Moving beyond terrestrial frameworks: patterns and drivers of aquatic plant biogeography.

Abstract

Biogeographical regionalisation aims to delimit biogeographical areas (bioregions) that reflect the shared natural history between biota and the Earth across time and space, providing a foundation for evolutionary and macroecological studies, as well as conservation efforts. Traditionally, the macroecology of aquatic plants has been based on biogeographical frameworks defined for terrestrial taxa, such as terrestrial ecoregions and phytogeographical domains. However, aquatic plants constitute a polyphyletic group of tracheophytes with varying degrees of adaptation to the aquatic environment. Due to these singularities, macroecological, evolutionary, and conservationist investigations demand a specific biogeographical foundation that reflects their environmental requirements and natural history. The regionalisation tool Infomap Bioregions was utilised, which delimits biogeographical units through bipartite networks, uniting geographical areas and interconnected species into bioregions. The method allows for the inclusion of phylogenetic relationships, identifies indicator species, and evaluates the weighting of restricted-range species in the formation of bioregions, providing complementary information for the interpretation of spatial patterns. Therefore, a global regionalisation for aquatic plants is proposed, exploring community assembly mechanisms from the perspective of taxonomic and phylogenetic beta diversity. Given that the Neotropical region harbours the greatest richness of endemic aquatic plant species in the world, it is also proposed to delineate specific biogeographical regions for this area, highlighting the contribution of restricted-range species and the environmental determinants of biogeographical boundaries. Eleven taxonomic and seven phylogenetic global bioregions were identified, which partially diverge from classical terrestrial regionalisations. The congruence between the scenarios is greater in tropical bioregions, whereas temperate ones exhibit significant divergences. It was also observed that community structure is predominantly determined by taxonomic turnover rather than phylogenetic turnover, and that this difference increases positively with latitude, showing similar organisational patterns among tropical and among temperate bioregions. For the Neotropical region, 18 bioregions were identified for endemic aquatic plants. The boundaries of these units were associated with pronounced heterogeneity in precipitation gradients and topographic roughness. The weighting given to restricted-range species was fundamental in representing the high degree of endemism in the region. These results reiterate the importance of proposing taxon-specific regionalisations, providing a more robust biogeographical framework to support macroecological and evolutionary studies for aquatic plants, thereby guiding assertive conservation efforts for these taxa.