It's known as one of Alaska's worst ecological disasters: Arctic foxes introduced onto scores of the Aleutian Islands through the early...

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ANCHORAGE — It’s known as one of Alaska’s worst ecological disasters: Arctic foxes introduced onto scores of the Aleutian Islands through the early 20th century by fur farmers ended up eating millions of nesting seabirds.

Before the invasive predators died out or were killed off in most areas, they almost drove the Aleutian Canada goose to extinction and decimated other species throughout the 1,200-mile archipelago.

But the damage went further, delivering more than silence to the Aleutian shore. A recent scientific study of 18 islands — nine with foxes and nine without — has revealed a previously unknown ecological link between land and sea.

Seabird poop.

Minus the seasonal drizzle of guano as nitrogen-rich fertilizer, soil on many of the Aleutian Islands simply lost the capacity to support what had been lush grassland, said Vernon Byrd, supervising biologist with the Alaska Maritime National Wildlife Refuge and one of the study’s five authors.

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The Aleutian Canada goose has made a strong recovery where foxes were exterminated.

What once was an ocean of beach rye and other dense growth changed into a scruffy tundra dominated by dwarf shrubs and leafy plants.

It’s as though a tall forest had been shrunk to scrubland, Byrd said from his office in Homer, Alaska.

“I was surprised that the connection was so strong,” Byrd said. “It just has a big effect, and I think it’s easier to see on islands because it’s a simpler system.”

The study has broad implications about the danger that nonindigenous species such as foxes pose toward isolated island habitats such as those found in the Aleutians, according to the paper, published in March in the journal Science.

By knocking out key species — as foxes ate up seabirds, for example — such invasive critters can cause the food chain to unravel, affecting everything from the microbes in the soil to the creatures winging overhead.

The findings also give urgency to the refuge’s continuing program to eliminate foxes from a dozen islands where they don’t belong, Byrd said.

“We’re down to the big islands now, and it’s really hard to get them off.”

Two years ago, teams of trappers worked the perimeter of Adak Island, in a $150,000 project of hunting down and killing foxes one at a time. They returned to check last year. Byrd fears that a few foxes remain.

“It’s pretty much brute force,” he said. “It’s really hard work.”

Teams will focus this year on Tanaga and Kanaga islands. A survey found foxes burrowing high on Tanaga’s volcanic slopes, possibly subsisting on ptarmigan. On other islands, the foxes always had concentrated along the shore.

“That was a shocker and a real paradigm shift in terms of what we look for,” Byrd said. “We may have to do something completely different on Tanaga.”

Russian trappers introduced Arctic foxes on certain Aleutian Islands in the 1700s. By the early 1800s, Native Aleut people reported that seabirds were disappearing from Atka and Attu islands, but fur traders considered birds little more than free-range feed. The industry peaked in 1929, when more than 400 farmers harvested fox pelts worth $900,000 across Alaska, according to a report by retired federal biologist Edgar Bailey. Fur prices later plummeted, and the industry crashed.

The U.S. Fish and Wildlife Service began eradicating foxes from the islands in 1949, employing a variety of methods ranging from poisoning to traps. By the early 1990s, the foxes had been exterminated from 17 islands and had disappeared from most others, but the animals remained on more than 20 of the Aleutian Islands, many of them large.

Byrd, who joined the refuge staff in the late 1960s, first noticed something was amiss as he surveyed for places to reintroduce the endangered Aleutian Canada goose.

“Vegetation just didn’t have the lushness that you would have expected, except in areas where there were large bird colonies,” he said. “I began to think that it’s possible that foxes might have had an indirect effect on the vegetation.”

Over time, Byrd teamed up with well-known biologist Jim Estes, who spent years studying Aleutian sea-otter ecology. With other scientists, Estes and Byrd organized the study that compared vegetation and soil nutrients from fox-infested islands with places that remained fox-free.

Among other things, they found that the dense congregations of seabirds on fox-free islands produced 63 times more guano than the meager bird populations on fox-infested shores — almost 13 ounces per square meter versus about one-fifth of an ounce.

The soil contained much higher levels of nitrogen brought from the sea in the guano. The scientists also found greater nitrogen concentrations in the grass, leafy plants, mollusks, songbirds, insects and spiders.

To further test the guano connection, the scientists tried fertilizing areas on one big fox-infested island. Over three years, the amount of grass growing in the plots increased 24-fold and thoroughly dominated the other plants, the scientists wrote.

“The findings demonstrate that fox-free islands are strongly subsidized by marine-derived nutrients,” they wrote.

The authors concluded that introduced predators “can ultimately have dramatic indirect effects on entire ecosystems and that these effects may occur over large areas — in this case an entire archipelago.”