Wild inbred butterflies risk extinction.

Butterflies in the scattered meadows of Finnish islands are providing the most direct evidence yet that inbreeding contributes to extinctions in the wild.

Considered alongside a variety of ecological factors, inbreeding accounted for 26 percent of a butterfly population's risk of becoming extinct in the course of a year, report Ilik Saccheri and his colleagues at the University of Helsinki. Their analysis appears in the April 2 Nature.

Glanville fritillary butterflies on the Aland Islands have fluttered into a longstanding argument about whether inbreeding matters in the real world. It clearly bedevils captive populations, enhancing expression of harmful recessive genes and hampering reproduction. Moreover, in laboratory experiments on colonies of fruit flies and mice, inbreeding increased extinction rates.

Although field studies have linked inbreeding to declines among song sparrows and adders, some researchers argue that, in nature, inbreeding proves trivial compared to crushing blows from weather changes, the demographics of a population, and especially human encroachment.

The island butterflies offered a powerful test case. Saccheri and his colleagues collected data over 4 years from some 1,600 meadows, each with its own, largely self-contained butterfly population. About 200 of these populations became extinct in any year, and wandering butterflies recolonized more than half of the vacancies.

The Helsinki team sampled genetic variability in butterflies in 42 meadows. The seven populations that disappeared during the next year had 28 percent less genetic variation than the groups that survived.

After factoring in such variables as original population size, meadow size, and abundance of nectar plants, the researchers still found genetic variability highly important.

Nevertheless, Saccheri emphasizes, "Man is the primary cause of population decline."

The statistical power of the butterfly study, with its many populations, may not be easy to match in examining inbreeding's effect on other extinctions. "Simply because people don't detect it doesn't mean it isn't going on," Saccheri warns.

Katherine Ralls of the National Zoological Park in Washington, D.C., says, "When I saw their data, I was a little jealous, speaking as someone who studies mammals." She welcomes the butterfly findings as important and convincing.

This and other field studies may nudge the debate on inbreeding versus environmental factors to a new level, says Tim Caro of the University of California, Davis. "It becomes an important issue as to which is the most important when you're trying to make a management decision," he says. Even though one of his specialties, the cheetah, became a poster child for inbreeding during the 1980s, Caro blames other menaces for the animal's rarity in the wild. "I think the cheetah was a false case of this phenomenon that is certainly now going on in nature."