What if beavers built no dams?

Beavers chew down small trees with their big front teeth.

They use the sticks to build dams across streams. What if the beavers were gone?

A beaver is a large rodent with a flat, scaly tail and front teeth that never stop growing. It eats the bark of trees like willow and aspen, and it builds dams of logs, sticks and mud across streams. Behind a dam, the stream fills up into a pond.

This lab asks: what if a forest had no beavers, so no one built the dams? First we look at the beaver's place in the food web. Then we look at the job a food web cannot show.

The North American beaver (Castor canadensis) is a herbivore that feeds on woody plants in winter and on herbaceous and aquatic plants in summer. It is also an ecosystem engineer: it dams and diverts streams and fells trees, and its dams, lodges, burrows and canals mark wherever it lives.

This lab removes the beaver from a feeding-link model of a Yellowstone-style forest and compares the result with what agencies and university researchers describe in real landscapes. The gap between the two is the point of the experiment.

In Olly's forest, the beaver eats willow and aspen trees.

In real forests, beaver dams make ponds. Fish, birds and many other animals live in them.

In our model forest, the beaver eats only one food: willow and aspen. Nothing in the model hunts it.

In real life, beavers do much more than eat. At Yellowstone, the National Park Service says the ponds and flooding beavers create help willow and aspen grow, and those plants are the beavers' favorite winter food and building material. In New York's Adirondack Mountains, a beaver family, called a colony, has five or six members on average.

In the model's 13-species forest web, the beaver has a single food link (willow and aspen) and no predators. With nothing removed, the model reports 0 species affected.

Real beavers alter their surroundings far beyond that link. NPS describes Yellowstone's beaver as a keystone species that affects habitat structure through damming and diverting streams and felling trees; the resulting ponds favor willow and aspen. SUNY ESF reports Adirondack colonies of 1 to 12, averaging 5 to 6, each with an effective territory about 805 m in radius.

Olly takes the beaver out of the food web.

The model finds just one change: more willow and aspen, because no beaver eats them. Is that the whole story? Let's see.

Remove the beaver in the model and only one species is affected: willow and aspen could become more common, because the animal that ate them is gone. Nobody else changes, because nothing in the model eats beavers.

Try it the other way round. Take out the willow and aspen instead and the beaver could disappear, because it has no food left. That affects 5 species. The beaver depends on the willows, but in the model the willows hardly depend on the beaver.

With the beaver removed, the model's only response is top-down release: willow and aspen could become more common. Gone 0, fewer 0, less food 0; 1 species affected, the smallest cascade of any animal in the forest web.

Removing willow and aspen instead shows the dependency running the other way: the beaver loses its only food and could disappear (wave 1), while insects, snowshoe hare, elk and songbird find less food (wave 2), 5 species affected. In feeding terms, beavers look nearly unimportant. The next section explains why that is misleading.

Olly's model only knows who eats whom.

It does not know about dams and ponds. But the ponds are homes for lots of animals!

The model counts only meals, so it misses the beaver's other big job: building. A dam holds back water and makes deep water for the beavers, and their tree felling and dam building create wetlands. The U.S. Fish and Wildlife Service says these beaver wetlands give homes to many other fish and wildlife.

That is why the model says willows would do better without beavers, while the Park Service says beaver ponds help willow grow. Both can be true at once: beavers eat some willows, and their ponds and flooding help make a place where willows do well.

The model's rules work only on feeding links (plus willow as songbird nesting habitat). Ecosystem engineering, physical modification of habitat that changes resources for other species, has no representation, so the model's 'willow more common' result runs opposite to the NPS observation that beaver ponds create conditions favorable to willow and aspen.

The model also ignores beavers' predators (SUNY ESF lists coyotes, bobcats, foxes and occasionally large birds of prey), changes in water tables and streamflow, and the many wetland species absent from the web. It is the right tool for asking who eats whom, and the wrong one for asking what beavers do.

Some beaver dams are very long and tall. The ponds behind them can be huge!

People now help beavers return, so their ponds can help other animals.

Beaver dams can be huge. In New York's Adirondack Mountains, SUNY ESF describes dams up to 90 m long and up to 3.6 m high, holding back ponds of up to 80 hectares. The U.S. Fish and Wildlife Service says beavers help conserve water during droughts, and that nearly half of North America's threatened or endangered species rely on wetlands.

Yellowstone's aerial surveys counted 49 beaver colonies in 1996 and 121 in 2024. The Park Service says better counting explains part of the rise, and that the increase is likely linked to the return of willows, at least in the north of the park.

SUNY ESF describes Adirondack dams from flimsy latticework to structures 90 m long and 2.4 to 3.6 m high, impounding ponds of 0.4 to 80 ha. USFWS calls beavers nature's engineers, whose dams provide deep water for protection, food access and underwater den entrances, and whose wetlands house many fish and wildlife species; it adds that wetlands support almost half of species on Earth and that nearly 50% of North America's threatened or endangered species rely on such aquatic environments.

Yellowstone's parkwide aerial surveys counted 49 colonies in 1996, 127 by 2007, and between 51 (2017) and 121 (2024) since 2011. NPS links the long-term increase partly to better counting and dispersal from a 1986–1999 release of 129 beavers north of the park, and likely to the resurgence of willow since the late 1990s.