The paper is devoted to an important problem of modern ecology, namely, the reaction of native ecosystems to the activity of invader species, with the European beaver (Castor fiber L) as an example. This species transforms the environment as a result of its foraging and constructive activities. Higher vascular plants (macrophytes) in water bodies and streams are one of the basic components of biotopes. They serve as a limiting factor for the spread of such invasive species as C. fiber, because macrophytes determine the basis of this animal’s diet.