Beavers

While kayaking in a town wetland a couple months ago I noticed a slow-moving trail of bubbles ahead of me. As a herpetologist, I recognized the bubbles as a sign there was a Common Snapping Turtle walking through the debris at the wetland bottom, so I decided to follow the bubble trail toward shallower water to try and get a glimpse of the marvelous beast. As I approached, the bubbles moved away and my attempts to keep up with the animal became increasingly laborious. Bit by bit the pace of my pursuit increased until I came to the realization no earthly snapping turtle could possibly move so fast and, at the exact moment of this realization, the beaver I was pursuing tired of the chase. With a heart-stopping “THWUNK!” the beaver surfaced next to my boat and plunged the back of its tail hard against the surface of the water, drenching me in a most startling manner before vanishing into a cloud of sediments. It was actually the second time I had mistaken a beaver for a snapping turtle, the first having been a much more alarming encounter in which I saw what looked like the hind claw of a snapping protruding from a small cloud of sediments. Hoping to get some underwater video of the turtle I crawled out onto a log, hung my torso downward, and was about to reach my arm into the water with a GoPro when the beaver and I became fully aware of one another. Hilarity ensued.

Having an interest in freshwater turtles, I spend a remarkable amount of my time in close proximity to beavers. In some cases, the turtles and beavers are both simply drawn to the same wetland habitat, but quite often the turtles wouldn’t be there at all were it not for the beavers having arrived first. Beavers are true ecosystem engineers and apart from humans, there are no other species in which just a single individual can alter habitat to such a large extent. At a landscape scale, beaver populations help maintain a mosaic of habitat patches that promote and increase biodiversity. I touched on this subject briefly in my blog post last month about Eastern Newts, and while I think most people reading this will be aware that beavers build dams and create wetlands, not nearly so many people understand how important beaver wetlands are in a forested landscape.

 

Beavers are very industrious and many don’t build dams at all, but when they do the basic progression is pretty straightforward. First beavers build a dam across a small flowing stream and water backs up behind it. Most of the flooded trees die and many others are cut down by the beavers themselves. Gradually, wetland plants arrive and species by species a new ecosystem forms that includes many plants and animals not otherwise present in a forest or its streams due to their predilection towards standing water (ie: Wood Ducks, American Bitterns, Painted Turtles, etc). When one beaver dies another takes its place and their dams can be maintained and improved upon over the course of beaver generations, but eventually the dam will wash out and the wetland will drain, leaving a meadow in its wake.

The water held back by beaver dams does more than just create pond-like environments for wildlife. By slowing the water, beaver ponds trap and accumulate sediments from further upstream, having a cleansing effect on water quality. When the dam inevitably fails (an event that can be either sudden or gradual), most of the water within wetland will drain, leaving behind many years of accumulated sediments. Those sediments make up incredibly rich soils that are the basis for highly productive meadow habitat with the original stream running through the middle. These meadows are biodiversity hotspots within the forested landscape and are important places for many woodland species to forage.

Some wildlife species have more of a love/hate relationship with beavers. Wood Turtles, which are a semi-aquatic species that spends about half of its time in streams and half of its time in upland habitat, do not find anything particularly attractive about beaver ponds. In fact, Wood Turtle overwintering habitat can be made unsuitable by beavers, but in the future when a dam fails and the wetland drains, the resulting meadow provides some of the most valuable foraging habitat Wood Turtles are likely to encounter. So at a landscape scale, the presence of beavers helps to maintain a diversity of habitat types available to many species, including Wood Turtles.

 

While beavers do fell many of the trees in the areas they flood, many others are left standing and become snags that are important habitat to birds such as woodpeckers and flycatchers. The tree felling behavior itself benefits wildlife as well. Alders, for example, are a favored food of the beavers. After alders are cut down, the new shoots that form at the base of the trunk are valuable food to other species, such as moose, which do not forage on the mature trees. Furthermore, whenever a mature tree is felled by beavers, a high diversity of other plant life will grow in its former shadow, including fruit-bearing species like raspberries that attract many forms of wildlife ranging from birds to bears.

Ecosystem management requires knowing who the key players are there is no denying that in many forested environments beavers are critical to the survival of numerous other species. Study after study has shown that habitat altered by beavers has greater abundance and diversity of plants, birds, mammals, reptiles, amphibians, and insects. In some places where beavers were wiped out, reintroduction has become a very high priority to facilitate restoration on a larger scale. In Vermont, where I live, beavers were reintroduced back in the 1920s and most people today are completely unaware the species was ever absent. Personally I find it hard to imagine a landscape without the beaver and shudder to imagine how many fewer turtles there must have been back before the beaver’s glorious return. Even though beavers startle me half to death at least once or twice a year, it’s a comfort just knowing they are there.