Distribution and richness of aquatic plants across Europe and Mediterranean countries: patterns, environmental driving factors and comparison with total plant richness

Questions What are the geographic patterns of ?-diversity of aquatic plants and what are the main driving factors? Are richness trends for aquatic plants similar to total plant richness? Is the Mediterranean area a hot spot for aquatic plants? Location Europe and the Mediterranean Basin. Material We listed vascular aquatic plant presence or absence for 44 countries. We also compiled total plant species richness and geographic and environmental variables for each country. Methods We first analysed country ordination based on their aquatic flora constrained by environmental variables (dbRDA), and selected the environmental variables best explaining species patterns (BEST analysis). Total species richness patterns were studied using maps and latitudinal gradients. We used generalized additive models (GAM) to detect the main environmental factors driving species richness, both for aquatic plants and total plants. Results The BEST analysis identified a single variable that best explains aquatic plant species distribution: evapotranspiration. However, richness of aquatic plants vs latitude varies and no clear trend was observed. No relation was found between total plant and aquatic plant richness. Aquatic and total plant richness peak between 40 degrees and 50 degrees N, and values were intermediate at low latitudes. GAM related aquatic plant richness with water resources and rainfall, while total plant richness is mainly driven by evapotranspiration and temperature. Hydrophytes were relatively more abundant at higher latitudes than helophytes and the ratio correlated with evapotranspiration. Conclusion Southern and western Europe hold the highest aquatic plant diversity, although no clear latitudinal species richness patterns were found. Aquatic plant richness is mainly driven by water-related variables. Total plant richness exhibits a latitudinal pattern influenced by the Sahara desert, which depresses richness at low latitudes. Best predictors of total plant richness patterns are waterenergy variables.