"We were able to show that an increase in temperature has led to a change in the types of host plants the butterfly uses," Rachel Pateman of the University of York told environmentalresearchweb. "Previously largely restricted to using rockrose in Britain, warmer summers have enabled the butterfly increasingly to use Geranium species. Geraniums are widespread in the landscape and this has enabled the butterfly to expand its distribution in Britain rapidly as the climate has warmed."

The brown argus (Aricia agestis) is at the northern edge of its range in Britain, where its larvae have typically fed on the rockrose (Helianthemum nummularium) in the chalk grasslands of southern England. In continental Europe the larvae have more commonly fed on members of the geranium family.

In the 1980s the brown argus was scarce and declining in England. But the butterfly has extended its range northwards in Britain by 79 km over the last 20 years, moving rapidly into areas where the rockrose does not grow. That's an expansion rate 2.3 times faster than the global average of 16.9 km/year.

Pateman and colleagues from the University of York, the Natural Environment Research Council Centre for Ecology and Hydrology, and Butterfly Conservation used brown argus distribution records collected by amateur butterfly enthusiasts over the last 40 years. These showed that during the 1990s a greater proportion of the butterflies were found in places where only geranium species were available as host plants. These increases were linked to warmer summers.

The team also looked at the effect of climate on butterfly density across 200 fixed transects in Britain. Brown argus feeding on both rockrose and geranium plants had a higher population density in warmer summers. Analysis of this finding and the distribution results indicate that the rockrose is a better host under cooler conditions, say the researchers.

Rockrose typically grows on areas of short turf on warm southerly facing slopes. It is a perennial and tends to grow in greater density than the dove's-foot cranesbill (Geranium molle), which is an annual. This means it's able to support larger and more stable brown argus populations. In the early 1980s the few records of geranium-feeding brown argus populations were mainly from sand dunes, which also have a warm microclimate.

Summer temperatures in Britain were on average 0.78 °C warmer from 1990 to 2009 than from 1800–1989. The researchers say this would have increased the ability of geranium-containing sites to support brown argus population growth – the butterfly population density grew by more than five times between 1976–1985 and 2000–2009.

"Changes in the way species interact with each other are predicted to affect the ability of species to shift their distributions in response to climate change and now we have shown this in nature," said Pateman. "Usually changes in species interactions are predicted to constrain the rate at which species can shift their distribution but in this case it has helped the butterfly to shift rapidly."

Now the team, who reported the results in Science