Floristic homogenization by native ruderal and alien plants in north-east Spain: the effect of environmental differences on a regional scale.
Aim: To evaluate the relative potential contribution of native ruderals and aliens to plant homogenization at a regional scale, after taking into account the effect of diverse environmental distances. Location: Catalonia (north-east Spain) Methods: We have used the flora module of the BDBC project (Catalonian Database of Biodiversity), which provides information on plant species distribution per 10×10 km Universal Transverse Mercator (UTM) cell. Pairwise floristic similarities of: (1) total, (2) native non-ruderal, (3) native ruderal, and (4) alien vascular plant species have been calculated for a particularly well-sampled subset of UTM cells, using a modified version of the Simpson index. These similarities have been compared per UTM pair using Mantel tests, before and after considering their relative association with geographical, climatic and landscape distances from linear regression models. The floristic similarity of both total and native non-ruderal species was also correlated with the proportion of alien and native ruderal species after discounting the effects of environmental distances. Results: The proportion of variance explained by environmental correlates was highest for the floristic similarity of native non-ruderal plants and lowest for that of aliens. In all plant groups, climatic distance was the main significant variable of species similarity. Geographical distance was only significant for total and native non-ruderal species and was of secondary importance in both cases. Landscape distance was not significant in any case. Similarities among both aliens and native ruderals were significantly higher than among native non-ruderals, but these differences disappeared after removing the effect of environmental distances. Main conclusions: Species similarity between sites may depend on differences in environmental factors other than geographical distance. This has to be taken into account when exploring the implications for biotic homogenization. In the case of Catalonian flora, the potentially homogenizing effect of native ruderal and alien species seems to be associated with their lower dependence on geographical distance and climatic factors compared with those of native, non-ruderal species.