Global warming is rescuing the once-rare brown argus butterfly, scientists say.

Man-made climate change is threatening the existence of many species, such as the giant polar bear. But in the case of the small drab British butterfly, it took a species in trouble and made it thrive.

It's all about food. Over about 20 years, the butterfly went from a species in trouble to one that is pushing north in Britain, where it found a veritable banquet.

Now, the butterfly lives in twice as large an area as it once did and is not near threatened, according to a study in Friday's issue of the journal Science.

Decades ago, the brown argus "was sort of a special butterfly that you would have to go to a special place to see, and now, it's a butterfly you can see in regular farmland or all over the place," said study co-author Richard Fox, an ecologist at Butterfly Conservation, a science and advocacy group in Britain.

Rachel Pateman of the University of York in England and her colleagues found that thanks to warmer summers, plants that were previously rarely used by the brown butterfly to lay eggs now make good hosts.

Listen to an interview with Rachel Pateman on CBC Radio's Quirks & Quarks.

In the past, the argus has used the rockrose, or Helianthemum nummularium, to lay eggs, because it grows on south-facing slopes, but thanks to global warming, it has been able to branch out and use different kinds of plants, which has allowed it to expand its habitat.

Primarily, it has started laying eggs in plants in the geranium family, such as dove's-foot cranesbill, which are much more widespread than the less common rockrose.

Global warming helping the brown Argus is unusual compared to other species, and that's why scientists are studying it more, said study co-author Jane Hill, a professor of ecology at the University of York in England.

Winners and losers of climate change

Biologists expect climate change to create winners and losers in species. Stanford University biologist Terry Root, who was not part of this study, estimated that for every winner like the brown Argus there are three loser species, like the cuckoo bird in Europe.

Hill agreed that it is probably a three-to-one ratio of climate change losers to winners.

As the world warms, the key interactions between species break down because the predator and prey may not change habitats at the same time, meaning some species will move north to cooler climes and won't find enough to eat, Root said.

"There are just so many species that are going to go extinct," Root said.

What makes the brown Argus different is that it found something new to eat, something even better than its old food that is much more available.

"It's almost like the whole of the buffet is now open to it," Hill said.