It’s tough to be a woodrat. First, of all, it’s name is a problem. Sure it’s a rodent, but so are chipmunks and squirrels. But those three letters — r-a-t — conjure up images of plague-spreading Old World rats. In reality, the woodrat is a New World rodent, rarely seen by most people, and a cousin of the pack rat.

And there’s another reason why it’s tough to be a woodrat. They’re disappearing. They are a threatened species in both Pennsylvania and Maryland.

No one is exactly sure of the cause, but scientists in Pennsylvania are working on a theory that its demise may be closely related to habitat fragmentation.

Scientists have long known that as forests are fragmented and individual patches shrink, they gradually lose species diversity. Many interior forest birds can’t live within about 300 feet of a forest edge because of pressure from predators and competing species. As forests shrink, those edges are drawn closer to the forest center — like a tightening noose — until there’s not enough interior left for the forest birds to remain.

Similarly, fragmentation may be affecting the woodrat.

Woodrats are scattered through forested Eastern mountains where they live in caves, boulder fields and rocky ledges where there are deep crevices into which they retreat and build their nests.

Needless to say, there is plenty of woodrat habitat in Pennsylvania. In the late 1940s and early 1950s, Pennsylvania Game Commission surveys found them plentiful. “Every rock outcrop had woodrats on it,” said John Hall, a professor of biology at Albright College.

But in the 1970s, Hall began noticing fewer and fewer woodrats in the caves where he was doing his studies. Caves that always had woodrats were suddenly vacant. Since then, Hall and other biologists have tried to explain the animal’s demise, but no explanation seems a perfect fit.

Scientists a few years ago thought they had a major new clue in the mystery. New York wildlife biologists attempted to restore their woodrat population by importing 30 animals from West Virginia. Soon, all the animals were dead. Follow-up studies showed the animals became infected with a raccoon roundworm.

Woodrats apparently got the parasite because, being consummate collectors, they had gathered dried raccoon feces and carried them back to their nests. But many biologists are reluctant to blame the woodrat demise squarely on a parasite. After all, raccoons have always been near woodrat sites.

Another suspect is the gypsy moth. In recent decades, the moth — a native to Europe — has expanded its range from Massachusetts and Connecticut through New York, New Jersey, Pennsylvania and into Maryland, Virginia and West Virginia.

Gypsy moth caterpillars are voracious eaters and are particularly fond of dining on the leaves of oak trees. Defoliated oaks have fewer acorns, a favorite woodrat food. The loss of woodrats from the Northeast through Pennsylvania, “seems to correlate very nicely” with the spread of gypsy moths, Hall said. But much of eastern Pennsylvania is now recovering from the initial impact of gypsy moths, and oak trees are rebounding. Woodrats are not.

Now, biologists are putting together a complex — but theoretical — picture to explain the woodrat’s demise. In it, a whole series of problems accumulate to drive the woodrat out of its historic habitat.

Fundamentally, those factors have cut the woodrat’s potential range through fragmentation. For example, raccoon and woodrat populations probably overlap more today than in the past. Raccoons tend to be a forest “edge” species which thrives where fields — or suburban developments — meet woodlands. As valleys have been developed, raccoon populations have increased and gradually moved up the sides of the mountains, bringing them into ever greater contact with the woodrats.

Unable to move down the sides of the mountains, woodrats are increasingly restricted to areas near the ridge tops, which have been especially hard hit by gypsy moths and are not the best locations for acorns. In addition, some speculate abundant deer — another edge species — have eaten much of the understory vegetation which also serve as food. And past logging on the ridges may have reduced the amount of trees left to rot, and therefore the amount of fungi — another woodrat food.

“The woodrats aren’t left with much,” said Jerry Hassinger, wildlife diversity supervisor with the Pennsylvania Game Commission. “They have to spend a lot more time looking for food to get what little they can get.”

And, in doing so, they may be more vulnerable to predators that have benefited from forest fragmentation, such as great horned owls and red foxes. Recent radio tracking work showed that woodrats spend more time on the ground moving through open areas — and exposed to predators — than previously thought.

Historically, Hassinger suggested, a long ridge top would contain many suitable rocky habitat pockets for woodrats. Hassinger likened historic woodrat colonies along a ridge to a string of lights; occasionally, a light might blink off, but that colony would be repopulated by neighboring woodrats, and the “light” would come back on. Now, Hassinger says, the lights may be blinking off faster than the colonies can be repopulated.

“What has happened is that it has become more dangerous for the young woodrats to travel from their family area to a new area,” he said. “There are a lot of predators in between, whether it is a great horned owl, red fox or a road where they are run over. You name it, the likelihood of a successful dispersal from a natal area to an unoccupied but acceptable rock island is less likely today than it was historically.”

Eventually, species in such circumstances just “wink out” altogether, Hassinger said. “It may take a long time for them to do so, as their populations become smaller and more isolated from one another, but they flash out, one after another.”