The Further You Are, the Closer You Get: Environmental Similarity Connects Aquatic Invertebrate Communities Across the Terrestrial Landscape in a Large River Network.

Biodiversity is structured on the landscape through a combination of spatial, environmental, and stochastic factors. In freshwater river networks, the interaction of the linear river network and the three-dimensional terrestrial landscape creates multiple pathways for ecological connectivity that affect aquatic community structure through habitat filtering and dispersal dynamics. Here, we use the high throughput capacity of environmental DNA (eDNA) metabarcoding to explore the influence of spatial relationships and environmental heterogeneity across an expansive and diverse landscape. We collected invertebrate eDNA samples from 70 aquatic habitats in the lower Colorado River Basin (LCRB), ranging from small, free-flowing headwater tributaries to anthropogenically-altered high order rivers, spanning five major drainage regions. Communities displayed high alpha and beta diversity among the drainage regions, structured by a combination of spatial, environmental, and stochastic factors. A novel distance-decay framework adapted from population genetics revealed the influence of terrestrial pathways of ecological connectivity among communities throughout the network. Hierarchical clustering identified patterns of convergent similarity in headwater tributaries despite extreme river network distances separating sites. These results reveal the dual influence of spatial and environmental metacommunity dynamics in aquatic invertebrate communities in the LCRB and the specific influence of interbasin terrestrial connectivity across large spatial scales. Overall, this highlights how terrestrial landscapes can shape convergent patterns of biodiversity in river networks.