Physicochemical and morphological degradation of stream and river habitats increases invasion risk

One of the key challenges of invasion biology in aquatic systems is determining the environmental conditions under which non-indigenous species establish populations in new habitats. It is widely believed that environmental degradation of streams and rivers may facilitate susceptibility to invasion; however, this has not yet been demonstrated consistently across a wide range of taxonomic groups. We analyzed macroinvertebrate data from 398 stream and river sites in Germany in order to test whether morphologically and physicochemically degraded stream and river habitats are more prone to invasion. Further, we identified the most important environmental variables facilitating invasion. The study confirmed that invaded sites were significantly more degraded than sites where only indigenous species were recorded. In both streams and rivers, invaded sites featured increased maximum temperatures, chloride and total organic carbon concentrations and a decreased morphological habitat quality. In streams, additionally the variables minimum temperature, oxygen, orthophosphate and ammonium contributed to greater degradation. In rivers also nitrate concentration was increased at invaded sites. Generalized linear models indicated that chloride was one of the most important variables that favored invasibility in both streams and rivers. In streams, the most indicative variables for invasion risk also included orthophosphate and maximum temperature. In rivers, in addition to chloride, morphological habitat quality was important. Our results confirm that the physicochemical and morphological intactness of riverine systems is a safeguard against invasion of aquatic non-indigenous macroinvertebrates. Based on this knowledge, management strategies can be developed to reduce invasion risk.