Guest Essay by Kip Hansen — 16 November 2023
Once in a while one comes across a really good bit of research – research that tells us something we didn’t already know. Cara Giaimo, writing at the New York Times, reports clearly in a piece titled “Leave It to Beavers? Not if You’re a Wolf.” It is about a paper published in the Proceedings of the Royal Society B – Biological Sciences, 8 November 2023, Volume 290 Issue 2010. The paper is titled: “Wolves alter the trajectory of forests by shaping the central place foraging behaviour of an ecosystem engineer”.
The title uses a catchy phrase — “an ecosystem engineer” — in place of the simpler and more direct “beavers”. However, it isn’t used just as click-bait. The authors, from the University of Minnesota and the University of Manitoba (Canada), are studying something a little more complicated than wolves and beavers — they study how central place foragers and prolific ecosystem engineers (beavers) change the nature of their immediate environments differently under predation by wolves.
From the Abstract:
“Wolves (Canis lupus) and beavers co-occur across most boreal ecosystems in North America and Eurasia. Wolves are the primary predator of beavers wherever the two species co-occur, and beavers are important seasonal prey for wolves. Although wolves are primarily cursorial predators, they often use ambush strategies to hunt and kill beavers. Still, a substantial proportion of predation is the result of opportunistic encounters]. Because wolves are apex predators and beavers are ecosystem engineers, wolf predation on beavers can have outsized ecological effects. For example, by killing dispersing beavers, wolves alter the creation and recolonization of wetlands, and in turn, alter all of the ecological effects associated with beaver-created wetlands. Logically, wolf predation could also have indirect ecological effects by shaping beaver foraging behaviour.”
“Central place foragers” (CPF) – a term used in the title of the paper — are animals that forage for food from a central place, in a big circle around a central point. Many honeybees are CPFs – collecting nectar from flowers in a certain area surrounding their hive. Beavers cut trees and drag them back to their pond that contains/surrounds their lodge.
Thomas D. Gable and his co-authors study the predation of beavers by wolves and the effect that that predation (or the beaver’s fear of predation) on the forest environment in the immediate area of a beaver lodge.
“We searched 27 741 clusters of GPS locations from 51 wolves during 2015–2022. In doing so, we identified 543 wolf-killed beavers and 1909 instances where wolves attempted to ambush beavers. Of the 543 wolf-killed beavers, 135 (25%) were killed on feeding trails and we recorded the length of feeding trails at 128 of these kills. Of the 1909 ambushing attempts, 949 (50%) were at feeding trails. All other kills and ambush attempts occurred at other beaver features including beaver dams, scent-mounds, feeding canals and lodges.”
As you would suspect, the further a beaver travels from his lodge and home water, along established paths created by the dragging of this food supply back to the lake/pond, to forage food from a limited supply of appropriate trees, the more dangerous it becomes for the beaver in the presence of wolves. If the pond is close, the beaver can dash back into the protection of the water upon becoming aware of a wolf in the area. But at further distances, the researchers found that wolves tend to more successfully ambush the beaver along the path that leads back to safety.
Of particular interest is the elimination of beaver forage trees near the ponds. It appears that the beaver have eaten themselves out of house and home in panel (a). They dare not travel further than the white line in (c) and (d) for either fear of, or actual, predation by the wolves. The more wolves, the smaller the circle. You can see that the beavers do not cut the conifers (pines, fir, spruce). But they cut nearly everything else, almost denuding the shores of their ponds. That then gives conifers a chance to begin or continue to grow and eventually take over the shores of these current or ex-beaver ponds.
In (b), you can see stands of aspens, a preferred food for beavers, at the edge of the are they have already cleared. The distance to those aspens is between 20 and 30 meters – possibly representing the furthest distance a beaver feels safe to travel from the safety of that pond.
We already know that beavers create meadows by flooding areas. The trees in the new pond die (drown). The new pond then, over time, silts up, and eventually become a new meadow. Trees and bushes that are not shade-dependent then begin to sprout up in the meadow, and on it goes.
So, here we have the larger environmental situation: The beavers create ponds by flooding low lying areas through the damming of streams. Then the beavers begin harvesting preferred trees for food…but only up to a distance that is safe enough to limit the predation by wolves. After some time, the shores of the beaver pond become denuded of forage trees but not conifers which allows additional conifers to take root and grow in a “fairy ring” circling the pond. At some point, the pond is no longer tenable for the beavers – they have consumed all the appropriate trees within safe foraging distance and the beavers must move on to another area where forage is available. There they build dams, flood the area, and begin to forage the shores of the new pond and on it goes.
I have emailed the corresponding author asking if they see that last effect—multiple conifer-ringed abandoned beaver ponds—along streams and ponds.
I’ll let you know what he says.
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This little study was not so little, and depended on data kept by the Voyagers National Park rangers on beaver pond locations—17 years worth. And seven years of GPS collar location tracks for wolves in the area, follow up by physical inspections of suspected ambush sites. Add to that trail cams near beaver paths for two further years. Terrific field work.
Interesting to me is the data on attempted kills vs successful kills. The wolf seems to have gotten the beaver only about 1/5th of the time.
I like beavers – but then, they haven’t flooded my land. My wife visits the beavers every few days, counts pups, etc. from a convenient railroad bridge that spans their pond. But not everyone appreciates those busy little buggers. I have a neighbor that had beavers flood 5 acres of high value property – the state authorities forbade him to remove the dam or the beavers.
The featured image is by Derek Otway via unsplash.
Let’s hear your beaver Wows and Woes in comments.
Thanks for reading.
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