Extreme weather is behind record lows in butterfly populations

Dr Aldina Franco/UEA/PA Wire

By New Scientist staff and Press Association

British butterflies could be under threat from increasingly frequent episodes of extreme weather. In fact, heat waves, cold snaps, and heavy rain may have already contributed to reported butterfly population crashes.

Researchers analysed data from the UK Butterfly Monitoring Scheme (UKBMS), which contains information on butterfly populations collected from more than 1,800 sites across the UK over 37 years.

The team found that rainfall level during the cocoon __life stage of butterflies adversely affected more than a quarter of butterfly species in the UK.

But the greatest harm was caused by extreme heat during the “over-wintering” __life stage, which had an impact on more than half the species.

This may be due to increased incidences of disease. Or it could be that extreme hot temperatures act as a cue for butterflies or their larvae to come out from over-wintering too early and subsequently be killed off by temperatures returning to colder conditions, said study co-author Aldina Franco, from the University of East Anglia.

Dr Aldina Franco/UEA/PA

Hot weather was found to benefit warmth-loving adult butterflies, leading to a positive population change in more than a third of species.

However, on balance the bad effects of extreme weather events outweighed the good.

“Years with extreme warm summers and winters may have mixed effects. For example, this year was terrible for butterflies,” Franco said.

Although the summer was warm the number of butterflies counted during the Big Butterfly Count was particularly low. “Our study indicates that this could have resulted from the detrimental effects of the warm winter.”

“The study has demonstrated previously unknown sensitivities of our UK butterflies to extreme climatic events, which are becoming more frequent with climate change,” said lead author Osgur McDermott-Long, of the School of Environmental Sciences at UEA.

“Some of these effects are undoubtedly putting future populations at risk, such as extremely warm winters,” he said. “However we’ve seen that warm and even climatically extreme hot summers may actually benefit butterflies.”

Journal reference: The Journal Of Animal Ecology, DOI: 10.1111/1365-2656.12594/full