Cooperative Fish and Wildlife Research Units Program: all
Education, Research and Technical Assistance for Managing Our Natural Resources

As one of the greatest threats to freshwater biodiversity, dams are being removed in greater number to restore riverine systems. Dam removal studies have primarily focused on taxonomic responses to larger dam removals with little work on the functional trait responses of fish to small dam removals, limiting the application of results to different biogeographic regions. We explored taxonomic and functional trait responses of fish assemblages to two small dam removals over ten 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. 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 about one year suggesting rapid colonization by lotic species after habitat change. In contrast, no change in assemblage structure was found for the undammed river, suggesting that dam removal was the driver of the shifts in assemblage structure observed in the dammed river. Based on CWM, we showed a shift in life history strategies 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 the predictions made by life history theory. We found no change in the CWM of periodic strategists or RaoQ of all life history traits at any site after dam removal. This study suggests 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. We also demonstrate that combining both taxonomic and trait-based approaches can improve our understanding of the impacts of dam removal on fish assemblages and provide complementary tools to uncover general ecological patterns as well as provide data more applicable to local management than one approach alone.