Beaver engineering makes a difference

Posted: Sunday, February 04, 2007

Some organisms physically modify the landscape they occupy; they are often called "ecosystem engineers." The activities of these engineers can have ecologically important effects on other organisms and even whole ecosystems.

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The effects can be either positive or negative.

A good example is the beaver. We say "busy as a beaver," but we commonly don't really recognize just how busy beavers are. They are ecosystem engineers extraordinaire.

Their most obvious activity is dam-building. Some dams are hundreds of feet long. Others are very tall. I once hauled my 16-foot canoe up and over a dam that was considerably higher than the length of my canoe - it must have been the work of several generations of beavers.

Beaver dams flood low-lying areas, killing most of the resident vegetation, and change free-running stream reaches to ponds of slow-moving water, with attendant decreases of stream flora and fauna.

On the other hand, beaver ponds create important rearing habitat for fish such as sockeye salmon, foraging areas for herons and kingfishers, mating and egg-laying sites for dragonflies and damselflies, as well as for frogs, toads, and salamanders. Dead trees in flooded areas create snags that provide nest sites for cavity-nesting birds and mammals.

The debris and sediment that accumulate behind a beaver dam gradually build up and fill the pond. Eventually, the beavers eat themselves out of house and home and then leave; the dam collapses, and the area is drained as the stream carves a new route.

The accumulated sediments are especially rich in certain nutrients, and the long period of saturation and low oxygen levels significantly change soil chemistry in the area.

The filled pond becomes a meadow with a wide array of herbaceous plants increasing the species diversity on the landscape.

Beaver meadows reportedly lack mutualistic mycorrhizal fungi, on which many trees depend for good growth, so the meadow stage of vegetation succession may endure for many decades.

Eventually, small mammal consumers of the necessary fungi may enter the meadow and deposit fungal spores in their scats, facilitating invasion by trees. Then forest may again occupy the site.

The duration of a beaver pond is typically measured in years or a few decades, but the ecological effects of the dam can endure for centuries, long after the beavers have moved on.

To build their dams, lodges, and stashes of winter food, beavers have to cut down lots of trees, chop them into manageable pieces, and haul the pieces to the building site or storage area.

However, not any tree will do. Beavers are selective loggers. In many regions, they favor aspen and cottonwood over other species, but not every aspen or cottonwood is equally acceptable.

The bark of various trees contains differing amounts of compounds that help protect the tree against herbivores - insects as well as beavers. One type of protective compound is tannin.

Tannins are feeding deterrents, and some tannins reduce digestibility by tying up proteins. Beavers avoid cutting trees with lots of tannins in the bark, when they have a choice.

By selectively harvesting mostly the low-tannin trees near their pond, they alter the composition of the remaining forest and the annual fall of leaf litter, and therefore change the community of decomposers, which do not feed equally well on all leaves, and the rates of decomposition.

Aspens and cottonwoods often send up sucker shoots from cut stumps, and these juvenile shoots tend to have more feeding-deterrent compounds than adult trees.

In short, beaver cutting induced the plant to put more defensive chemicals into its new shoots. As a result, beavers invading a new stream system encounter mostly adult trees with fewer defenses, and there they often prefer smaller trees.

But in an area long occupied by beaver, most of the trees are juveniles from sucker shoots with more defenses, and here the beavers tend to select larger, older trees. So harvest patterns change in direct response to previous harvesting.

Thus, beavers engineer changes in habitats, plant communities, decomposer communities, and soil chemistry.

By selectively removing certain trees, they also indirectly affect the foraging opportunities of other herbivores, including moose, porcupines, hares, voles, and many insects.

By providing new habitat, they allow new organisms to move into previously unoccupied area, increasing the species richness of the region.

Between the early 1600s and 1900, industrial trapping of beaver nearly exterminated the species from its formerly broad range in North America. Even though beaver populations have begun to recover, their population is far lower than it once was.

That means that many stream ecosystems have lacked their main native engineer for a century or two. And because serious scientific studies of ecosystems developed only in the past few decades, we don't really know the full extent of what beavers can accomplish and what their near-extirpation meant for the North American landscape.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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