Biotic and physical responses to biomimicry structures in a Rocky Mountain incised stream

Abstract

An increase in stream degradation resulting from land use change has motivated an increase in restoration efforts across the globe. Post-restoration monitoring is still lacking, however, and does not always incorporate biotic responses to changes in the physical template. Beaver mimicry structures (BMS) are becoming a popular tool to restore degraded streams throughout the American west, but relatively little is known about how these installations influence both biotic and abiotic factors, with consequences for ecosystem functioning. We monitored basal resource deposition and macroinvertebrate density, biomass, and production to quantify functional responses to BMS installations. We compared conditions at BMS sites to naturally occurring beaver dam and reference riffle sites in a low-gradient stream in southwest Montana. Thermal ranges were contracted, and daily maximum temperatures increased at BMS sites compared to reference riffle sites. Deposition of fine sediment and basal resources was similar at beaver and BMS sites, and both were higher than reference riffles. Densities and production of macroinvertebrates were higher at the BMS sites compared to reference sites and similar to beaver sites due to changes in physical habitat and basal resource availability, reflected by increases in production of shredders (beaver) and collector-gatherers (BMS). In this study site, changes to the physical template using BMS appear to have strong impacts on biotic functional responses, creating habitats similar to target conditions of natural beaver dams. Future research should consider the extent of degradation and temporal limitations of monitoring schemes to incorporate BMS into standard restoration practice. Functional response metrics provide an important and mechanistic approach to determine the efficacy of process-based stream restoration practices.