Realistic basin shape variation modulates local riverine biodiversity via altered connectivity

Understanding the drivers of biodiversity in the worlds rivers, which are known to be hyperdiverse relative to their coverage area, has been an enduring goal in ecology. While regression-based empirical studies have identified a suite of environmental factors that are correlated with riverine fish biodiversity, these insights are often system-specific and inconsistent across regions. In contrast, a more limited body of studies have suggested that network connectivity of rivers affects fish biodiversity by limiting dispersal, and basin shape may modulate these relationships. The few theoretical papers that have explored basin morphology effects have tended to use extreme network shapes and inconsistent methods, thus limiting general insights. Here, we build on these results to demonstrate that river basin morphology, as measured by log aspect ratio, can alter both network connectivity and biodiversity in simulated, all-else-equal scenarios. First, we quantify variation in log aspect ratio across the worlds 100 largest rivers and use this empirical range of shape variation to guide synthetic experiments. In particular, we use Optimal Channel Networks (OCNs) constrained to basins with log aspect ratios within realistic range to study how shape alters connectivity profiles when node number and basin area are held constant. By coupling OCNs with dendritic neutral models, we demonstrate that variation in aspect ratio and concomitant changes in connectivity lead to substantial changes in simulated biodiversity. Finally, we use Earth Movers Distance to establish that basin-shape-induced changes in node-level connectivity distributions are predictive of transformations in node-level distributions of and {beta} diversity. Overall, elongated basins such as the Mekong River feature lower species richness (-diversity), higher turnover ({beta}-diversity), and less variable distributions of both quantities relative to a square reference basin. Furthermore, approximately one third of the worlds largest rivers are elongated enough to potentially feature statistically-detectable, shape-mediated variation in connectivity and biodiversity.