Current climate overrides historical effects on species richness and range size of freshwater plants in Europe and North America

1. The latitudinal diversity gradient (LDG) hypothesis suggests that species richness should be highest at low latitudes, whereas Rapoport’s rule states that largest ranges ought to be found for species at high latitudes. However, there is no consensus over these patterns and their underlying drivers in the freshwater realm.

2. We investigated species richness and mean range size of freshwater plants in 50 km × 50 km grid cells across Europe (40°N to 71°N) and North America (25°N to 78°N), supplemented with data based on 1° latitudinal bands for mean range size. We were especially interested to find out whether there are similarities and differences in these ecogeographical patterns and their underlying drivers between the continents due to their contrasting historical characteristics, spatial extent and topography. 

3. First, we used partial regression to reveal whether species richness and mean range size of freshwater plants have a linear or quadratic relationship with latitude...