Rodents Key Players in Forest Health

Mice, Voles and Shrews, Oh My:

By TODD McLEISH/ecoRI News contributor

Photo by Will Collette

When University of Rhode Island teaching professor Christian Floyd brought students in his mammalogy class to a nearby forest in September to set 50 box traps to capture mice and other small mammals, he was surprised the next morning when more than half of the traps contained a live white-footed mouse.

“I usually expect to catch three or four, and on a good year we’ll get about 12, but you never get 50 percent trap success,” URI’s rodent expert said.

“White-footed mouse populations fluctuate in boom and bust years, and this year seems to be a boom year.”

Floyd speculated that abundant acorns, recent mild winters, and healthy growth of concealing vegetation were probably factors in the unusual numbers of mice captured this year. But whatever the reason for their abundance, healthy mouse populations are a good sign for local forests.

A new study by scientists at the University of New Hampshire concluded that small mammals such as mice, voles, shrews, and chipmunks play a vital role in keeping forests healthy by eating and dispersing the spores of mushrooms, truffles, and other fungi to new areas.

According to Ryan Stephens, the postdoctoral researcher at UNH who led the study, all trees form a mutually beneficial relationship with fungi. Healthy forests are dependent on hundreds of thousands of miles of fungal threads called hyphae that gather water and nutrients and supply it to the trees’ roots. In return, the trees provide the fungi with sugars they produce in their leaves. Without this symbiotic relationship, called mycorrhizae, forests would cease to exist as we know them.

Different fungal species enhance plant growth and fitness during different seasons and under different environmental conditions, so maintaining diverse fungal communities is vital for forest composition and drought resistance, according to Stephens.

But fungal diversity declines when trees die because of insect infestations, fires, and timber harvests. That is why the role of small mammals in dispersing mushroom spores is so critical to forest ecology.

To effectively support healthy forests, Stephens said these animals must scatter spores of the right kind of fungi in sufficient quantities and to appropriate locations where tree seedlings are growing. But not every kind of small mammal disperses all kinds of spores, so it’s imperative that forest managers support a diversity of mammal species in forest ecosystems.

“By using management strategies that retain downed woody material and existing patches of vegetation, which are important habitat for small mammals, forest managers can help maintain small mammals as important dispersers of mycorrhizal fungi following timber harvesting” and other disturbances, Stephens said. “Ultimately, such practices may help maintain healthy regenerating forests.”

Distributing mushroom spores isn’t the only important role played by mice and voles in the forest environment. They are also tree planters.

“Almost all rodents cache food — they have a cache of acorns, seeds, maybe truffles, little bits of mushrooms,” Floyd said. “Our oak forests are probably all planted by rodents. They scurry around and dig holes and bury things.”

White-footed mice, which Floyd said are the most abundant mammal in Rhode Island, are also voracious consumers of the pupae of gypsy moths.

“For a mouse, gypsy moth pupae are like little jelly donuts; they’re a delicacy,” he said. “The theory is that when mouse numbers are high, they can regulate gypsy moth populations.”

Mice, voles, shrews, and chipmunks are also the primary prey of most of the carnivores in the forest, from hawks and owls to foxes, weasels, fishers, and coyotes. These small mammals are a vital link in the food chain between the plant matter they eat and the larger animals that eat them.

Are these small mammals the most important players in maintaining healthy forests? Probably not. Floyd believes that accolade probably goes to the numerous invertebrates in the soil. But this new research on the dispersal of mushroom spores by mice and voles may move them up a notch in importance.