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Beavers: Landscape Engineers

Beavers: Landscape Engineers
Illustration by Adelaide Tyrol

When my sisters visit from Ireland, I try to play tour guide, but I’m occasionally at a loss for what to do next. During a visit in the late 1990s, my sister Grace said she would love to see a beaver. At that time, I lived close to a beaver pond and often quietly waited for beaver sightings. Alas, the rodents failed to cooperate for Grace’s visit, although she was able to see their engineering work. I was disappointed for her, but not surprised. Many of my own encounters ended with at most a fleeting glimpse, and a loud slap of a leathery tail on water.

When I returned to the pond years later, the beavers had departed – but the dam remained. Seven feet tall and made of sticks and mud, the dam had an upstream arch that spanned more than 50 feet of stream valley. According to Tom Wessels in his book, “Reading the Forested Landscape,” old beaver dams can last for decades. Wessels points out that beavers engineer more than mere dams, however.

“Beavers are the only animals, other than humans, who will create entirely new ecosystems for their own use,” he writes. “And often, like humans, once they have depleted an area’s resources, they will abandon their holdings and move on.”

Beyond dams and lodges, beavers sometimes dig canals to aid their movement, as well as to float saplings and limbs to stock their under-water larders. Some tree species die after being submerged in beaver-made ponds, becoming habitat for woodpeckers and other wildlife. Some favored food trees, such as big-toothed aspen, resprout from their stumps, producing early successional habitat and multiple delectable stems for beavers to eat. Eventually, when the beavers exhaust their supply of food trees within easy distance from their pond, they seek out new wetlands. In their wake, they leave an enriched ecosystem that benefits other wildlife.

In areas where beavers can resettle along the same water system, their ponds can serve as aquatic habitats for decades. Well established beaver populations provide a complex combination of active ponds, abandoned ponds, and beaver meadows in various phases of succession. These create a diverse set of habitats that increases biological diversity across the landscape.

Abandoned beaver ponds accumulate silt and fallen leaves, forming rich soil that eventually fills the pond basin. Light from the canopy gap and well-watered, rich soils support lush communities of grasses and wildflowers called “beaver meadows,” which store an abundance of carbon. This soil continues to build as grasses grow, live, and die. Beaver meadows may remain open for decades, even if the beavers don’t re-flood the area, due in part to a lack of mycorrhizae necessary for tree colonization.

Another important physical impact on the landscape, is the animals’ effect on groundwater. Beaver ponds are far deeper than undammed streams, and pond water saturates surrounding soils. This raises the groundwater table for some distance around the pond. The pond, together with the higher water table, stores a huge volume of water. During dry spells, water seeps from the pond and riparian water table to sustain streamflow. Rainstorms that might otherwise have quickly scoured and eroded streambanks recharge the pond and water table. Flooding from small storms is contained by the combined water storage capacity, and erosion caused by larger storms is reduced.

I was surprised to learn that beavers also live in – and engineer – salt marshes. In his studies of beavers in Washington State, researcher Gregory Hood found the animals constructing dams in tidal marshes that were submerged completely during high tide but retained water as the tide went out. These dammed marshes provided far more habitat for juvenile fish than similar marshes lacking beaver dams. Beavers sometimes pay a high price for their marine existence, however. In a 2019 article, Ben Goldfarb (author of the popular book about beavers, Eager) described beavers suffering and dying from salt intoxication after consuming too much sea water.

Although my sister and I didn’t see beavers during our rambles in Vermont, Grace may now have some hope for beaver sightings closer to home. In Scotland, the reintroduction of beavers in 2009 has resulted in an increase in lake levels, higher retention of organic matter in streams, and reduced flooding. In 2016 the Scottish government deemed the reintroduction a success, and in 2019 declared beavers a protected species. It seems Scottish beavers are having positive impacts, much like their North American cousins.

Discussion *

Apr 13, 2021

Thank you James for your kind and thorough comment!

You are certainly correct that beavers have negative impacts; many land owners would likely have less kind words for me than your thoughtful comments.

That said, by most estimates, beavers have returned to dramatically smaller population levels than their pre-European hunting numbers.  Doubtless, they have dammed far fewer streams in recent years than they did before European colonization.

By extension, many of those streams would not have been suitable trout streams in that earlier time frame.  Removal of beavers may well have significantly expanded brook trout range in streams. It’s interesting to consider.

And they certainly remove trees both directly and by inundating their roots.

So is the return of the beaver a correction to a more natural state? If yes, this begs a second question: do we like this more natural state? I don’t think I’m smart enough to fully answer that question to the satisfaction of all.

There are so many interesting questions….but only so much space in a 750 word essay.  You may enjoy “Eager” by Ben Goldfarb.  He covers some interesting ground….and far more of it than I did.
Cheers
Declan

Declan McCabe
Apr 08, 2021

The author of the article has done an excellent job of describing the pluses associated with beaver—- even mentioning some pluses I hadn’t thought about. However, it is unrealistic to say or imply that everything beaver do is a plus.  In substantial portions of New York State (certainly in the southwestern part of the State where I live) populations have expanded in many, many instances to nuisance or overstocked levels resulting in the killing of large desirable trees that are adjacent to their impoundments.  The need for a place to exist leads them to even take residence in existing ponds and lakes that benefit very little if at all from their presence.  They find highway culverts an easy place to establish an impoundment.  The State Division of Wildlife now routinely issues beaver destroy permits to private landowners and highway departments. After all, fur trapping has diminished to the point that it no longer seriously keeps beaver populations in check.

There’s another negative impact of beaver ponds that needs to be acknowledged. Wild populations of trout here in the mid-Atlantic States are more and more confined to the very headwaters of a limited number of stream systems especially if the stream systems are not of high gradient such as those found in more mountainous areas.  These lesser gradient (but not low- low gradient) headwater streams that historically had wild populations of trout are also where in my experience beaver populations prosper and proliferate quite successfully because their dams are less likely to get washed out (literally blown out) during periods of high runoff. Unfortunately the shallow impoundments created by beaver result in significantly warmer summer water temperatures—- warm enough so that cold-water-requiring trout can no longer survive in the stream except possibly in an even smaller piece of it.  Any significant fishery for these fewer remaining and typically small trout becomes essentially non-existent. The truth is that if there is no real trout fishery in these headwater streams, there is less concern for what is to become of any wild trout that they may hold.  And the situation for wild trout populations in general in many parts of the East can only get worse with climate change.

It may at first seem ironic that if you have beaver ponds on headwater streams in the Rocky Mountains, survival of wild trout is not in jeopardy at all (at least not at present) and you will in fact have some outstanding trout fishing in these ponds BUT the same is not true in much of the eastern United States because our streams are not fed in summer months by snowmelt.  Here in the mid-Atlantic region high gradient mountain streams and a relatively few lesser gradient streams notably fed by large year round springs are the principal hold outs for decent wild trout populations.

In conclusion, I will be among the first to say that beaver are of great benefit to a large array of species (waterfowl and wading birds immediately come to mind) but let’s also recognize that beaver can have negative environmental impacts too, especially when checks other than the amount of habitat available to them limits their populations.

James Pomeroy

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