large-bodied generalist (Papilio zelicaon), respond favorably to warmer conditions. Performance of populations related to climate was evaluated in seven peripheral populations spanning 1.2° latitude (160 km) using: (1) population density surveys, an indirect measure of site suitability; and (2) organismal fitness in translocation experiments. There was evidence that population density increased with temperature for P. zelicaon whose population density declined with latitude in 1 of 3 sample years. On the other hand, E. propertius showed a positive relationship of population density with latitude, apparently unrelated to climate or measured habitat variables. Translocation experiments showed increased larval production at increased temperatures for both species, and in P. zelicaon, larval production also increased under drier conditions. These findings suggest that both species may increase at their range edge with warming but the preference for core-like conditions may be stronger in P. zelicaon. Further, populations of E. propertius at the range boundary may be large enough to act as sources of colonists for range expansions, but range expansion in this species may be prevented by a lack of available host plants further north. In total, the species appear to respond differently to climate and other factors that vary latitudinally, factors that will likely affect poleward expansion.
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Acknowledgements This work was supported by the OYce of Science (BER), US Department of Energy, grant no. DE-FG02-05ER-64023. It also was funded by the Endangered Species Recovery Fund of World Wildlife Canada and Environment Canada, and by the University of British Columbia. Thank you to J. Myers, GOERT, J. Heron, and R. Bennett for consultation and to the following for site access: Department of National Defence (A. Robinson); Government House (F. Spencer); CRD Parks (T. Fleming and M. Simpson); BC Parks (D. Closson and W. Woodhouse); Nature Conservancy of Canada (T. En-nis); and the High Salal Strata Corporation (M. Rabena). The following people assisted on this project: D. Beauchamp, G. Chavez, G. Crut-singer, J. V. Hellmann, L. LaTarte, T. Marsico, K. McKendry, and N. Vargas. The following provided valuable comments on the manuscript: R. Bennett, P. Ehrlich, T. Marsico, T. Ricketts, E. Zakharov, and two anonymous reviewers.
- Climate change
- Erynnis propertius
- Geographic range
- Larval production