The Beaver — Nature's Hydrologist, Part 2

Excerpted from: Water - A Natural History by Alice Outwater, 1996.

North American Beaver, Castor Canadensis

Beaver Engineering

The beaver is a clever engineer, but its brain is embarrassingly small — smooth and un-convoluted except for the well-developed olfactory lobe. The beaver's ratio of brain size to its body weight is the lowest found among mammals.

Beavers don't have much gray matter and they don't see well. However, there is abundant evidence that much of the building technique appears to be learned during their long childhood.

Virtually all beavers build dams and behind each dam the water slowly backs up and cover the land. A rush of insects, animals and plants transforms this sheet of water into a place where every level, nook and cranny is teeming with life.

Environmental Transformation

Virtually all beavers build dams and behind each dam the water slowly backs up and cover the land. A rush of insects, animals and plants transforms this sheet of water into a place where every level, nook and cranny is teeming with life.

Ecologically, wetlands are an example of an ecotone — a transition between two diverse communities. Uniquely, an ecotone contains organisms native to each overlapping community as well as organisms characteristic solely of the ecotone itself. This so-called edge effect — the increased variety and density of community junctions — is what makes wetlands so productive. The beaver's role in this system is to build the dams that make wetlands, increasing the edge between waterways and dry land.

What Is A Wetland?

Beaver created wetland

In a wetland, the food web is dense and the niches are varied. The crush of insects, animals and plants in the wetland is directly attributable to the lowly algae, the primary producers at the base of the food web. Every tablespoon of wetlands' water is crowded with millions of organisms that make u a highly diverse planktonic community. Also found are excretions, feces and corpses along with the debris that washes into supension from the surrounding land.

The plankton includes — besides the algae, or phytoplankton — zooplankton and bacteria. These are respectively the plants, animals and scavengers of this small kingdom.

Phytoplankton can be seen as analogous to the grass that grows in the meadows, and zooplankton as the beasts that graze on them. the web of life in a wetland provides something for every appetite. Zooplankton make up the 12-cource meals that the insects eat.

Cleaning Water and Removing Pollutants

The wetlands' underwater world does a remarkable job of cleaning the water. Not only to planktonic bacteria consume the water's organic contaminants, trillions of tiny phytoplankton use the inorganics to make food. The water is also cleaned by sedimentation. When muddy water from streams and rivers rushes into the stillness of the wetlands, the silt in the water adheres to the stalks of water plants and settles to the bottom. Wetlands thereby clarify water and prevent the soil from washing downstream. The area where a stream once ran becomes covered with a rich blanket of organic matter.

Water Storage

Graphic of North American beaver anatomy

When beavers make a series of dams and ponds within a drainage basin, the water cycle in the entire watershed is affected. Wetlands act like a sponge, soaking up water during storms and releasing it slowiy at drier times. The wetlands that ring a beaver colony's dams and ponds can hold millions of gallons of water. When streams are swollen and muddy with melted spring snow, wetlands reduce flooding and erosion downstream by absorbing much of the excess flow. By providing a flexible reservoir, wetlands reduce flooding from summer storms as well.

When herbaceous plants are actively growing, they make up much of the beaver's diet. In the winter, beavers switch to woody plants and the food they have stored over the winter. Beavers do not necessarily use the same trees as construction material and as food. Inedible material is more likely to be used as the cap of a beaver family's food cache, the upper part which is frozen in the ice, while the cache itself is composed of edible, high quality branches, which remain unfrozen and accessible

North American Beaver Loss

19th Century Beaver Tophat

19th Century Beaver Tophat

When Europeans came to America, they trapped beavers by the millions to ship beaver pelts to European countries, especially for making hats. The beaver loss was dramatic. Since the arrive of Europeans, the beaver population of the United States has dropped from perhaps 200 million to 10 million. If each of the lost beavers had built only a single acre of wetlands, then an area of more than 300,000 square miles — a tenth of the total land area of the United States &mdash was beaver-built wetlands. Now these wetlands are gone.

When the beavers were removed, their old dams slowly collapsed and the streams were released from the series of ponds and impoundments. Each watershed lost wetlands and the water that had once seeped quietly down to the aquifer now flowed to the sea — and much more rapidly. Some of the springs and freshets that had bubbled through each watershed began to dwindle, while others disappeared entirely. The water table soon dropped and wetlands disappeared.

Not only was there less water in the land but the water quality changed for the worse.

Today, the beaver has returned in part, but its numbers are nothing like what they once were and we have forgotten that beaver wetlands once enlivened the now arid rangelands of the West.

Living With Beavers

Having a family of beavers on your property can be an amazing benefit, but as with any human/wildlife interaction it can have its problems too. Depending on how close your house is to a stream, pond or basin flooding could be an issue. If you tear down their dam they will only come back more determined than ever. If you put a hole in the damn or take down a part of it, the sound of flowing water will trigger them to fix it. One solution is to put a pipe called a "limiter" above and below the dam. The pipe will allow water to slowly and quietly flow through the dam while still keeping enough water behind it for their lodge, protection, food cache and to maintain the new ecosystem. This will require the use of a professional.

Beavers also have a talent for clear cutting expensive, landscaped trees. Willow, aspen, birch and maple trees are among their favorites. Depending on how close your house or other structures are to large trees, falling trees can be another concern. A solution to these probems is to pick which trees you want to save and wrap a 3-4 ft. barrier of galvenized welded wire around the tree leaving enough space between the fence and the tree so that the beavers cannot reach it. This will prevent them from girdling or cutting down your valued trees while leaving others for them to use for building and food supplies. Beavers are not very good climbers so fencing in your yard is another option.

If a happy medium cannot be reached and you are adamant about wanting beavers off of your property, call a professional trapper who will humanely catch and release them into a different stream system several miles away. There is no need to resort to fatal measures. Make sure to call a professional and do not attempt to do it yourself because beavers are not small, weighing in at up to 60 pounds, and can be aggressive when threatened.

With a little patience and a little work, humans and wildlife can live peaceful lives together.

Further Reading:

 Beavers — Nature's Hydrologist, Part 1
 Garter Snakes — The Gardener's Friend
 Wisconsin Native Salamanders
 Goundhog or Woochuck: All The Facts
 Voles, Both The Good and The Bad