Let’s face it, rodents have a public relations problem. Even the cutest of them — say, the eastern chipmunk or the white-footed mouse — are more tolerated than encouraged by humans. And the least popular rodent — that would be the Norway rat, an introduced species now abundant on Martha’s Vineyard as well as nearly everywhere else — is positively reviled as a disease-carrying scavenger. This is not a popular group.
But rodents are impressively versatile animals, and by virtue of their habits and their abundance, they are players to be reckoned with in terms of their ecological importance. There are a couple rodent species of uncertain status here, but it’s safe to say the Island supports at least seven: the eastern chipmunk, with its snazzy stripe; that scourge of the bird-feeder, the eastern gray squirrel; the white-footed mouse (the Vineyard has its own endemic subspecies); the meadow vole, like a furry, short-tailed sausage; the muskrat, common within its limited freshwater habitat; the notorious Norway rat; and the meadow jumping mouse, probably common here but so cryptic that I’ve never seen a live one in the wild. Eastern cottontail rabbits, though rodent-like, are actually in a different family, but most of what I’ll say about rodents applies to bunnies as well.
Our rodent species differ substantially in where and how they live, but the members of this group also share some very basic ecological similarities. Although some rodents reach impressive size (beavers, for example, which don’t occur on the Vineyard but probably once did, may weigh more than 60 pounds), rodents are generally small animals. Our species range from a bit under an ounce (the abundant white-footed mouse) to a couple of pounds (muskrat). Typical rodents also have short lives and very high reproductive rates. Most everything else about the lifestyles of these animals relates in some way to these fundamental facts.
Take diet, for example. Widespread and abundant species, like most of our rodents, can’t depend on rare or thinly distributed food sources, so it is not surprising that mammals have varied diets and rely mainly on very common vegetable food sources such as grass seeds or acorns. Perhaps the most versatile diner among our rodents is the Norway rat, which is the quintessential omnivore: there is very little, animal or vegetable, that these animals won’t eat, and this versatility is one secret of their universal success as an invasive species. But even milder-seeming rodents like our white-footed mouse eat a wide range of plants and are partially predatory, eating grubs, caterpillars, and other invertebrates when they’re available.
Diet, in turn, leads to an often overlooked ecological role for rodents. While these mammals may reduce their activity during the winter, they don’t truly hibernate, and the need to ensure a food supply through the winter months means that most of our rodents have at least some tendency to gather and store food. Given their reliance on seeds, this means that rodents are highly effective agents for dispersing plants across the landscape, carrying (and dropping, and forgetting about…) large numbers of potential seedlings. Virtually anywhere you look on the Vineyard, the mix of plant species is shaped in part by the preferences of rodents for consuming or collecting particular kinds of seeds.
Small size and fecundity also play into a more familiar role played by rodents: as prey items for larger predators. Barn owls didn’t get their name by chance: they like to hang out around barns, and the reason they do is that rats also like barns. You might say that these nocturnal birds are optimized for preying on these almost equally nocturnal rodents. In woodlands, the white-footed mouse is a favored prey item for the eastern screech-owl, a surprisingly common bird on the Vineyard. And in open habitat, the plump meadow vole is a frequent target for the northern harrier hawk. The list of predators goes on and on — the small size of rodents make them vulnerable, but their fecundity allows them to tolerate the enormous pressure of being everybody’s lunch.
To be sure, rodents fit awkwardly with human preferences. Rats, with their willingness to eat anything and their close association with humans, truly are public health threats, and even the cute white-footed mouse is, as is now well-known, a host for immature deer ticks and a repository of Lyme disease. Disease issues aside, the abundance and voracious appetites of rodents result in considerable economic harm from spoiled food or damaged vegetation.
And yet, these small animals deserve appreciation for their resourcefulness. They play a vital role in the food web, converting plant material into a form that hawks, owls, and snakes can consume. And in their quasi-agricultural role as harvesters of seeds, rodents actively shape our landscape.
Despise them, if you must. But don’t for a moment assume they aren’t a critical cog in the Vineyard’s ecology.
