Patterns and mechanisms underlying ecoregion delineation in North American freshwater plants

Aim The regionalized patterns of biodiversity distributions are actively studied in terrestrial and marine ecosystems, but much less is known on the geographical patterns of ecoregions founded on freshwater taxa. Here, we studied, for the first time, how well existing freshwater ecoregions describe the geographical distribution of inland water plants. Location Greenland, continental Canada and USA. Taxon Freshwater vascular plants of all taxa and multiple functional groups (i.e. growth forms). Methods Using newly available fine‐grained data on freshwater plant distributions, we studied how ecoregions founded on fish are suitable for freshwater plant regionalization across North America. Specifically, we calculated internal homogeneity and distinctness among neighbouring ecoregions in relation to species replacements and richness differences. We also explored how a complex suite of ecogeographical characteristics affect ecoregion delineation of freshwater plants using spatially explicit regression routines. Results We found a clear geographical patterning of ecoregion robustness for North American freshwater plants, with communities being more internally homogeneous and more similar to one another in Polar and Subtropical inland waters. The degree of internal homogeneity and ecoregion distinctness were almost equally driven by species replacements and richness differences. Considering different life‐forms, ecoregion delineation performed best for emergent and floating‐leaved plants. Finally, within‐ecoregion homogeneity and distinctness were best explained by annual mean temperature and terrain ruggedness, respectively, with mean water alkalinity, ecoregion area and late Quaternary glacial legacies having supplementary effects. Main conclusions Our findings suggest that selection through climate filtering (e.g. mean annual temperature) is likely the main mechanistic driver of freshwater plant ecoregions. Geographical regionalizations founded on a particular organismal group may not be directly applicable for all taxa but can be a good basis for further adjustments. Our study is a promising starting point for further investigations of geographical delineations for freshwater taxa other than fish.