Arb Notes for Feb 17th - Beaver Fever

February 20, 2012 at 3:46 pm
By Owen McMurtrey


Beaver with Big Tree 

Two weeks ago, when the Arb naturalists were joined by retired Arb Manager Myles Bakke for a winter turkey-tracking outing, I also, for the first time, bore witness to some remodeling undertaken by a family of beavers.  

During the fall, a group of beavers moved into the floodplain forest at the end of the dike trail in the Lower Arb.  There are likely several beaver families working throughout the Arb, along the Cannon, Spring Creek, and occasionally in small ponds in the floodplain, but I certainly haven’t ever seen a group of beavers do so much work in so little time. There were several stumps from downed trees greater than a foot in diameter. One stump was so large that my arms probably would not fit around the base.  Seeing the trees, now laying ignominiously in the mud covered from top to bottom with gnawing impressions, I imagined an army of beavers overwhelming and subduing these giants in Lilliputian fashion under cover of darkness.  Actually, it may have just been one beaver working on and off for as little as a few days.

But why would the beavers bother to take down such large trees? Isn’t it possible that the tree would crush them? Myles indeed claims he has seen evidence of beavers crushed by the very tree they were gnawing upon. The bigger the tree, the greater the danger.  But sometimes the danger is worth it - they drop such large trees because a beaver’s diet is made up almost entirely of bark, particularly during winter.  By bringing down a few big trees, these beavers have developed a food cache that will last for months.  While beavers often create dams that significantly alter water levels on small rivers, these beavers have only constructed a small lodge that seems to have a small impact on the flow of water in the Cannon.

Sad Beaver 

So how much wood does a beaver actually eat? According to estimates by Shaler Aldous, a scientist who studied beavers for the U.S. Biological Survey in Ely, MN in the 1930s, beavers eat between 22 and 23 ounces of wood per day.  A 5.2 inch diameter alder provides 580 ounces of bark and twigs for a beaver, enough for one beaver for three weeks.  A tree with diameter of ~18 inches, if fully utilized as the ones we observed had been, could provide food for the entire winter.

-Owen McMurtrey, ’12, on behalf of the Cole student naturalists.

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