Islands have their own unique ecosystems, but their isolation also makes them uniquely vulnerable to invaders from the mainland. Landscapers who want to use Japanese barberry in their garden designs aren’t welcome on Monhegan Island, for example. Bamboo doesn’t have any friends on Swan’s Island. Bittersweet is considered more bitter than sweet on Islesboro.

Monhegan’s ongoing battle against barberry is probably the best known among the wars over island invasive species. First introduced several decades ago by gardeners attracted by its brilliant red berries, rapid growth and use as an imposing hedge, barberry spread quickly until by 2003 it infested 40 percent of the island’s 360 acres of forest. It was aided, ironically enough, by another invasive species-deer, which had been introduced on the island in the 1950s.

“On Monhegan, the growing deer population had a major impact,” explains Dr. William Livingston, a biologist and associate professor of forest resources at the University of Maine who has studied forest health on several Maine islands, including Monhegan. Deer ate most of the vegetation on the forest floor, but left barberry alone, he says, removing the plant’s competition. The deer herd was removed in the late 1990s, but by then barberry was firmly established.

Residents have been mounting regular Wednesday morning forays against the stubborn, sharp-thorned invader for almost a decade. The bushes are cut and the stumps treated with an herbicide. Lucia Miller, a longtime summer resident and one of the leaders in the barberry battle, says the effort is paying off slowly but surely. “We’ve made very good progress,” she says. “Bill Livingston was out last summer and was very optimistic. Getting rid of the deer made an enormous difference.”

But bringing one problem under control has created new ones. Barberry enriches the soil, and as it’s removed new invasives-asiatic bittersweet and white swallowwort-are moving in. “The swallowwort is closely related to milkweed,” Miller notes, “and produces very similar seed pods. The Monarch butterfly needs milkweed to reproduce. It lays its eggs on the milkweed, and the larvae feed on the seed pods. But the swallowwort lures Monarchs, too, and the larvae can’t eat it, so they starve.”

Livingston says islands are particularly predisposed to problems with invasive plants and animals that take advantage of the changing ecology and uses. After millennia of dominance by red spruce, farmers cleared the forest for pastures and fields in the 18th and 19th centuries. The end of island agriculture in the early 1900s allowed white spruce to regrow in the open fields, he notes. The white spruce is now dying and giving way to climax stands of red spruce again, but the entire process opens opportunities for new species to slip into the cracks. Add the human element, and all sorts of mischief are possible.

“We never had raccoons out here until 40 years ago, when someone imported a few because they thought it was a good idea,” says Dexter Lee, first assessor on Swan’s Island. Raccoons can be formidable predators along the shores of lobster pounds, where they drag lobsters ashore and split the undersides for the tomalley, but Swan’s Islands raccoons haven’t yet risen above the nuisance level.

Japanese knotweed, better known in Maine as bamboo, is another story. “It’s not terribly widespread yet, but wherever it’s planted it takes hold and spreads,” Lee says of the plant, which was introduced to the island as a garden ornamental. “The stuff is almost impossible to kill. We’ve got stands of it here and there all over the island.”

On the plus side, Lee adds, “We don’t have any skunks yet, thank goodness. Or porcupines. Don’t want them, either.” Muskrats have long flourished on Swan’s but have not become pests, unlike on Monhegan, where the islanders have to invite trappers out to the island periodically to bring the population back under control.

Not all invasive species are unwelcome. “Strictly speaking, lupine is a nonnative¬† plant,” points out Steve Miller, longtime executive director of the Islesboro Islands Trust. “I doubt many people would want to see that eradicated.”

Miller takes a somewhat philosophical approach to the issue of invasive species. “I’m not personally convinced that any of them is outcompeting others,” he says. He can run off a long list of nonnative species on the island, ranging from honeysuckle to white-tailed deer. “Purple loosestrife is one I do question,” he notes. “Is it in fact driving out native island species? It’s hard to tell.”

Japanese barberry is also well established. “It’s probably the worst of them,” Miller says. “Bittersweet is pretty thick in places, too.” He finds a certain irony in residents who spend time and money to rip out stands of bittersweet, only to replace it with lawns of grasses that are themselves nonnatives.

Islanders may as well get used to the newcomers, in Miller’s view. “They’re here to stay, I’m afraid,” he says. “Trying to eradicate them all would be like spooning out the ocean.”