Processes underpinning fish species composition patterns in estuarine ecosystems worldwide

Abstract

Aim We present the first global biogeographical regionalization of estuaries, assessing how dispersal limitation and/or environmental filtering mechanisms drive the patterns of fish assemblage composition among and within biogeographical regions. Location Estuaries worldwide.Methods A bootstrapped hierarchical cluster analysis was applied to define biogeographical regions based on pairwise beta diversity (bsim) of fish assemblages among 393 estuaries worldwide. Variables representing dispersal limitation and environmental features were used to disentangle the possible effects of assembly processes acting among and within biogeographical regions, through hierarchical partitioning of variation. Finally, we assessed the contribution of species to beta diversity through a method newly developed in this article.Results Estuaries were grouped into seven major biogeographical regions explained by geographical dispersal variables. Dissimilarity in species composition within each biogeographical region was mainly determined by ecosystemconnectivity dispersal variables, with a minor influence of environmental variables (chiefly sea surface temperature). Marine species contributed highly to beta diversity between estuaries but less than expected by chance, whilst freshwater species had low contributions yet also less than expected. Estuarine (i.e. brackish) and diadromous species had low contributions to beta diversity but higher than expected by chance.Main conclusions Fish assemblage composition in estuaries seems to be driven by dispersal limitation processes both among and within biogeographical regions, with only a minor role of environmental filtering evident within biogeographical regions. These findings contrast with the acknowledged importance of environmental variables as drivers of species richness patterns. Irrespective of biogeographical region, the ecosystem affinity of a species determined its contribution to beta diversity between estuaries. The new method presented here offers new opportunities for assessing the effects of taxonomic and functional aspects on beta diversity (bsim) across taxa and ecosystems, and thus contributes to improving knowledge of assembly processes.

Environmental drivers of taxonomic, functional and phylogenetic diversity (alpha, beta and gamma components) in estuarine fish communities

“…Our results support a similar conclusion for the three diversity facets and revealed that changes in estuarine γ‐diversity were essentially induced by an increase of β‐diversity in large estuaries. Since entrance width and river flows are commonly used to assess the estuarine connectivity, our results are also congruent with previous findings that stress the positive effect of connectivity on fish taxonomic diversity (Henriques et al., ; Vasconcelos et al., ). Moreover, we found that the taxonomic β/α ratio was lower in shallow estuaries with high tidal range probably in response to the predominance of marine conditions at the rising tide and to the very low water level at low tide.…”

“…Latitude ( Lat ., degree) was taken at the mouth of the estuary and is related to regional environmental factors, such as productivity or temperature (Henriques et al., ). The continental shelf width ( C. shelf , km) was chosen to reflect the large‐scale availability of marine habitats, as advocated by Nicolas et al.…”

“…For several years, great advances have been completed to identify processes that drive diversity patterns in estuarine fish communities (e.g. Henriques et al., ; Nicolas et al., ; Sheaves & Johnston, ; Vasconcelos et al., ). However, to our knowledge no study attempted to combine the three diversity facets.…”

“…Therefore, environmental drivers underlying fish communities are linked to local estuarine conditions, as well as to marine, geographic and climatic processes that influence assemblages of marine and freshwater species that can colonize estuaries (e.g. Attrill & Power, ; Henriques et al., ; Vasconcelos et al., ). For instance species richness increases with estuary area and connectivity level with the adjacent ecosystems (Nicolas et al., ; Vasconcelos et al., ).…”

“…chemical pollution, nutriment enrichment, changes in water supply), causing alteration of ecosystem functioning (Bowen & Valiela, ). The specific responses to all these environmental factors are expected to vary depending on species ecological traits and their evolutionary history, notably between marine, estuarine, diadromous and freshwater species (Henriques et al., ). Since these fish species are unevenly distributed within estuaries, environmental changes should induce modifications in the three facets of regional diversity, including in its local and turnover components.…”