White pine seeds were found to be especially important in helping voles survive and reproduce in greater numbers. Photo by Alessio Mortelliti.
Small mammals like squirrels, chipmunks, mice, and voles play a surprisingly significant role in determining the tree composition of most forests. That role is based largely on their choice of which seeds to eat. By consuming the seeds of some species (thereby denying them a chance to grow) and hiding other seeds (which helps them germinate), the animals determine which tree species thrive or decline. Conversely, the availability of different seeds also influences the population cycles of some mammals.
Those are among the findings of a series of research studies conducted by Alessio Mortelliti, a University of Maine biologist who combined data from a 33-year population study of rodents with experiments in the Holt Research Forest, Penobscot Experimental Forest, and Acadia National Park.
“Small mammals can reach extremely high densities – in some years up to 100 individuals per acre,” said Mortelliti, assistant professor in Wildlife Habitat Conservation. “They can eat every single seed of the species they like, which means they can have a massive impact on forest regeneration.”
By pointing trail cameras at plates of different tree seeds left in the forest, Mortelliti found strong preferences among different animal species. None of the animals liked the seeds of balsam fir, for instance, which Mortelliti said is one reason why there are so many balsams in Maine forests. Paper birch seeds were also avoided by most of the animals. Mice had a preference for red oaks, which voles ignored, and all of the animals liked white pine, spruce, and maple seeds.
In another study, the researcher found that different seeds affect the vole population differently, and the effects are largely dependent on the voles’ population density. White pine seeds were found to be especially important when vole populations were low by helping the voles survive and reproduce in greater numbers. The animals ate only paper birch seeds when the vole population was high and few white pine seeds were available.
Mortelliti hopes to use his research findings to help forest managers minimize the impact of small mammals on the most commercially valuable tree species. “With a little more research, we should be able to figure out how to optimize timber production by understanding how small mammals are affecting the regeneration of the forest,” he said. “We hope to be able to give timber companies prescriptions for the best way to manage the forest so they can maximize the regeneration of the trees they want.”
He said this will become especially important as the climate changes and new tree species expand their ranges northward.
“Tree species composition is going to change, and small mammals will play a key role in affecting the expansion,” Mortelliti said. “They’re the gatekeepers. They’re going to decide which plants will regenerate. The results of our studies will tell us what tree species will be blocked by the animals and which will be favored, and that will inform the management actions that can be taken to deal with this process.”