COS 189-1 - Inferring riverscape dispersal processes from fish biodiversity patterns

Abstract: Dispersal patterns are recognized as a main determinant of biodiversity structure, particularly in rivers. The dendritic organization of rivers, waterflow direction, large distance immigrants from the outlet, and fragmentation by dams combine to produce a complex dispersal scenario. Unraveling the role, magnitude and spatial scale at which these different dispersal sources determine metacommunity diversity is challenging and demands a large amount of spatiotemporal information, which is rarely available. Here, we incorporated alternative dispersal hypotheses in coalescent and lottery models, contrasting their predictions with observed fish diversity in the Negro River basin of Uruguay, South America. Evidence from different models but supporting the same hypothesis was combined, finding support for i) a dispersal constrained by the riverscape—with no role for direct immigration from the metacommunity; ii) an asymmetric neighbor dispersal, sharply decaying in upward but not in downward river directions; iii) an outlet as a source of individuals that affects diversity even at distant communities; and finally iv) a strong effect of dams, indicating that fish diversity at the riverscape level is now affected by dam constraints on individual movement. Both observed alpha (r=0.58) and beta (r=0.54) diversity were well predicted by the selected model, with a positive and a U-shaped association with communities centrality. Process stochasticity—variation in diversity metrics along realizations of the same assembly process— in alpha and beta diversity were negatively and hump-shaped associated with communities centrality. The ongoing fragmentation of rivers worldwide demands a mechanistic understanding of the role of large-scale dispersal on ecosystem diversity and stability. Selecting models and combining evidence from different models allows the present research to advance on the relative support for hypotheses with a comparatively limited amount of empirical information, thus highlighting the potential of theoretical-empirical approaches for identifying the mechanisms that shape biodiversity.