Four alternative ways to identify compound climate extremes and their relevance to ecological impacts: a case study of UK butterflies

Given observed and predicted increases in the frequency and intensity of many climate extremes, researchers have shown an increased interest in the climate extremes and their impacts on ecosystems because of the profound effects. However, most previous studies on the responses of ecosystems to climate extremes focus on droughts and summer heatwaves, and relatively little is known about the effects of other kinds of extremes, such as winter heatwaves, extreme wet periods, and cold waves.

In this study, we identify four types of extremes (two temperature (heatwaves and cold waves) and two precipitation ones (droughts and extreme wet periods)) and present 4 alternatives to identify compound extreme events. We demonstrate the relevance of the different types of year-round (compound) events for ecological studies by demonstrating their impact on the abundance of 34 UK butterfly species across each species' life stages (hibernation, egg, larval, pupal, and adult) over a 45-year period. We chose this example as these species are expected to respond rapidly to climates due to their ectothermic nature and short life cycles.

The results show that considering different types of year-round (compound) extreme events is relevant from an ecological point of view as at different stages, other extremes have more impact on the survival of individuals. For instance, statistics show that heatwaves and droughts during the pupal and adult stages appear beneficial for butterflies in England, with around 30% of univoltine species showing significant positive influences, whereas extreme wet periods during the pupal life stage cause negative population change for 26% of univoltine species. Our study demonstrates that considering different forms of extremes during all seasons of a year may bring interesting new insights for ecologists. However, we did not seek any eco(fysio)logical explanations of the obtained results.