| Abstract | | The distribution, abundance and habitat characteristics of an alien species, Elodea canadensis, were surveyed in watercourses in Slovenia. The accompanying plant community was also examined. Distribution and abundance of macrophytes were assessed in reaches of different length, and habitat assessment is based on 12 parameters of the Riparian, Channel, and Environmental (RCE) Inventory. E. canadensis thrived in 132 out of 1,227 reaches examined and in 12 of the 39 watercourses surveyed. It grew in the company of a variety of species, most frequently with different species of Potamogeton. It was rarely found as the prevailing, and never as the only species in any reach. Canonical correspondence analysis (CCA) of reaches with E. canadensis revealed that the presence of detritus, retention structures and properties of the riparian zone explained most of the variance in community composition. Habitat analysis of well-established stands of E. canadensis showed that its preferred habitats were watercourses flowing through agricultural landscape, with a narrow, more or less disturbed riparian zone, with moderate presence of retention structures, and with sediment that was a mixture of gravel, sand and silt with either coarse or fine organic matter. It was not found in the parts of streams with the most dynamic flow, and was absent from watercourses in the karst region, due to the frequent and extreme water level fluctuations. The alien species E. canadensis did not express its invasive character in heterogeneous watercourses with rich macrophyte communities.
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