Although beavers play an important role in the ecosystem, they can also cause problems that are sometimes more than a nuisance. Beaver dams can actually cause flooding. This can wipe out land that farmers need for crops or livestock. On the other hand, they create wetlands that absorb large amounts of water, which counteracts the effects of heavy rainfall and can prevent potential floods. In addition, beaver dams act as natural filters that keep sediment and toxins from flowing into streams and on into oceans.
For willow flycatchers, an endangered species that relies fairly heavily on wetlands and riparian systems, it is often thought that beavers are very important. The picture below is from Carmen Valley in Tahoe National Forest in California. This was a picture I took of a willow flycatcher territory in 2007--he never acquired a female but it seemed like a pretty sweet place to hang out.
I returned to the territory in 2008 to resight an unbanded male here, and again no female. I wasn't able to check it out in 2009 and didn't even know if there was a male here again. But just this past week I was in Carmen Valley again and took the following photo of the same territory:
Most of the willow is gone or dead, and at least 3 beavers were seen swimming in the area at the same time. So in this case, it seems the beavers did some damage. Maybe its just too many beavers for one area.
Where's the willow flycatcher you ask? Hes still here--I saw him (or at least someone) flycatching like crazy over his new lake front property and hes taken up residence behind where I stood for the photo where there are a few willows in some kind of drainage ditch. I'm curious to see if he stays--but I have no idea why he would. Carmen Valley, in my opinion, has several other great places with more willow that he could occupy. If he is really the same bird--is he that site faithful that he won't move after these drastic changes?