Beavers at Acadia: the keystone species that builds ponds and wetlands

The beaver is a keystone species: take it out and an ecosystem can collapse, the way an arch falls when its top stone is removed. The National Park Service says that at Acadia National Park the North American beaver is considered one of the park’s busiest residents.

A beaver (Castor canadensis). NPS photo.

Ecosystem engineers

Beavers are often called “ecosystem engineers” of the forest. They keep building dams and lodges, carrying mud and stones in their fore-paws and timber in their teeth, and their dams turn small streams into ponds and lakes that other animals use. Ben Goldfarb, in Eager (2018), calls beavers continent-scale forces of nature, partly responsible for shaping the land Americans built their communities on. Francis Backhouse, in Once They Were Hats (2018), suggests that in a changing climate, water stewardship may prove to be the beaver’s greatest gift to humankind.

A beaver lodge at Witch-hole Pond, Acadia National Park. NPS photo.

Where they live

Beaver habitat is always near or in water, in riparian ecosystems that include streams, rivers and lakes, and near aspen, birch, poplar and willow, which supply both building material and food. A lodge may be surrounded by water or sit partly on land. It is made of branches and mud that set hard to protect the beavers from predators, has underwater entrances and houses two adults and their young.

Built for the job

The beaver is the largest rodent in North America, and it works mostly at night.

• Body and tail. Average body length is two feet without the tail, and weight is 35 to 60 pounds. A beaver can hold its breath underwater for about 15 minutes. The tail is flat, broad and scaly, around a foot long, and is used for balance, for swimming and for making noise to warn others of approaching predators. A two-layer coat has a dense grey undercoat and a rough outer layer that mats the hairs and traps air; oil from glands spreads over the guard hairs, giving the brown color and shine, and a new layer grows as winter arrives.
• Feet. The large hind feet are webbed for swimming and give stability on land while cutting trees, and they hold nerves that sense vibration from predators. The front paws have five widely splayed, hand-like toes for gripping logs and branches.
• Senses. The eyes have three eyelids that guard against twigs; a beaver sees clearly underwater but poorly on land. The small ears hear well, and in water they lie flat and a membrane seals the ear canal. Nose and ear valves shut underwater. A highly developed sense of smell is used to choose food and to recognize family members, and castoreum, from two glands at the base of the tail, grooms the fur and marks territory.
• Mouth and teeth. A flap in the mouth closes underwater and the lips close behind the oversized front teeth, so a beaver can carry food and building material without drowning. It has 20 teeth: 16 molars for grinding and four large, orange, iron-enriched incisors for cutting trees. The incisors keep growing all its life, so beavers keep chewing to sharpen them.

A beaver’s head in the water. NPS photo.

Detail of a beaver’s mouth and teeth. NPS photo.

What they eat

Beavers are herbivores and prefer wood from trees such as aspen, willow and cottonwood. They also eat aquatic plants such as pondweed and water lilies, along with grasses, algae, moss and various fruits and berries. They do not hibernate, so they cut and store large amounts of bark and roots in their homes for winter, when food is scarce.

A beaver swimming. NPS photo.

Family life

Beavers tend to work alone but live in family groups called colonies, made up of a monogamous adult pair and their young, called kits. Pairs mate for life, though a beaver whose mate dies will take another. Mating usually happens in the water from January to March, and gestation averages 120 days. Females usually give birth between May and June to two to six kits weighing, on average, 8 to 24 ounces. Kits swim from day one and usually stay with their parents for two years, and offspring start mating at two to three years old. Wild beavers live 16 to 24 years.

A parent and kit. NPS photo: Kent Miller.

Sources

• National Park Service, Acadia National Park, “Acadia’s North American Beaver: The Ultimate Keystone Species,” https://www.nps.gov/articles/north-american-beaver-acadia.htm; rewritten in hubnx's own words. The page quotes Ben Goldfarb, Eager: The Surprising Secret Life of Beavers and Why They Matter (2018), and Francis Backhouse, Once They Were Hats: In Search of the Mighty Beaver (2018); both are paraphrased here.