BAR HARBOR – A scientist says the once-abundant beaver population on Mount Desert Island may be in decline.

Allan O’Connell, who works for the U.S. Geological Survey, told a weekend seminar at Acadia National Park that he has witnessed a decrease in the number of active beaver dams and lodges on the island in the past few years. And that has led him to believe that MDI’s beaver population may be at risk.

He and others say they don’t know why such a decline may be occurring. But the park will monitor the area before moving forward with any plan to deal with Acadia’s beaver population.

O’Connell spoke Saturday during the last of a series of seminars called to discuss the resources within the boundaries of Acadia.

Beaver are the second-largest member of the rodent family and are known for their trademark broad flat tail and large teeth used for gnawing through trees. They use mud and sticks to slow the flow of water, often creating elaborate dams to flood an area they find hospitable for habitation, and cutting down trees such as aspen and poplar for food.

They have been widely regarded as a nuisance animal in much of Maine and are subject to trapping as a means of population control.

Beaver dams can flood private property, block roads and trails and in some cases interfere with sources of drinking water. While trapping for beaver is permitted on MDI, trapping isn’t allowed within the boundaries of Acadia National Park.

“The beaver population here, in my opinion, has gone down in recent years,” O’Connell told a group of 25 gathered at park headquarters.

“It’s very hard to prove that scientifically, though.”

And that difficulty, O’Connell said, comes from uncertainty in determining just exactly what a “normal” population of beaver is for Mount Desert Island.

Around 1900, O’Connell said, beaver had been nearly trapped out of existence on the island, and there was little hope of the animal making a comeback on its own.

Later, an Acadia National Park superintendent had beaver reintroduced to the island by transporting healthy beaver from mid-Atlantic states and placing them in various places on the island. The beaver population swelled, and by 1930 had become a nuisance – flooding roads and cutting down sapling trees.

The 1947 Bar Harbor fire, O’Connell said, destroyed much of the beaver habitat and food supply and sent the beaver population into another decline. By 1960, according to estimates, there were about 200 beaver living on the island. By 1970 that number increased to 300, with beaver activity found in all island watersheds. But by 1997 the beaver population had dropped once again to about 100, with beaver activity found in only 45 percent of island watersheds.

O’Connell said it isn’t clear what is leading to the possible decline in beaver numbers, but added that development on the island may be limiting habitat. He also said there is some concern that the beaver population on MDI may be inbred, and therefore particularly subject to disease and genetic defect.

“There’s some indication here that the beavers are starting to use less than preferred food,” O’Connell said. “And so the question becomes, `Is this a natural population?”‘

While beaver are often seen as a nuisance to development and landowners, they also play an integral role in the environment by creating and maintaining wetlands for other species.

But there is no definitive way to determine just what level of beaver numbers the state and National Park Service should manage for. And he noted that the park service has often maintained that it tries to be cautious in its management of species within park boundaries.

“At some point,” O’Connell said, “you have to make a decision as to what it is that you want to perpetuate here. At what previous population level do you want to manage for?”

Karen Svenson, a scientist with The Jackson Laboratory in Bar Harbor, said she undertook a study to determine the genetic health of the beaver in the park, but the results of her study were inconclusive. In the hope that mice genetics would be closely related to beaver, Svenson tried to analyze the genetic characteristics of the beaver in the context of what is known about mice genetics.

The point of the study, she said, was to help determine whether MDI beaver were indeed inbred and suffering from genetic disease or defect. But Svenson said the two species aren’t closely enough related for the study to provide any conclusive results.