Shaping species richness: ecological filtering in native and non-native plants along elevation gradients in the Swiss Alps

Ecological filtering, imposed by climatic constraints or competitive interactions, might shape distribution patterns of native and non-native plants differently along elevation gradients. However, patterns of species richness that might reflect these processes have rarely been compared between native and non-native plants. All angiosperms and gymnosperms were recorded along and away from three road-corridors that vertically extended >1100 m in a major valley of the Swiss Alps. Non-native species richness declined with elevation and distance from roadsides. Native richness was humped across the elevational range and varied little away from the road. Conversely, richness of native ruderals was distributed similarly as for the non-native plants whereas mountain species numbers strongly increased with elevation. Consistent with directional ecological filtering, non-native species either dropped out or exhibited progressively wider elevational tolerances if recorded at higher altitudes. A similar pattern was found for native ruderals and the opposite pattern for mountain specialist species, i.e. widening ranges in top-down direction. Furthermore, elevational limitations of species within these groups were mostly relaxed along the road-corridor compared to adjacent habitats. These results led to the following conclusions: (i) elevational richness patterns among the native plants strongly differ based upon ecological functional groups, (ii) richness patterns potentially reflect directional ecological filtering both in bottom-up (native ruderals, non-natives) and top- down (mountain plants) direction, (iii) species among groups with asymmetric richness distributions across elevation seem to be inhibited from laterally invading non-roadside habitats, especially towards their elevational range limits, (iv) understanding the processes of directional dispersal and biotic/abiotic components of ecological filters will help towards predictions of future distributions of plant groups, including the non-native group