Winter warming facilitates range expansion: cold tolerance of the butterfly Atalopedes campestris

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Abstract
Our ability to predict ecological and evolutionary responses to climate change requires an understanding of the mechanistic links between climate and range limits. The warming trend over the past half-century has generated numerous opportunities to develop much-needed case studies of these links. Species that are only limited by climatic factors are likely to shift range quickly during periods of warming. Such species directly impact recipient communities and indicate trends that will become more widespread. Because minimum temperature (T min) is rising at twice the rate of maximum temperature, species with this range-limiting factor may be especially responsive to global warming. In this study, I test the hypothesis that rising T min has directly affected the range of a skipper butterfly. Atalopedes campestris has moved northward rapidly this century, recently colonizing eastern Washington where January T min has risen 3°C in 50 years. The results show that: A. campestris' range lies completely within the −4°C January average minimum isotherm, and that recently colonized areas were below this threshold earlier this century. In acute cold stress experiments, −4 to −7°C proved to be a critical thermal limit: median supercooling point was −6.3°C, and minimum lethal temperature (LT50 with 12-h exposure) was −5.7°C. In chronic cold stress experiments, survivorship declined sharply in diurnally fluctuating thermal regimes typical of the current range edge. High mortality occurred under constant 0°C conditions as well as in fluctuating regimes, implying that thermal insulation from snow would not protect A. campestris. There was no evidence of evolution in cold tolerance at the range margin, despite strong selection. Thus, winter warming was apparently a prerequisite for the range expansion. Characteristics of this species that seem to be associated with its rapid response are that it is an opportunistic species, it is not habitat or dispersal limited, and it is constrained by T min.