Beavers: The Keystone of North America
Before the arrival of the first people to North America, meandering rivers braided through the landscape, pooling up within earthen and brush dams and gently spilling into vast spongy wetlands. The slowly moving water hydrated otherwise barren land and fostered a vibrant ecosystem, legions of grazing bison, vivacious waterfowl, and one particularly adept hydrological engineer—the beaver.
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The North American beaver, Castor canadensis, emerged around 1 million years ago and quickly spread across the continent, from Alaska to the Florida panhandle. Where there was fresh water, there were beavers.
When Europeans arrived in North America, the naturalist Ernest Thompson Seton estimated that up to 400 million beavers busily built as many as 65 million dams in waterways across the country.
With the arrival of the Europeans also came a rapacious demand for beaver pelts.
A fur trader dispaying his catch. Photo credit: North Dakota State Historitcal Society
Beaver pelt hats were de rigueur at the time, not just among earlier European settlers but also aristocrats across the pond. European merchants set up trading posts throughout North America. By the 19th century, the fur trade became one of the primary economic drivers in the US.
Also, by the 19th century, beavers were nearly extirpated across the entire continent. Like many good things, we don't always see their true value until they are gone.
Beavers arguably have the most outsized impact on their environment than any other creature. They are the quintessential keystone species.
Any architect or builder will expound on the importance of a keystone, the wedge-shaped piece that sits at the apex of a stone arch. Without the keystone, the arch collapses. In ecology, the keystone species is of equal significance.
Using their gnarly orange incisors, beavers whittle away at riparian trees until they topple over. They then place felled trees and the occasional stone in streams and rivers to form dams. The dams create ponds, which spill into wetlands. The result is a new ecological niche where aquatic and terrestrial species thrive.
In the intermountain western US, wetlands comprise about 2% of the land area, yet these spongy oases host 80% of the region's biodiversity. As a consequence of the fur trade, the lower 48 states lost over 1 million acres of wetlands.
Beaver-built wetlands are not only valuable for their contribution to biodiversity. Wetland systems are an essential balancing force in water dynamics. They provide a steady inflow of water through groundwater recharge while mitigating flooding and sedimentation. Wetlands capture overflow from spring snowmelt and rain and release it back into the landscape during the dry summer.
Two sections of Sevenmile Creek, Utah — where beavers have been active (left) and where heavy grazing is allowed, effectively deterring beavers. Photo credit: Stacy Passmore
The beaver ponds that feed wetlands play a similar role. Each pond holds an estimated 3.5 acre-feet of surface water or 1.1 million gallons. The weight of that much water forces an even greater amount into the underlying aquifer. This water resurfaces downstream, feeds otherwise dormant springs, and provides much-needed irrigation for agriculture.
In the US, our most significant food production hubs rely on groundwater. The Ogallala aquifer brings life to agriculture across the great plains, representing 30% of all crop and animal production in the US. The aquifer's water supply has dropped by as much as 75% and is on pace to dry up in the next 70 years.
California's central valley is facing the same fate. In the last 100 years, the Central Valley extracted more water than was replenished by more than 122 million acre-feet or 45 trillion gallons. This area produces a cornucopia of crops totaling 25% of the nation's food production.
With the same enthusiasm ecologists laud beavers, farmers and ranchers tend to vilify them.
Beavers and agriculturists gravitate towards the same terrain. Some of the most fertile farms in the world are along rivers and streams in flat valley bottoms. These are the same rivers and streams that beavers instinctively dam to create wetlands in the adjacent valley plains. Besides the spontaneous flooding of farmers' fields, beavers are also known for rerouting irrigation ditches. Farmers respond by trapping and removing beavers. The hydrological consequences of expelling beavers are not immediately apparent as they slowly ripple through the landscape.
Ecologists and land managers have tried to reintroduce beavers into now brittle landscapes. In the late 1940s, beavers were parachuted into remote backcountry in Idaho. Today, beaver dam analogs (BDAs) are a more accepted approach. BDAs are manmade beaver dams strategically placed within streams. They are used to kickstart the beaver colonization process and, more often than not, attract new beaver residents.
Despite these efforts, there is still a fundamental disconnect between how humans understand water and the wizardry with which beavers govern waterways. Until we can let go of micromanaging nature for narrow outcomes, straitjacketing waterways, and controlling the overland flow of water with concrete, we will continue to be at odds with the hydrological genius of beavers.
Beavers built the complexity and resilience of early North American ecosystems. More than ever, our landscapes need to return to the rich life-supporting habitats only beavers can create. The first step is to recognize them as our allies.
