Butterflies in the U.K. have been in decline for decades, and climate variability—a side effect of global warming—may be to blame.
By Nathan Collins
The gatekeeper butterfly. (Photo: Aldina Franco/University of East Anglia)
Although bees’ troubles tend to receive more attention, butterflies have also had a tough go of it lately, particularly in the United Kingdom, where populations have been slipping away for four decades. Now, researchers suggest climate change may be the culprit, but not in the usual way: Growing variability in temperature and precipitation brought on by climate change, rather than overall higher temperatures, may be to blame.
“Most attribution of climate change impacts on biodiversity … is based upon the observed or projected changes in mean [that is, average] climate,” Osgur McDermott Long, a research student at the University of East Anglia, and his colleagues write in The Journal of Animal Ecology. The effects of climate variability—in particular, the trend toward more heat waves, cold spells, heavy rain, and droughts—haven’t been studied as much, despite the fact they’re likely to grow more common and more severe in the years to come. The question is, could such extremes already have had an effect on butterflies?
Unseasonably warm winters led to population declines in around half the species studied.
In short, yes. The researchers first gathered data from the United Kingdom Butterfly Monitoring Scheme, which has tracked dozens of butterfly species around the U.K. since 1976. The team combined that information with weather data from the European Climate Assessment and Dataset project, from which they inferred the existence and length of extreme climatic events—for example, the number of days with precipitation in the top 2.5 percent for a given site. They then looked at how extreme temperatures and precipitation affected each of 41 different butterfly species.
The results were fairly complicated, owing in part to varied butterfly lifecycles—for example, “multivoltine” butterflies go through two or more generations per year, while “univoltine” species go through only one—but some general trends emerged. Most importantly, unseasonably warm winters led to population declines in around half the species studied—45 percent of univoltine and 67 percent of multivoltine butterflies—even though very hot weather seemed to benefit adult butterflies in summertime. Butterfly pupae (more commonly known as chrysalises) were particularly sensitive to high rainfall, while drought had less of an effect. Multivoltine species were generally more sensitive to extremes of temperature and precipitation than univoltine species, the team found.
“[B]utterflies could potentially benefit from increasing temperatures in the U.K. in the future but warmer and wetter winters and increases in severe weather events that have also been predicted could be detrimental to the survival of many of its butterfly species,” the researchers write, “and further research is needed regarding the balance of importance that these variables could have and whether the benefits of warmer summers will be outweighed by the detrimental winter effects.”