Species like silver-studded blue and high brown fritillary butterflies have been on the decline around the British isles. A team of researchers from the University of York say they know the answer as to why that is.

The answer is climate change.

Warming weather is causing several species of butterfly and moth to emerge earlier in the year than they naturally would. This could be beneficial to species with fast, multiple breeding cycles in a year and ones that are more flexible about their habitat.

The reason is that they can grow in numbers before the winter and expand their range toward the north, the scientists say. Other species aren’t so lucky, however. So-called habitat specialists and species with a single annual life-cycle are shrinking in numbers and disappearing from northern parts of the country within their historical range.

“Single generation species that are habitat specialists (like the rare High Brown Fritillary butterfly) are most vulnerable to climate change because they cannot benefit from extra breeding time and emerging earlier may throw them out of seasonal synchrony with their restricted diet of food resources,” the researchers note.

“Our results indicate that while some more flexible species are able to thrive by emerging earlier in the year, this is not the case for many single generation species that are habitat specialists. These species are vulnerable to climate change,” says Prof. Jane Hill, a biologist at the University of York.

“Because butterflies in general are warmth loving, scientists predicted that the range margin of most species would move north as a result of global heating. However this hasn’t happened as widely or as quickly as expected for many species,” explains Callum Macgregor, an entomologist and ecologist at the University of York.

“Our study is the first to establish that there is a direct connection between changes in emergence date and impacts on the habitat range of butterflies and moths,” Macgregor adds. “This is because emerging earlier has caused some species to decline in abundance, and we know that species tend only to expand their range when they are doing well.”

Even as some species will benefit from warming weather, others will lose out. And this is as true of butterflies as other animals. “These changes remind us how pervasive the impacts of climate change have already been for the world’s biological systems, favouring some species over others,” stresses Prof. Chris Thomas, who is director of the Leverhulme Centre for Anthropocene Biodiversity at the university.

“The fingerprint of human-caused climate change is already everywhere we look,” Thomas adds.

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