Do the ecological impacts of dam removal extend across the aquatic–terrestrial boundary?

Abstract

AbstractThe longitudinal influences of dam removal on river ecosystems are increasingly well described, but impacts to aquatic–terrestrial connectivity are largely unknown. Before and four successive years after dam removal, we quantified density and trophic metrics of nearshore tetragnathid spiders and riparian swallows and their potential emergent‐aquatic insect prey at two experimental reaches located within the former impoundment (one unrestored, one with channel restoration) and one control reach above an intact lowhead dam. We observed substantial annual variability in response variables; however, few patterns appear to be strongly linked to dam removal. We found negligible changes in emergent‐insect biomass or community structure, although there were slight changes in the relative abundance of feeding modes (e.g., increased predators, decreased collector‐gatherers). The most pronounced response was a ~9.9 times average decline in spider densities after dam removal at the restored reach. Stable‐isotope analysis (13C, 15N) indicated that changes in aquatically‐derived energy (i.e., nutritional subsidies derived from aquatic primary producers) to riparian consumers generally showed comparable patterns system‐wide; swallow aquatically‐derived energy declined more in the restored vs. control reach by the final year of the study. Trophic position (TP) of spiders showed little change post‐dam removal, while swallow TP converged slightly between control and experimental reaches after dam removal. Collectively, we interpret our results as evidence that lowhead dam removal can prompt relatively nuanced shifts in aquatic–terrestrial trophic dynamics, regardless of intensive channel‐restoration activities. In contrast, we found that broader‐scale forcings (e.g., discharge, regional temperature) may be more influential in governing cross‐boundary trophic interactions than the perturbations related to lowhead dam removal. This study furthers current understanding of the effects of dam removal on integrative, food‐web metrics of ecosystem structure and function.