Fish assemblage and functional trait responses to small‐dam removal

Abstract

Abstract Dams are one of the greatest threats to freshwater biodiversity and efforts to remove dams to restore riverine systems are increasing. However, dam‐removal studies have primarily focused on taxonomic responses to large dam removals with little work on the functional trait responses of fish to small‐dam removals; such a focus limits the application of results in regions with different taxonomic compositions. We explored taxonomic and functional trait responses of fish assemblages to two small‐dam removals over 10 years using a Multiple Before After Control Impact design at a dammed and an undammed river. Eight life‐history traits were used to calculate functional diversity (RaoQ) and determine the position of each fish species within a multivariate life‐history space relative to three life‐history strategy endpoints: opportunistic, periodic, and equilibrium. The distance of each species relative to these endpoints was used to calculate community weighted means (CWM), allowing us to examine the shift in life‐history strategy of a given assemblage after dam removal. Based on life‐history theory, we predicted a decrease in the CWM of equilibrium strategists and an increase in the CWM opportunistic strategists after dam removal. For the dammed river, we observed changes in assemblage structure at both the control and impact sites driven primarily by shifts from a lentic to lotic assemblage, with the most apparent change occurring in the formerly impounded sites. These changes tended to occur within 1 year, suggesting rapid colonization by lotic species after habitat change. By contrast, no change in assemblage structure was found for the undammed river, suggesting that dam removal was the primary driver of the shifts in assemblage structure observed in the dammed river. We found no change in the CWM of periodic strategists or RaoQ of all life‐history traits at any site after dam removal. Based on CWM, life‐history strategies shifted in response to dam removal at impounded sites where equilibrium strategists decreased and opportunistic strategists tended to increase after the impounded sites changed from a stable lentic environment to an unstable lotic environment, supporting predictions made by life‐history theory. Our results suggest that small‐dam removal may provide ecological benefits by restoring fish assemblages to a more natural riverine state and reversing the negative effects of dam construction on the ecosystem. We demonstrate that combining both trait‐based and taxonomic approaches can improve our ecological understanding of the impacts of dam removal on fish assemblages and provide relevant data for local management.