Effects of beaver expansion in semi-natural and modified watercourses in the Tatra Mts foreland, Polish Carpathians: implications for river restoration.

Following centuries of absence, beavers were successfully introduced to the Polish Carpathians in the 1980s and have since colonized watercourses of different size, degree of human modification and ecological status. In the foreland of the high-mountain Tatra massif, beaver colonization in the last 10 years has been particularly rapid in but its success, patterns and effects vary between highly dynamic large rivers with extensive riparian areas and smaller streams. In large rivers, beaver activity is focused on modification of adjacent floodplain areas, extensive use of vegetated islands, and ‘creative’ adaptation and use of large wood accumulations and particularly channelized structures in heavily changed reaches. In smaller, low-gradient streams, morphological changes introduced by beavers can be dramatic compared to channel size but largely depend on underlying substrate and interactions with local human population (including land ownership). In most cases, unrestricted beaver activity indicates the potential of these animals for rapid and spontaneous restoration of modified rivers and streams; however, its long-term impact on riparian forests should be monitored.