Patterns and mechanisms underlying ecoregion delineation in North American freshwater plants

Abstract

AbstractAimBiogeographical regionalisations are actively studied in different ecosystems, because they increase our understanding on fundamental broad□scale patterns and can help us in the establishment of conservation areas. Thus, we studied how well existing freshwater ecoregions describe geographical delineation for inland water plants and which ecogeographical gradients explain them.LocationNorth America, excluding Mexico and remote islands.TaxonFreshwater vascular plants of all taxa and different functional groups.MethodsUsing newly available fine–grained data on freshwater plant distributions, we calculated internal homogeneity and cross–boundary heterogeneity among neighbouring ecoregions. We further integrated measures of community dissimilarity to assess whether the degree of within–ecoregion homogeneity and distinctness are driven by their relationships to species replacements and richness differences, and explored how a complex suite of ecogeographical mechanisms and plant life forms affect ecoregion delineation using spatially explicit regression routines.ResultsWe found a clear geographical patterning of ecoregion robustness for North American freshwater plants, with their communities being more internally homogeneous and more similar to one another in polar and subtropical inland waters. Surprisingly, 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 Ice Age legacies having supplementary effects.Main conclusionsOur findings emphasise that geographical regionalisations founded on a particular organismal group are not applicable for all taxa. Our study is a promising starting point for further investigations of geographical delineations for different freshwater taxa. These updated regionalisations can then be used for conserving different biotas in freshwaters, which are currently among the most threatened ecosystems in the world.Statement of significanceIn biogeographical regionalisation biota is categorized to meaningful geographical units, such as ecoregions. However, ecoregions delineated for a particular group of organisms may not be applicable to another assemblages. We studied how ecoregions founded on fish are suitable for freshwater plants across North America. Our findings suggest that these ecoregions did not show consistent robustness for freshwater plants in North America. This study is a promising starting point for further investigations of geographical delineations for different freshwater taxa, having also value in conservation planning and management.