The main impact of climate change on organisms is commonly thought to be increases in thermal stress during the hottest summer months. However, when the geographic range of an organism is limited by cold tolerance, increases in winter temperatures may have a dramatic impact on a population's range. An excellent example of this phenomenon has recently been discovered by Lisa Crozier, who found that the sachem skipper butterfly, Atalopedes campestris, which formerly resided to the west of the Cascade mountains in the United States, has recently colonized a part of central Washington State, where the minimum winter temperature has risen from -7°C to-4°C since 1950. Crozier determined that the butterfly's range had expanded to match the -4°C isotherm, thus she wondered if -4°C represented a physiological threshold.
Crozier collected specimens from butterfly populations in Washington State and California. Back in the lab she held the butterflies under cyclical thermal and lighting conditions designed to mimic the natural habitat in central Washington, and developed a set of experiments to analyze cold tolerance of the butterfly at different stages of development.
First, Crozier measured the insect's supercooling point, which tells us the absolute minimum temperature that the insect could survive. By testing acute cold exposure on almost 200 insects, Crozier found that the supercooling point ranged from -17.3°C to -3.4°C with an overall mean of -6.8°C. However, the supercooling point of intermediate-stage caterpillars was significantly higher than larvae or adults, which is relevant, as the intermediate-stage caterpillars are the ones present when the temperature begins to drop in the Fall.
Next she determined the lethal temperature, at which 50% of the insects died, by slowly cooling them to sub-zero temperatures, holding for 12h and then slowly rewarming to room temperature. The lethal temperature ranged from-5°C to -6°C for third instar caterpillars, and the overall mortality ranged from 5% at -4°C to 95% at -7°C!
Crozier then acclimated the butterflies to temperatures simulating several winter conditions, similar to those experienced in their natural environment. She found that the butterflies were able to survive the cold until the daily cycle's low temperature reached -4°C or below, at which point the death rate rose sharply. These results confirm that the butterflies are unable to acclimatize to temperatures below -4°C, the temperature of the isotherm that defines their range expansion.
Because butterflies in areas of higher elevation in the Cascade mountain range live in winter conditions where the temperatures are relatively stable at 0°C, Crozier also included acclimation to constant 0°C in her winter simulations. The butterflies had survivorship levels that were identical to those of specimens held under 4°C to -4°C daily cycles,suggesting that the mean temperature, and not the extreme lows of the treatment, may be responsible for the observed survivorship.
In summary, all of these results suggest that the butterflies are unable to survive habitat temperatures that are cooler than -4°C, and that 20-30% of the population will die at temperatures between 0°C and -4°C. The results of these studies strongly suggest that the range expansion of A. campestris into central Washington is directly related to the warming of wintertime low temperatures in this area. Understanding thermal physiology is an essential part of our ability to ascribe a causal relationship between climate change and species range shifts, and this paper provides a beautiful example of how this can be done.