Advection and habitat loss interactively reduce persistence: maintaining threatened riverine populations while restoring natural flow regimes

Modification of flow regimes and habitat degradation are the strongest, most common, and often co-occurring human activities affecting riverine populations. Ongoing efforts to restore peak flow events found under pristine flow regimes could increase advection-driven dispersal for many species. In rivers with extensive habitat loss, increased advection could transport individuals from remnant populations into degraded downstream areas, causing restored flow regimes to decrease persistence of threatened species. To resolve the capacity for such washout effects across imperiled taxa, we evaluate population growth in spatial matrix models of insect, fish, and mollusc taxa experiencing advective dispersal and either long-term habitat loss or temporary disturbances. As a case study to quantify advective dispersal in threatened species, we use intensive mark-recapture methods in a Rio Grande population of the federally endangered unionid mussel Texas horhshell (Popenaias popeii). Among unionids, the most threatened freshwater taxa of North America, we find high levels of annual downstream emigration (16-51%) of adult P. popeii, concomitant with strong immigration from upstream habitats. For different taxa experiencing such advective dispersal during specific life stages, our population model shows that washout effects strongly reduce population recovery under high levels of habitat loss. Averting this negative consequence of restoring hydrology requires simultaneously restoring or protecting long, contiguous stretches of suitable habitats. Across taxa in heavily impacted systems, we suggest integrating hydrodynamic studies and field surveys to detect the presence of advective dispersal and prioritize areas for habitat restoration to enhance population persistence.