Mice as a Keystone species

Basic ecology uses simple diagrams to illustrate how biomass is distributed in ecosystems. These “ecological pyramids” lump all primary producers (plants) at the base of the pyramid. The second level is necessarily smaller since it is sustained by plants, and consists of herbivores or plant-eating animals, such as deer, rabbits and many rodents. Above the herbivores sit first order predators, such as weasels and snakes. And above them sit hawks, owls and other top carnivores.

The importance of plants in any ecosystem is evident from any ecological pyramid. The role of herbivores may be secondary, but is equally critical. Without the pyramid’s second tier, there would be no predators and hence no animal community.

Mice are keystone species in almost every ecosystem. In forests, fields and deserts, mice represent food to predators of all sizes. They link plants and predators in every terrestrial ecosystem. Weasels, foxes, coyotes, hawks, owls, skunks, shrews, bobcats and bears all eat mice. Name a predator, and it probably eats mice.

But not just one generic mouse supports nature’s complex webs of life. More than 70 species of mice, including 18 species of deer mice and 20 species of voles, occur in virtually every habitat in North America.

Deer mice and white-footed mice (members of the genus Peromyscus) inhabit almost the entire continent. These twin species are cute, classic mice with brown fur, white bellies, long tails, prominent ears and big black eyes.

Peromyscus are equally at home in underground burrows, tree cavities and birdhouses. Most of my nest boxes shelter these mice all winter long, often in heat-conserving social groups of three to five individuals. (Beware when cleaning out dusty mouse nests. Peromyscus harbors the potentially fatal hanta virus. Do not inhale dust from a Peromyscus nest.)

Voles (members of the genus Microtus) are also common and widespread, but most stick to within a few inches of ground level. Small beady eyes, tiny ears concealed by fur, and short tails characterize these shaggy little mice.

That brings me to my favorites, the jumping mice. Jumping mice are tiny mammals (they weigh a bit more than a chickadee) with huge rear feet and a tail that’s longer than the body. Their bellies are white and the fur on their back (what you glimpse as they bound out of view) is a rich rusty brown.

Though jumping mice can walk and run, when hurried or frightened, they bound away in zigzag fashion. If just moving through open vegetation, each hop measures only several inches. But when frightened, they can jump 6 to 10 feet. The only time I’ve actually seen them jump is while mowing trails in a hayfield.

Jumping erratically and unpredictably helps these mice elude just about any predator. And that’s good news because they’re on the menu of most vertebrate predators.

The most fascinating part of jumping mouse biology is unfolding right now, and it probably explains why I’ve seen several recently. They’re in a feeding frenzy to fatten up for hibernation. Jumping mice are true hibernators. For the next four to six weeks they will devour seeds, berries, roots, nuts, underground fungi and invertebrates – up to half their body weight each day.

By late October or early November, they’re ready for the big sleep. Though they live and nest above ground during the spring and summer, fall sends them underground. They dig a burrow that may extend three feet below the surface and build a grapefruit-sized bed of grass and leaves.

When the den is complete, jumping mice plug the tunnel and assume a distinctive sleeping position. Jumping mice curl into a tight ball by placing the nose between the hind legs and wrapping the tail around the body. Hibernation lasts up to six months. Body temperature drops to 35 to 40 degrees, and the heart and breathing rates slow drastically. In late April or early May, jumping mice emerge to a bountiful crop of lush spring vegetation and a deadly array of hungry predators.

Dr. Shalaway can be heard on Birds & Nature from 3 to 4 p.m. Sundays on 620 KHB Radio, Pittsburgh or live online anywhere at www.khbradio.com. Visit Scott’s website www.drshalaway.com or contact him directly at sshalaway@aol.com or 2222 Fish Ridge Road, Cameron, WV 26033.