WARMER WINTERS DRIVE BUTTERFLY RANGE EXPANSION BY INCREASING SURVIVORSHIP

As the climate warms, many species are moving to higher latitudes and elevations. However, range shifts can be caused by many factors. These factors are unknown in most cases. The specific role of climate in these dynamics needs study to better predict future consequences of global warming. This case study evaluates whether warming is driving the northward range expansion of a skipper butterfly ( Atalopedes campestris ). Recently colonized areas have warmed 2–4°C over the past 50 years. To assess the importance of climate change for population persistence in these areas, I compared population dynamics at two locations (at the current range edge and just inside the range) that differ by 2–3°C. Population growth rate at these two locations over two years was positively correlated with January mean and annual mean temperatures. To determine whether larval overwinter survivorship could explain this correlation, I transplanted larvae over winter to both sites. Larval survivorship was very low at both locations, but significantly lower at the range edge, probably because lower lethal temperatures frequently occurred there. To estimate the direct effect of cold stress on larval survivorship, I applied a previously derived hazard‐rate model based on laboratory experiments. With input from field‐measured daily mean temperature, the model accurately predicted transplant survivorship at both locations over two winters. Combined results from population and larval transplant analyses indicate that winter temperatures directly affect the persistence of A. campestris at its northern range edge, and that winter warming was a prerequisite for this butterfly's range expansion.