Environmental filtering effects of beaver recolonization on fish communities in mountain streams and their management...

The recolonization of the Eurasian beaver in the Carpathian Mountains after nearly two centuries of absence is driving substantial changes in the structure and functioning of mountain stream ecosystems. As a keystone ecosystem engineer, beavers alter hydromorphology, thermal regimes, and sediment dynamics, thereby creating novel environmental filters that influence fish community. However, the spatial extent and ecological consequences of these filters at the catchment scale remain insufficiently quantified. We combined Kohonen artificial neural network with Indicator Species Analysis to examine patterns in fish assemblages across 192 sites within a 919 km² mountain river catchment. We identified four assemblage types linked to stream characteristics and beaver engineering, three of which were defined by indicator species reflecting dominant environmental filters. Cold-water specialists indicated undisturbed, high-gradient headwaters with low temperatures, coarse substrates, and limited aquatic space. Ontogenetic niche shifts were evident, with trout young-of-the-year associated with environmental conditions distinct from those of juveniles and adults. Beaver-modified segments formed a separate assemblage dominated by species tolerant of elevated temperatures, reduced dissolved oxygen, slower flow velocities, and increased fine sediment deposition. These conditions excluded cold-water, lithophilous species, demonstrating that beaver activity alters both the strength and identity of environmental filters shaping fish communities. In the context of ongoing beaver population expansion and climate-driven warming, beaver-induced environmental filters are likely to play an increasingly important role in determining the distribution, persistence, and functional composition of cold-water fish communities in mountain river systems, with direct implications for river management and conservation planning.