The potential role of biotic interactions in stream restoration

Biotic interactions perform an important role in structuring freshwater communities; however, these are rarely considered during stream restoration. Degraded stream communities are often dominated by organisms with shell or case protections, such as snails, which are less vulnerable to predation than desired organisms, such as mayflies. Unprotected organisms may be preferentially eaten, limiting biotic restoration success after the physical restoration of degraded stream communities. We investigated whether restoration success depended on differences in vulnerability to predation by testing whether consumption-based interaction strengths (IS) between vulnerable colonizing Deleatidium spp. mayflies and upland bullies (Gobiomorphus breviceps) changed with different densities of protected Potamopyrgus antipodarum snails and whether snails affected Deleatidium behavior. Deleatidium and fish IS were stronger with fewer Deleatidium present, a destabilizing effect on vulnerable prey populations, and less total prey biomass was consumed with higher densities of protected relative to vulnerable prey, potentially having long-term negative effects on predators. Furthermore, mayflies appeared less on surfaces with high snail densities, potentially due to altered resource use. The combination of these biotic interactions could form a barrier to successful biotic restoration by preventing colonization of desired vulnerable prey, through priority effects within the established community. Therefore, considering biotic interactions in stream restoration is important to overcome biotic restoration barriers.