Patterns of exotic and native plant species richness and cover along a semi-arid Iberian river and across its floodplain

Patterns of native and exotic species richness and cover were examined together with the composition of species assemblages along a typical Mediterranean river and across its floodplain. Major goals were to characterize species distribution regarding diverse environmental and habitat features, and to explore floodplain landforms relations using plant assemblages and invasion responses as biological indicators of aquatic-terrestrial connectivity. A floristic survey was undertaken during May and early June 1998 in six sites of a southern Iberian basin. Vegetation data were gathered from three transects in three landforms: inundated and frequently flooded banks (FF), infrequently flooded valley floors (IF), and rarely flooded higher floodplains (RF). We used 2x2 m plots to record each species present and visual estimates of percentage cover of each species. Cover of native and exotic species assemblages were also estimated for each plots, for the transects, and for the landforms. The total number of vascular plant species found was 287 of which only 26 were exotic. Despite the high environmental and geomorphological variability, the hierarchical classification showed that species composition varied across the floodplain, rather than along the river. The IF and RF landforms were the most species-rich, both for native and exotic species group, whereas the cover trends of exotic and native plant species were opposed, with exotics being consistently more abundant in FF, and native cover increasing towards RF landforms. We found positive correlations between native and exotic species richness on the floodplain scale, and on a smaller scale in FF and IF landforms, thus supporting the hypothesis that in aquatic and riparian ecosystems of semi-arid landscapes, species-rich communities are preferentially colonized by exotic species. However, this pattern was not consistently coupled with a high cover of exotic species, and it was not sustained for the dry areas of the RF landforms.