Native and Non-Native Populations Respond Unevenly to River Barrier Removals.

The multi-faceted threats of habitat degradation and biological invasions are increasing concerns for the integrity of freshwater ecosystems and their associated services globally. Removal of anthropogenic riverine barriers has gained momentum over the last three decades, motivated by the maintenance requirements of the aging infrastructure, while also functioning as a primary means of restoring river connectivity to promote ecosystem recovery and native biodiversity. Although barrier removals are intended to benefit native species dispersal, these actions can simultaneously facilitate non-native species spread and increase biological invasion risk, creating a conservation paradox. Using a systematic review and meta-analysis, we synthesised the patterns and trends in responses between native and non-native populations to riverine barrier removals globally. Since 1998, research interest in the effects of barrier removals has rapidly accumulated, but with a major geographic imbalance towards North America (80% of studies). Fishes were the most studied taxon (74%), followed by riparian and aquatic plants (24%) and invertebrates (15%). The removal of dams dominated (85%), compared to road culverts (7%) and weirs (2%). Results from 2840 reported effect sizes showed that both native and non-native population sizes generally increased after barrier removal, but the positive effect was strongest for native species. Moreover, these population responses were highly context-dependent, with taxonomic grouping, sampling directionality and time since removal affecting outcomes. Non-native population growth outstripped that of native species in the first few years post-removal and non-native invertebrates responded the most favourably to barrier removal. While native species typically benefit from barrier removals overall, our analyses highlight context-specific responses that can simultaneously benefit non-native populations and erode long-term conservation outcomes. As river restoration efforts continue to gain traction globally, further planning and research are required to anticipate and assess long term population recoveries, to reconcile conservation quagmires around interacting global environmental changes.