Warmer winters help butterflies breed more

Warmer winters can lead to increases in butterfly populations, researchers say, but only in species that can breed more than once a year. As well as understanding natural variation in butterfly populations, the findings may help scientists predict the insects' response to climate change. Higher temperatures in late winter and early spring mean that many butterflies can start flying — and reproducing — earlier in the year. Most species produce just one clutch of eggs every summer; but, for species that produce a second round of eggs, getting the first brood out early makes a second brood possible. This is the first time the timing of a butterfly's first and second broods has been linked.

"We were surprised that the association hadn't been demonstrated before, because it seems to be so obvious," says Angus Westgarth-Smith from Brunel University, whose research features in Ecological Entomology. "If the butterflies can breed earlier, there's more time for two generations — potentially even three." This means that warmer winters can cause an increase in the numbers of butterflies that breed more than once per year, such as the Small White, the UK's most commonly seen butterfly, and the Common Blue. This is because the second brood is more likely to be laid early and is therefore more likely to survive. For butterflies that only breed once, like the Gatekeeper, the warmer weather is not so helpful; scientists from Brunel University and the Centre for Ecology & Hydrology (CEH) found no such increase.

Small White (Pete Eeles).

The scientists studied the effects of a natural climate cycle, the North Atlantic Oscillation (NAO), which has a strong influence on the average temperatures of British winters: so much so that it was at least partly responsible for the freezing winters of 2009 to 2010. Looking at population records for British butterflies, the scientists found that when there was a warm winter in the UK, twice-breeding butterflies increased in number. "This may be a model for the effects of long-term climate change," Westgarth-Smith suggests. "We think that there is a likelihood that warmer weather could favour species that have two or more generations."

Single-brood butterflies don't have such long flight seasons compared with their double-brood cousins, so their population size is less likely to be affected by warmer winters. The number of broods they produce does not depend on the weather either. In fact, warmer winters may harm some butterfly populations. The scientists found that some species of once-breeding butterflies had lower populations after a warm winter. This may be because they need cold winters for dormancy — a period of inactivity by the insect during its transformation from caterpillar to butterfly, while waiting for the right weather conditions.

Information for the study was collected by the UK Butterfly Monitoring Scheme using datasets going back 34 years. One site, the Monks Wood nature reserve in Cambridgeshire, which once hosted a CEH research station, has butterfly records dating back to 1973. "The UK Butterfly Monitoring Scheme is probably one of the best insect biodiversity databases in the world," says Westgarth-Smith. "It's weekly, whereas the other good butterfly datasets are annual. I'd put the Butterfly Monitoring Scheme right at the top."