The beaver is a large, semi-aquatic rodent known as an “ecosystem engineer” due to its dam-building activities. Beavers are strict vegetarians with a highly varied, seasonal diet. Their diet shifts dramatically between soft aquatic plants and the inner bark of trees, depending on the time of year and the availability of plant life around their wetland habitat.
The Preferred Summer Diet: Soft Vegetation and Aquatic Plants
During the warmer spring and summer months, beavers prioritize soft, herbaceous, and aquatic vegetation. This material is easier to digest and provides immediate nutritional value, making it favored over woody material when abundant and actively growing. Studies show that herbaceous material can account for up to 90% of a beaver’s feeding time during the summer.
The diet includes a wide array of plants such as water lilies, pondweeds, cattails, and rushes, which they access easily from the water. They also consume grasses, ferns, sedges, and the roots or tubers (rhizomes) of various plants found along the shoreline. The preference for non-woody material reflects its higher water content and lower fiber, making it a more efficient source of energy during the active foraging season.
Woody Material: Clarifying Bark and Cambium Consumption
While beavers fell trees, they do not eat the hard, indigestible wood or heartwood that makes up the bulk of the trunk. Instead, they focus on the nutrient-rich layer directly beneath the outer bark, called the cambium. The cambium is the living tissue responsible for the tree’s growth and is a concentrated source of sugars and nutrients. Beavers use their strong incisors to strip away the outer bark and scrape off the cambium layer, leaving the debarked wood for construction.
Beavers select specific deciduous tree species, including willow, aspen, birch, cottonwood, and maple. Aspen and cottonwood are often the most preferred species, though beavers adapt to other trees when necessary. They typically target younger trees and saplings because these have thinner bark and a higher ratio of palatable cambium to hard wood, making them easier to process. The twigs and leaves of these species are also consumed, adding to their nutritional intake.
Winter Survival: Food Caches and Seasonal Adaptations
As temperatures drop and waterways freeze, a beaver’s diet shifts to a greater reliance on stored food. Beavers do not hibernate, but they remain confined to their lodge and pond system under the ice. To sustain themselves, they create a large, submerged pile of branches near the entrance of their lodge, known as a food cache.
The cache consists of palatable branches from preferred deciduous trees, such as poplar, willow, and alder, which are cut in the fall and anchored into the mud. The submerged water keeps the inner bark fresh and accessible throughout the winter months. When hungry, beavers swim out from their lodge’s underwater entrance, retrieve a branch, and bring it back inside to consume the inner bark and twigs. These stored branches are the primary food source for up to five months.