Drivers of plant invasion at broad and fine scale in short temperate streams.
Riparian ecosystems have been described as highly prone to alien plant invasions; thus, disentangling the contributing factors of the invasion process is of utmost importance to conserving and managing these valuable ecosystems. In this study we examined the drivers of riparian plant invasion in 16 Cantabrian river basins (northern Spain) ranging from 100 to ca. 1050 km2. A complete flora was determined for five randomly selected sites within those basins. One hundred and thirty alien plant species were found across the 80 sampling sites, representing 21% of the recorded total flora. At site scale, the level of plant invasion, measured as alien richness (AR) and relative alien richness (RAR), was assessed in relation to a set of explanatory variables by means of Generalised Linear Mixed Models. This level of invasion was influenced by environmental variables such as the thermicity index, the average riverbed width and the number of plant communities and by human-related variables such as the distance to the nearest town and the proportion of surrounding urban land. At basin scale, industrialised river basins were more heavily invaded than non-industrialised basins, and they both differed in their alien plant composition. Given that some of the alien species occurring in Cantabrian streams are specially abundant (Crocosmia × crocosmiiflora) and/or form very dense stands (Fallopia japonica, Paspalum distichum), future research should focus on the drivers that influence the presence and distribution of these species of special concern.