Environmental filtering governs the spatial distribution of alien fishes in a large, human-impacted Mediterranean river

Aim To analyse the occurrence and abundance of native versus alien fish species in relation to climate, land use, hydrologic alteration and habitat fragmentation in a heavily invaded and human-impacted riverine ecosystem. To test whether co-occurrence patterns of native versus alien species are structured by environmental filtering or biotic associations. Location Mediterranean, Iberian Peninsula, Ebro River catchment. Methods We modelled freshwater fish distributions and their association with environmental conditions using a hurdle model-like approach involving boosted regression trees. Additionally, we applied a joint species distribution model to quantify the co-occurrence of native versus alien fish species that can be attributed to shared environmental responses or potentially to biotic interactions. Results Our results point to environmental factors, rather than biotic associations, as major correlates of the increase of alien and the decline of native fishes in the Ebro River. We observed contrasting patterns of native versus alien species along the upstream-downstream gradient. Alien species dominated in the lower reaches associated with warmer temperatures, higher shares of intensive land use and appeared facilitated by dams and river regulation. Native species richness was highest in the larger tributaries followed by a strong decline in the main stem which was related to the river network position and land use type. Fragmentation played a subordinate role in explaining fish richness and abundance patterns. Main conclusions Given the strong association with temperature, a further range expansion of alien fishes in the Ebro with future climate change may be expected. More local-scale factors related to habitat degradation and hydrologic alteration will further exacerbate the invasion success of many alien fishes. Further multiple, independent species introductions might mask isolation and fragmentation effects of dams on the future spread and distribution of alien fish.