Thermal regimes, but not mean temperatures, drive patterns of rapid climate adaptation at a continent-scale: evidence from the introduced European earwig across North America

Understanding the mechanisms by which an introduced species adapt to newly encountered habitats is a major question in ecology. A key method to address this question is to collect data on introduced species that have successfully invaded a broad diversity of novel environments, and analyze how their life-history traits changed with these new constraints. Here, we present and analyze such a unique data set in the European earwig Forficula auricularia L, an insect that invaded North America during the last century. We conducted a common garden experiment, in which we measured 13 life-history traits in 4158 individuals from 19 populations across North America. Our results demonstrate that the successful invasion of this species came with changes in 10 of their life-history traits in response to thermal regimes (winter-summer and autumn-spring temperatures), but with no change in response to the overall mean temperatures of the invaded locations. Importantly, we show that some of these changes are by-products of novel thermal regimes, whereas others reflect adaptive strategies of females to these constraints. Overall, our findings reveal the importance of thermal regimes over mean temperatures in climate adaptation, and emphasize that studying adaptive capabilities is crucial to predict the limits of biological invasions.