A mouse runs through the vegetation in Holt Research Forest in Arrowsic. Credit: Courtesy of Holt Research Forest

Scurrying through underground burrows and up tree trunks, small mammals such as shrews, mice, voles and flying squirrels often go unseen due to their size, stealth and nocturnal habits. But if we take a closer look, these creatures can tell us a great deal about the health of a forest.

“There are huge population shifts [in small mammals] from year to year,” said Jack Witham, associate scientist for University of Maine’s Center for Research on Sustainable Forests. “Most people just aren’t aware of the number of species that are out there because you don’t see them unless they come into your house and you trap them.”

Witham has researched small mammals for more than 35 years in the Holt Research Forest in Arrowsic. The 300-acre property, owned by Maine TREE Foundation, is a site of long-term ecosystem research in the midcoast region.

“Here, our dominant species is the white-footed mouse, and most people are familiar with them because they get them in their houses,” Witham said. “But they’re here in huge numbers — it’s just amazing — and those numbers vary widely from year to year. They seem to have a pretty good ability to sort of control their population size.”

The University of Maine’s College of Natural Sciences, Forestry and Agriculture has conducted studies in the Holt Research Forest since 1983. An oak-pine forest, it is home to at least a dozen different small mammal species, some much more common than others.

Once a year, researchers from UMaine trap small mammals in the forest to get an idea about population fluctuations. They also collect ticks from any small mammals they capture, and work with the Maine Medical Center Research Institute to record tick abundance and track tick-borne diseases in the forest.

Often an increase or decrease in a small animal population isn’t necessarily good or bad, but it indicates that something in the forest has changed. For example, if a red squirrel population is high for a long period of time, that may indicate that trees are producing an abundance of nuts for multiple years in a row.

“Often trees react to stress by producing more seeds,” Witham said. “So if the small mammal numbers are up, maybe that’s an indicator that there’s a very high seed production due to stress in the forest.”

Some of those stressors on trees have been linked to climate change, which has caused rising temperatures, drought and storms.

Witham will be sharing his knowledge and experiences in the upcoming virtual presentation “Small Mammals in Your Forest.” The free event is scheduled for 6 p.m., Thursday, March 4, and will be hosted by the Kennebec Estuary Land Trust.

During the presentation, Witham will describe some of the many small mammals you can find in a Maine forest, offering fun facts about each one. He’ll also explain some of the research projects he’s worked on over the years, examining how certain animals relate to aspects of forest health and the changing climate.

Some small mammal species are good indicators that a forest features a diversity of habitats and food sources, Witham said.

“Chipmunks need a diversity of habitats, so they’ll be present if the forest is healthy,” Witham said. “Red squirrels will also be present, and their numbers will go up and down depending on the availability of food.”

The arrival of a new small mammal species — or a spike in their population — can indicate an event has occurred in the forest. For example, meadow voles are rare in the Holt Forest because they prefer grassy areas for habitat and seeds. Since most of the forest has a closed canopy, that grassy habitat doesn’t usually exist. However, the meadow vole has shown up in Holt Forest when areas of the forest canopy have opened up, allowing grasses and other low-lying plants to thrive.

“In the past, when we’ve done some [tree] harvesting here, the meadow voles have shown up,” Witham said. “They’ve also shown up in relation to defoliation when we’ve had some severe gypsy moth outbreaks here.”

Climate change has been linked to one of the biggest small mammal changes they’ve seen in the Holt Research Forest.

“When we first started working here, we were trapping exclusively northern flying squirrels,” Witham said. “It was a gradual switch over. And now, for the past 10 years probably, we’ve only caught southern flying squirrels. So northern flying squirrels have been forced out, primarily by climate change.”

As winters grew milder in midcoast Maine, Witham explained, that opened up a window for southern flying squirrels to survive year round in the Holt Research Forest. And the two types of flying squirrels don’t coexist for long.

“The southern flying squirrel carries a parasite which is fatal to the northern flying squirrels,” Witham said. “Not everybody agrees, but many people think this is the primary reason they don’t coexist.”

The movement of flying squirrels has been documented in other areas of Maine as well, Witham said. A colleague of his at the University of Maine was recently conducting research on northern flying squirrels, which used to be fairly abundant around Orono and Bangor, and she had to travel all the way to Presque Isle to find any for the study.

“A similar change has been documented in Ontario as well,” he said. “There they’ve recorded a 200 kilometer shift in the range of the northern flying squirrel over time.”

Witham will touch on these topics and many more in his upcoming presentation “Small Mammals in Your Forest.” Registration is required, as the Zoom link will be sent out to registrants just prior to the event. To sign up, visit kennebecestuary.org/upcoming-events or call 207-442-8400.

Aislinn Sarnacki

Aislinn Sarnacki is a Maine outdoors writer and the author of three Maine hiking guidebooks including “Family Friendly Hikes in Maine.” Find her on Twitter and Facebook @1minhikegirl. You can also...