Signs of Spring 7: Beavers

Photo by Steve, Wikimedia Commons

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I just finished reading an extremely enjoyable book by Frances Backhouse entitled “They Once Were Hats: In Search of the Mighty Beaver.” Backhouse discusses the evolution and ecology of the North American beaver (Castor canadensis) and also  explores its place in the cultures and mythologies of a wide range of Native American people. She also describes the beaver’s near annihilation during two centuries of fur harvesting and its ultimate survival due to the efforts of forest rangers,  game wardens, writers, con-men and charlatans, and repentant trappers. She also describes in great detail how to skin a beaver (not for the squeamish) and exactly how a beaver hat was (and is) made!

Beavers were once extremely abundant all across North America. Unregulated trapping in the Eighteenth and Nineteenth Centuries, though, nearly drove them into extinction. It is estimated that prior to European colonization of this continent there were 200 to 400 million beavers living in a wide range of habitats from far northern Canada and Alaska down into Mexico. Every stream small enough for beavers to dam had beavers  living in them. Even streams that were too wide or too deep for them to dam often had beavers living in streambank dens (like the “bank beavers” I described last year on the nearby Kiskimenitis River (see Signs of Winter 8, January 24, 2019)).

Beaver dam. Photo by Hugo.arg, Wikimedia Commons

Almost all of the streams in the pre-European settlement North America, then, were chains of beaver ponds. It is difficult to imagine how different the landscape and ecology of our continent were when beavers were the uncontestably dominant sculpting force! The life cycles and distribution and abundances of fish, birds, mammals, trees and other plants were all influenced by the beaver modified landscape. The term we use today to describe an organism that has such a wide ranging impact on the other species of its community is “keystone species,” and beavers without question filled the role of the keystone organism that held its ecosystem together. As David Thompson (a Hudson Bay Company trader and mapper) put it in his early 19^th Century memoir, “This continent … may be said to be in possession of two distinct races of being, Man and Beaver.”

At the start of the Twentieth Century there were only 200,000 or so beavers left in North America. Most streams, then, no longer had beavers and lacked the ecological benefits of fresh beaver ponds. Today, thanks to controls on trapping and the establishment of wildlife preserves, there are approximately ten to fifteen million beaver across North America, and in both wilderness areas and human settled areas beavers are once again building their dams and ponds sometimes to the great annoyance of landowners whose ecological amnesia is so profound that they are totally intolerant of submerged fields and lawns, and driveways and roads.

Photo by R. Stevens, cynic.org.uk

Beaver dams and ponds help to control high water flow rates in the spring, and they also store water to keep streams flowing in the summer. Beaver ponds also slow down surface water flow rates sufficiently to allow the surface water to percolate down into ground water. This ground water, then, helps to sustain wide areas of plant growth even in seasonally dry climates. Along the Colorado River, for example, sites without beaver dams experience water table declines of two meters or more during a dry summer. These declines effectively remove water from the reach of the surrounding vegetative community. This results in a shrinkage of the riparian vegetation zone down to a narrow band of green very close to the stream bed itself. In areas with beaver dams, though, dry season water table drops of only five centimeters are observed. This means that ground water is well in reach of plant roots over a wide area around the ponded stream. This allows extensive vegetation to flourish and sustain a wide range of animal life even during a very dry summer season.

Beaver ponds also improve water quality by facilitating  sedimentation and the sequestering and processing of pollutants (including bacteria from the feces of wild and domestic grazing animals and nitrogen and phosphates from agricultural field runoff). Beaver dams also back up the large water volume flows in the spring so that soils far above the riparian plane actually get inundated and receive nutrient rich sediments during the spring floods! The ponds also generate areas of open water that support a wide variety of birds, mammals, insects and fish. Beaver dams and ponds have been shown in numerous studies to be extremely important to the overall abundance and success of wild trout and salmon populations throughout North America.

Beaver pond. Photo by NPS. Public Domain

In her book, Backhouse illustrated the beaver’s keystone role in its ecosystems by telling the story of Eric Collier. Collier emigrated from England to British Columbia in 1920. He saw, then, the vast world of a still mostly unsettled Canadian wilderness from which the beaver had been extirpated. He listened to the stories of Native American elders that described the abundance of fish and game and migratory birds that used to exist in the surrounding river valleys. Eventually, Collier built a cabin along Meldrum Creek, a small stream with a long chain of beaver meadows, with the intent of making a living as a trapper. But, he quickly found out, there was almost no game in the valley to trap. So he dedicated himself to making the valley back into a vital, active ecosystem. He and his wife laboriously cut and hauled logs and cemented them together with loads of mud and slowly rebuilt some of the beaver dams. Eventually the ponds behind these dams refilled and soon flocks of ducks and then geese returned. Eventually, even the muskrat and mink came back into the valley.

Collier worked on these dams for ten years. A provincial game warden, hearing of Collier’s project, thought that the original dam makers might be a better way to rehabilitate the Meldrum Creek valley. So, he donated two pair of beaver and released them into two of Collier’s ponds. Within a few years, Meldrum Creek was alive with beavers and the dams and ponds were almost all re-established. In a few more years the beaver population became so abundant that Collier, with great reluctance, was forced to trap some of the beavers in order to thin out their numbers.

Beaver ponds are not static entities. They have an extended life of their own as they move through a cycle of change called “pond succession.” Inevitably, any dam, either beaver or human constructed, will accumulate so much sediment in their impoundment basin that that pond will get shallower and shallower and, eventually, solidify. The solidifying ponds first become marshlands and then increasingly dry terrestrial heathland, then grasslands and then maybe even forests often with distinctive organic-rich, peaty soils. Beaver ponds, then, will turn into beaver meadows during this pond succession cycle.

Photo by NPS. Public Domain

Beavers abandon their dams when their pond water levels become too shallow to guard the entrance to their den or allow them to safely swim from their dens to their feeding areas. These beavers will then move on along the stream to find a new place to construct a dam and build a pond.  These abandoned meadows, though, for a significant period of time will be treeless grasslands often surrounded by a forest of dense timber. Settlers found these meadows and used them as hay fields and for planting crops. They were instantly usable farmland all thanks to the efforts of long departed beavers.

As beaver numbers increase here in the 21^st Century, we are seeing them returning to more and more of the habitats of North America that they had once occupied and dominated. There are several areas, though, that beavers are seemingly unable to re-occupy including some of the most protected areas of our country, our National Parks.

Rocky Mountain National Park (from Bear Lake). Photo by D. Mayer. Wikimedia Commons

Rocky Mountain National Park was a rich habitat for beaver. Most of the original beavers were trapped and killed prior to the dedication of the park, but even as late as 1940, the park still supported a population of 600 beaver. By the 2000’s, though, there were only 30 beaver left. The reason for this was a chain of unexpected consequences that started with the decision in the 19^th Century to eliminate large predators (wolves and cougars) from the park.

I have talked about the war on predators before (see Signs of Spring 11, May 16, 2019 and Signs of Spring 3, March 19, 2020). What initially seemed like a good idea, a humane idea, to remove large predators from an ecosystem to make them safer for other animals (and for humans!), turned out to be a terribly wrong-headed move that frequently led to ecological disaster.

In Rocky Mountain National Park (and also in Yellowstone) the removal of wolves from the park caused the elk herd to greatly increase. These elk, made increasingly incautious because of the lack of predators, began extensively feeding on the willows and aspens (both trees and seedlings) along the streams (which are very exposed, visible sites they had avoided when predators were about). These willows and aspens were also the trees upon which the beaver relied for food. As the trees along the stream declined, so did the beavers. Without beaver to build and repair dams, the streams, especially in their spring floods, ran faster and faster and cut a deeper and deeper channel. Eventually, the channel became too deep for beaver. Attempts to re-introduce beaver to the park have failed because the beaver quite simply cannot build a dam or lodge on these deep, fast flowing streams.  The park ecosystem, then, has slid past an ecological tipping point and can now no longer support beaver.

There are so many lessons in the story of the beaver, so many things that we should remember. This semi-aquatic rodent changes entire landscapes and makes life possible for many other species.  When beavers are removed from their ecosystems those systems can degrade so extensively and so rapidly that soon the beaver, and all of its dependent species, will never again be able to live in the degraded site.  We are up against many “tipping points” right now with regard to habitat degradation, species extinction and climate change.  We need to take them all very seriously!