Flies have Hawaii scientists buzzing : Study of a diverse species is helping researchers track the ups and downs of the islands’ environment.

Buzzing flies might be unwelcome at most outdoor gatherings, but David Foote does all he can to attract them. His invitation is a pink sponge tacked to a tree, soaked with a broth of rotting mushrooms and smeared with spoiled, mashed-up bananas.

“It’s only been a few minutes and they’re already coming in,” said Foote, a National Biological Service entomologist, as he recently inspected a lure on a soapberry tree.

One, two, three and more flies soon appeared in dark clusters on the sponge. They would hardly have warranted a second glance if they were not the product of an evolutionary explosion nearly unmatched in the world--one that makes them an exceedingly accurate barometer of the rise and fall of Hawaii’s tropical tapestry of life.

Over the next five years, Foote and his colleagues plan to track the abundance and type of flies that flit across this landscape and glean from them clues so exacting that they can learn the dominant age of soapberry trees now standing.

The tree holding the fly-speckled sponge stands within what Hawaiians call a kipuka, a green island of vegetation encircled by rough seas of hard lava. It is a fragment of native forest where such remnants are increasingly rare, invaded by foreign species imported by people and now and then swallowed by new lava flows.

It is also one of the few places where Foote can find certain species of drosophila, a lineage of Hawaiian fruit flies that scientists see as evolutionary giants.

A century and a half ago, the basic tenet of evolution, survival of the fittest, struck Charles Darwin after he noticed variations among finches in the Galapagos Archipelago off South America. Since the different sizes and shapes of the birds’ beaks suited a range of habitats and food, the diversity told Darwin that “one species had been taken and modified for different ends.”

Had Darwin come across the Hawaiian drosophila, he might have found that of all the species in the world, the insects may best illustrate the forces of evolution.

“What I’m looking for is a fairly slimy part of the log,” said Foote as he dug into the decaying bark of a fallen tree with a pocketknife, looking for the larvae of a drosophila species that feeds on microbes that decompose the tree.

The flies, no bigger than a fingernail, are, like Darwin’s finches, specialists. Perhaps a few flies riding an ancient wind first brought their kind to the Hawaiian chain, but that was all it took. Drosophila colonized volcanoes that sprouted into new islands, each colony evolving with its surroundings into its own species. Later volcanic eruptions split fly populations and erased others, creating divergent species with even more precise niches.

“We see this remarkable radiation of species suited to every nook and cranny that happened rapidly in evolutionary terms,” said Hampton L. Carson, a professor of genetics at the University of Hawaii. “There’s nothing else quite like it that we know of.”

There are 800 or more species of Hawaiian drosophila, each with its own habits. Certain species breed only in aging soapberry bark, others in the fruit. Another needs decaying koa trees.

Where there are no koa trees or where livestock have razed other native greenery, there will be none of the flies they support. No flies mean no grub for birds, nothing to pollinate fragile rain forest flowers. This is the principle--eyeing the flies as “indicator species” to detect the state of the environment--that guides Foote’s work.

But the Hawaiian drosophila is much more than the proverbial canary in the coal mine.

“The problem with many indicator species is that we don’t know what they indicate,” said Foote, counting picture-wing drosophila--flies with leopard-like spots--sitting immobile on a sponge. “Here we have something that’s easy to sample that tells us what’s happening out here, down to the plant species that’s in trouble.”

It’s no secret that exotic wild pigs, nonnative plants and even alien insects like mosquitoes and yellow jackets are choking Hawaii’s environments. But in a fenced section of Hawaii Volcanoes National Park where rangers have exterminated pigs and hacked down invading plants, the true rain forest is staging a comeback. So is its partner.

Under a branch shadowed by fronds of tree ferns, Foote found male flies, one per leaf, preening.

“These guys are all defending territories, waiting for females to show up,” he said. “What this represents here is recovery of an entire community.”