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Flow intermittence shapes fish metacommunity variability in a Neotropical ecoregion

Abstract:

Drying river networks comprise intermittent reaches that cease to flow or dry, and these networks are becoming more prevalent with climate change. However, their distribution, constituent species, and ecological processes remain underexplored in the Neotropics. Drying is a strong environmental filter that reduces α-diversity through the loss of taxa that lack adaptations to resist and recover from drying. β-diversity, however, can be high in river networks with intermittent reaches. In this study, we sought to understand the spatiotemporal dynamics of a fish metacommunity in a Neotropical drying river network through its annual hydrological cycle. To do so, we assessed the relationships of local and regional variables with fish metacommunity structural variability in the Chocó forests, a biodiversity hotspot in Northwestern Ecuador. Fish α-diversity was lower in intermittent reaches during the wet and dry seasons, whereas temporal β-diversity was higher in perennial reaches. Distinct fish assemblages were indicative of intermittent and perennial reaches, with assemblage composition being stable in intermittent reaches but more variable in perennial reaches. Lastly, metacommunity analysis suggested that environmental filtering was the primary community assembly process, with species being replaced along the environmental gradient of the river network. Physical distance influenced community structure, but only for fish with limited inferred dispersal ability, indicating that for these species, community dynamics may be shaped by hydrological connectivity rather than environmental conditions alone. Our findings suggest that fish assemblage variability in this Neotropical river network resulted from seasonal drying, species-specific habitat preferences, and dispersal abilities, partially confirming previous hypotheses on fish diversity in drying river networks. The results emphasize that preserving both intermittent and perennial habitats while maintaining longitudinal connectivity is essential to safeguard the ecological processes that sustain fish metacommunities in drying river networks.