Common brown butterfly (Heteronympha merope). Is it time to lay yet? Image: Paul Sunnuck, Monash UniversityFor the first time, climate change has been shown to alter the timing of a natural event – the emergence of the common brown butterfly

For the first time, a causal link has been established between climate change and the timing of a natural event – the emergence of the common brown butterfly.

Although there have been strong correlations between global warming and changes in the timing of events such as animal migration and flowering, it has been hard to show a cause-and-effect link. This is what Michael Kearney and Natalie Briscoe of the University of Melbourne, Australia, have now done.

The researchers compared temperature changes in Melbourne – where the butterfly is common – with recorded observations of the first brown butterfly to be seen in the spring since the 1940s.

With each decade, the butterflies emerged 1.6 days earlier and Melbourne heated by 0.14°C. Overall, the butterfly now emerges on average 10.4 days before it did in the 1940s, says Kearney. "And we know the rise in air temperature links to butterfly emergence in a cause-and-effect pattern."

The pair are confident in the cause-and-effect relationship for two reasons. First, they placed eggs of the butterfly, Heteronympha merope, in chambers where temperature could be controlled and found that each larval stage has a different response to warmer-than-normal conditions. …

Can the butterfly adapt to the changes? The female waits until the end of summer to lay her eggs, ensuring that the larvae's development stages align with the seasons. According to Kearney, if summers are longer, the female must "wait around", and it's unknown if her lifespan can cope.

"It also may be too warm for the developing butterfly to get through the larval stages," says Kearney. …

Global warming changes natural event: first causal link

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