The relationship between biodiversity and ecosystem functioning has primarily been studied from a local perspective, with the complementary motivation of understanding the impact of environmental perturbations on biodiversity and forecasting the ecosystem-level consequences of the ongoing biodiversity loss. Adopting a spatial perspective to the biodiversity-ecosystem function relationship (BEF) becomes necessary when dispersal and spatial flows of resources play a dominant role in structuring local diversity. While a spatial perspective on the BEF is interesting for many types of ecosystems, river networks may represent an especially relevant case. River networks are intensely structured by both spatial flows of organisms and resources, and inherently display strong environmental gradients, both in terms of resource availability and habitat stability between upstream and downstream patches. Therefore, we investigate the mechanisms underlying the distribution of biodiversity in dendritic riverine networks in relation to their intrinsic characteristic connectivity patterns and environmental gradients.
We show that classical biodiversity distribution in river networks fundamentally rely on ecological drift, which, due to its dependency on population size, acts more strongly in upstream than downstream patches. However, this pattern can be reversed if perturbations increase species turnover enough to slow down ecological dynamics (e.g., competitive exclusion, drift) in small patches compared to large ones. Furthermore, dispersal contributes to the formation but also to the disintegration of these diversity patterns in dendritic networks depending on the magnitude of dispersal and priority effects. We disentangle these drivers of biodiversity and conclude by demonstrating their respective significance in the context of invasion dynamics, a widespread threat to rivers around the globe.