If you've walked the Wetland Trail in the last month or so, you may have noticed an ever-increasing number of fresh-cut tree stumps near the water's edge, particularly in the last month. The construction project currently underway: a beaver dam, visible at low tide, being erected just below the trail bridge at the mouth of Hume Spring. The North American beaver (
Castor canadensis) is an important ecosystem engineer, more commonly associated with dams, ponds, and lodges on flowing rivers, but also known to build dams in tidal freshwater settings, as we see at Four Mile Run Park. And they have been making fast progress, well deserving of their reputation for being "busy beavers." The dam is now backing water up the Hume Spring tributary as intended, as can be seen from the photo at right (
photo courtesy of Russ Bailey).
Over the past few years, while the wildlife cameras we've installed in the wetland have captured photos of their nighttime activity, we don't yet see evidence that beavers are making the park a permanent home, though they probably have a lodge somewhere nearby. Very likely, the purpose of the dam is to make it easier, safer, and faster for them to access the wetland at low tide to support their diet of twigs, leaves, tree bark, and other plant material. Beavers are slower and more vulnerable on land, but they are fast swimmers, so they benefit from backing up water where, at low tide levels, the stream channel would otherwise be an exposed mud flat.
There may be benefits for other members of the ecosystem. Although there aren't many studies of the impacts of beaver dams in tidal freshwater, some research has shown changes in vegetation composition and increased fish abundance upstream of these tidal dams. And the tree chomping is more and more apparent. Fascinating changes happening at Four Mile Run -- stay tuned!
