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Rodents and forests

Northern Lower Michigan forests are inhabited by many species of flora and fauna. This biodiversity is generally correlated with complexity. More complex forests tend to be more species rich, with some forest ecosystem types generally more complex than others.

Understanding how complexity varies among different forest ecosystem types and putting this information into management are keys to maintaining well-functioning forests and reaping the ancillary benefits to biodiversity and society. Increasingly, forest planners and managers are realizing that they must understand how forests naturally work if they are to manage them in an ecologically sustainable fashion. Different types of forests need to be managed differently to maintain complexity, especially in their wildlife communities.

“Wildlife” is a colloquial term for non-domesticated animals. In most instances, the term wildlife refers to species of animals with backbones (vertebrates) that are of human interest because they are game species (e.g., white-tailed deer, ruffed grouse), are of aesthetic appeal (e.g., common loon, bald eagle, many songbirds), or are considered (by some) to be pests (e.g., American beaver, species of mice).

While black bear, elk, white-tailed deer, coyote, and bobcat may fall into the category of “charismatic megafauna,” the majority of our native wildlife are relatively small and nondescript. In fact, of the approximately 52 mammal species found in Northern Lower Michigan, 21 species (40%) are rodents (taxonomic order Rodentia, see “Mammals of the Great Lakes Region,” 2001).

Our largest rodent, the American beaver, is considered to be a “keystone species” — the presence of this species directly impacts habitat for a broad suite of other species. While beaver do kill trees in forests for food and building materials for lodges and the dams that create beaver ponds, these actions epitomize forest complexity. In fact, data from northern Minnesota presented in “Beavers: Boreal Forest Engineers” (2017) suggested that 124 species of native wildlife used beaver ponds. If a forest landowner is really interested in biodiversity, then they should appreciate most (perhaps not all) that beaver do.

Smaller species of rodents are also important for maintaining forest complexity. Deer mice, for instance, can be very abundant across some forest ecosystem types, especially oak forests. As such, deer mice are a common prey for a diverse array of predators, from raptors such as broad-winged hawks and great horned owls to other mammals such as foxes and many weasel species.

More surprisingly, however, is that deer mice themselves act as predators. For instance, different developmental stages of the exotic, invasive gypsy moth are often eaten by deer mice. In a paper in the journal Environmental Entomology (1974), data from New England suggested that small mammals, especially deer mice, preyed heavily on gypsy moth pupae, especially female pupae. After gypsy moth cyclical population eruptions occur, rodent predators, insect predators, parasites, and fungal and bacterial pathogens lag behind in abundance. Ultimately, these predators and pathogens reduce the number of gypsy moths until the next outbreak.

Deer mice can also influence a forest by preying upon the seeds of trees. In a paper in the journal Ecology (2010), data suggested that deer mice reduced both the emergence and establishment of seedlings in burned and unburned conifer forest. In other words, deer mice predation on conifer seeds generated a patchy pattern of tree regeneration and promoted forest complexity, not uniformity.

Another study in southern Michigan published in the journal American Midland Naturalist (1976) documented how red squirrels and fox squirrels can consume spring swelling buds of deciduous trees. Maple buds, in particular, were consumed when other foods (such as acorns and other nuts) were not found or were in a limited supply. Having a diversity of tree species in the forests provides these rodents with alternative food resources, especially during late winter and early spring.

Wildlife habitat has a species-specific definition. In other words, each wildlife species has its own habitat. When we speak of forest biodiversity, we therefore either need to talk about habitat among species individually or talk about expected forest patterns based upon how different forests naturally develop over time. Thus, biodiversity conversations often involve consideration of “ecosystems”: biological entities and their abiotic (non-living) environment.

The general consensus from the forest ecology literature is that most forest management activities tend to simplify forests, so explicit consideration must occur to devise ways to maintain complexity when forests are managed. Fortunately, considerable growth of our understanding of how to manage complexity has occurred in the past 30+ years. Transferring this information to private landowners must be a priority for all those involved in forest planning and management.

For more information on Michigan’s mammals, please see iNaturalist: https://www.inaturalist.org/projects/mammals-of-michigan.

Greg Corace is the forester for the Alpena-Montmorency Conservation District. For more information, including sources used in this article, Greg can be contacted via email (greg.corace@macd.org) or phone (989-356-3596 x102).

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