Moose Browse and Beaver Ponds
In Alaska and the Yukon, the largest members of the deer family are high valued. Northerners and tourists alike enjoy moose watching, and these animals are certainly in demand by people hunting with gun or camera. Because of their popularity, moose may also be more closely associated with management than any other northern animal. We manage forest fires with an eye toward moose browse; we manage wolf populations to increase the hunters' odds.
The more we can base management decisions on science (hard numbers and well-established hypotheses) the happier we are: so there's a good bit of research on moose populations and the effect of wolf and bear populations on the number of moose in a given area. But what about the beavers?
An aggravating, but interesting, aspect of research is that the pursuit of answers often turns up dozens of new questions. A recent study of how moose browsing affects boreal forest soils and plant communities did just that; it raised a question about how moose and beaver affect one another.
John Pastor, a researcher with the University of Minnesota in Duluth, studied with colleagues the moose of Isle Royale, a good-sized island in Lake Superior. They looked at plants in exclosures (forest plots from which moose were fenced out), comparing them to similar, neighboring areas to which moose had access. During the study, they had to worry about which other plant-eating mammals might affect their plant counts.
In this case, snowshoe hares and beavers were the only such herbivores the researchers had to worry about. Because Isle Royale is a well-studied place, they could determine that hare populations had never been high enough to affect vegetation in the exclosures. And because the study area was mostly on high ground, there was only one record of a beaver-cut tree in or near the study areas.
Thus reassured, Pastor and coworkers could dismiss the other herbivores from their study, but a new question was raised. If more beaver were present, how might populations of moose and beaver affect one another when they share the same area? Their report, published in a recent issue of the journal BioScience, speculates on the answer.
They think it depends on densities. Beaver and moose both eat deciduous trees, but moose like shrub-sized plants, while beaver prefer good-sized aspens that provide both food and building material.
When a beaver colony moves into an area, the animals quickly harvest the larger aspens. Cut aspens send up shoots from their roots; shoots too small to interest the beavers, but perfect moose salad. With the big trees now gone, more sun is available for shrubs and saplings to start growing, and these too provide fine moose food.
So far so good--for both kinds of animals. The moose don't eat what beaver prefer, and their munching on the competing shrubbery can make it easier for aspens to grow to a healthy size. In return, the beaver's hard work leads to better browse for the moose.
But if there are too many moose, or if the resident moose stay put long enough, they may consume the young trees before any can grow large enough to replace the beaver's needed supply.
Too many moose (or, for that matter, too many beaver) can produce a spruce forest, and neither animal will harvest conifers. When aspens, birches, and willows are eaten away by hungry herbivores, young spruce thrive without competition. Once a spruce forest has taken hold, both beaver and moose must move on--until a forest fire wipes out the spruce and sets up conditions for the deciduous trees to return.
So, just possibly, moose and beaver are good for one another, as long as there are the right numbers of each relative to the habitat. At any rate, it's a logical speculation, and one that some enterprising graduate student may pursue someday soon: the symbiosis (interrelationship) between two species of moose and beaver. For optimal moose management, we may need to inventory beaver ponds as well.
