"Voyage of the dammed" by Kevin Taylor doesn't mention one of the negative impacts of beaver — their indirect influence on native plants and animals when non-native species are present (HCN, 6/8/09). For example, beavers strongly prefer native cottonwoods over non-native salt cedar (tamarisk) and Russian olive. This selective foraging gives a substantial additional advantage to these invasive riparian exotics in competition with native plants.

In addition, beaver impoundments turn flowing streams into ponds, a habitat highly favorable to invasive fishes, such as sunfish and bass, which are a major factor in the decline of many Western native fishes. Before non-native fishes were introduced, beaver impoundments were good habitat for native fishes; now they are excellent habitats for increasing the abundance of non-native fish.
Beavers can provide many positive effects on Western wetlands by enhancing the natural functioning of riparian corridors. But before they are introduced, it seems prudent to first control invasive trees and fishes so that the positive effects of beavers are not overwhelmed by their indirect, detrimental effects on native plants and animals.
 
Phil Hedrick
Aravaipa Canyon, Arizona