Dark fragmentation: daylighting hidden disconnections in river networks

Jingrui Sun , Damiano Baldan, Ellen Wohl, Yushan Tang, Julian Olden

River fragmentation is a central driver of freshwater biodiversity loss, yet its true extent is often underestimated due to incomplete barrier inventories, static river maps, and simplified assumptions about barrier passability. We propose dark fragmentation as the hidden component of river-network disconnection arising from three interacting dimensions: inventory darkness caused by unmapped barriers, functional darkness from poorly characterized barrier function, and hydrological darkness from under-documented hydrological interruptions. Together, these dimensions broader mismatch between mapped connectivity from realized ecological connectivity. Dark fragmentation provides a framework for identifying where conventional assessments overestimate river continuity and misguide conservation decisions. We outline how these hidden components can be estimated using improved barrier mapping, passability assessment, hydrological monitoring, and scenario-based connectivity modelling. Recognizing dark fragmentation can help target field surveys, reveal hidden bottlenecks that limit species from accessing required habitats, improve restoration prioritization, and support more realistic freshwater conservation targets.