Retreat but no surrender: net-spinning caddisfly (Hydropsychidae) silk has enduring effects on stream channel hydraulics

Abstract

Animals and plants engineer their physical environment by building structures that create or modify habitat. Biotic effects on physical habitats can influence community composition, trophic dynamics, and ecosystem processes; however, the scales and mechanisms regulating the importance of biotic engineering effects are not well documented. We used a laboratory experiment with common and abundant silk net-spinning caddisflies (Trichoptera:Hydropsychidae) to investigate how biotic structures built in riverbeds influence fluid dynamics at micro spatial scales (1 cm) over 2 months. We made velocity measurements with acoustic doppler velocimetry around caddisfly silk structures to test how they influence flow velocity and whether these effects are maintained after the structure is abandoned. We found that caddisfly retreats reduced flow downstream by 85% and upstream by 17% compared to gravels without caddisfly retreats. We also found that experimentally abandoned caddisfly retreats could persist for at least 60 days, suggesting legacy effects of the structures. Although aquatic insects are rarely accounted for in hydrological models, our study suggests that small, but numerous caddisfly larvae could have substantial hydraulic effects. Future work could address variation in the magnitude and duration of biotic engineering among different silk-producing species, densities through space or time, and hydrologic regimes.