Climate change and species' range boundaries: A case study at the northern range limits of Atalopedes campestris (Lepidoptera: Hesperiidae), the sachem skipper

Climate change can have large impacts on the distribution and abundance of species: species are expected to shift to higher latitudes and elevations in response to warming. The goal of this study is to explore the mechanisms by which temperature affects the probability of persistence at a species' northern range boundary during range change. Atalopedes campestris (Lepidoptera), the sachem skipper butterfly, has recently colonized Oregon (1967) and Washington (1991) from California, and is continuing to expand in eastern Washington. Over the past 50 yrs, minimum temperature has risen 2–3°C. I use observation, laboratory experiments and field transplant experiments to establish the following: (1) Atalopedes' geographical distribution and overwinter survivorship is positively correlated with winter minimum temperature. (2) Summer population growth rate and overwinter mortality are both temperature sensitive and together define the range limit.; A census of relative population size in two populations near the range boundary over 2.5 years indicates that populations contracted more during colder winters. Summer population growth was greater in the peripheral population, despite cooler temperatures there. Atalopedes' geographical distribution is correlated with the −4°C January minimum isotherm. This isotherm is close to Atalopedes' supercooling point (−6.2°C), the minimum lethal temperature after 12 hr exposure (Lt ₅₀: −5.8°C), and the temperature at which daily mortality rate rises rapidly in chamber simulations of typical January conditions (daily max/min = 4/−4°C). Field transplant experiments confirm that overwinter mortality was significantly higher in the more peripheral population. Summer transplant experiments demonstrate that while fecundity and survivorship were not significantly different, larval development time was significantly slower outside the current range; the delay is sufficient to reduce expected voltinism from 3 to 2 generations/yr, which would have a large impact on summer population growth rate. In sum, these experiments suggest that winter mortality is the primary factor limiting the current range. However, the projected rate of ongoing expansion depends on both summer and winter temperature, and exceeds the expected rate of shift due to a change in mean temperature alone.