Guest Essay by Kip Hansen  —  23 November 2023

In my recent essay on beavers and wolves, I wrote about a study by Thomas D. Gable, Sean M. Johnson-Bice, Austin T. Homkes, John  Fieberg and Joseph K. Bump, titled “Wolves alter the trajectory of forests by shaping the central place foraging behaviour of an ecosystem engineer”.

The basic finding of the study was that, in Voyageurs National Park, the interaction between beavers, which are a central place forager species, and their major predator, wolves, creates a certain definite change to the forest.  In short, it creates areas that look like this:

From the images supplied in the paper, I posited that the beavers might have “eaten themselves out of house and home” in those ponds now entirely surrounded by inedible-for-beavers conifers and subsequently the beavers had to move on to other streams and ponds.   Some readers thought this reasonable and some objected to the idea. 

As the specific issue was not discussed in the paper, I said:

“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.”

Dr. Thomas Gable, the corresponding author, has kindly responded with many new images and his thoughts on the matter (which he has given me permission to quote):

“…your point about abandoned ponds is spot on, and that is precisely what we do see in Voyageurs. Most wetlands in the area are not occupied by beavers. For instance, we digitized a little over 7,000 beaver ponds from aerial imagery in our study area, which is about 1,500 square kilometers (excluding large lakes). Beaver density in the area is about 1 lodge/km^2 meaning there about roughly 1,500 beaver lodges (i.e., occupied ponds) or so in our study area, meaning that there are far more unoccupied ponds than there are occupied ponds! I have attached a marked up aerial image showing this using the results of our annual beaver survey last year (that is how we know which ponds are active).”

Dr. Gable also offered this insight: I would also suggest checking out this photo and post by the Wolves and Moose of Isle Royale who had a nice image of this posted online recently.”  I include a copy below –  but without the comments):

This image is much more dramatic with the fall colors setting off the conifer rings created by beaver central place foraging under predation of wolves.

Tom Gable also offers this further clarification:

“However, it is worth noting that conifer rings are not the only outcome of beaver foraging necessarily. As we note in the paper, although beaver foraging can push forests toward a coniferous composition, beaver foraging can also reset forest succession and promote the growth of early successional species (aspen, birch) or can push forests toward a composition of deciduous trees less preferred by beavers. So one would not necessarily expect to see a conifer ring around every wetland or to have that conifer ring to be uniform around wetlands.”

There are six additional images but no more data really, just for their beauty:

Again, fall images, making the conifers stand out.  At each pond, the ‘safety zone’ for the beaver seem to be of different sizes – maybe due to the time span of beaver colonization or increased predation at each site.

I’d like to mention that my interaction with Dr. Tom Gable is how science journalism should be done — with journalists asking pertinent questions and researchers freely communicating their views and supplying additional information. 

So, kudos to Dr. Gable and his team.  Nice work!

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Author’s Comment:

It is so nice to get straightforward answers to one’s questions.  And  to receive extra material (ideas and images) that help explain and expand our understanding.

In the previous essay, one commenter was quite insistent that the major thing about beavers that must be talked about is the intensive slaughter of North America’s beaver population, and the same for wolves. Of course, to discuss the large-scale population dynamics of both species one must include the fact that humans hunted and trapped both to near extinction.  But in that, and this, essay the research and my reporting was on the narrow issue of the effects on the environment of beaver/wolf interaction.

Thanks for reading.

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