Aquatic Insect Bioconstructions Modify Fine‐Sediment Entrainment and Mobility

The importance of two‐way interactions between animals and the physical hydraulic and sedimentological environment are increasingly recognized (e.g., zoogeomorphology). Caddisflies (Trichoptera) are a group of aquatic insects known for their bioconstructions, particularly cases built from fine sediment and silk. Caddisfly cases differ in size, shape and density from the incorporated sediment, and case construction may therefore affect the mobility of these sediments in rivers. However, although communities of caddisfly often use substantial quantities of sediment in case construction, the effect of these bioconstructions on sediment transport in rivers is unknown. We use a flume experiment to compare the bed shear stress required to transport (a) empty caddisfly cases and (b) individual sediment particles, following disaggregation from the case. The cases of three species were considered; two that construct different styles of tubular case (Potamophlax latipennis and Sericostoma personatum) and one that builds a domed case (Agapetus fuscipes). P. latipennis and S. personatum cases were easier to entrain than the sediment grains incorporated into them, whilst A. fuscipes cases were not. Despite their low mass, A. fuscipes cases required the most shear stress to transport them because their domed shape impeded rolling. These findings are important to understand how differences in case design between species, reflect different adaptation strategies to the turbulent hydraulic river habitat. Furthermore, the results suggest that un‐attached tubular caddisfly cases may be preferentially transported over other particles on the river bed and thus, where caddisfly occur in high abundance, they may increase fluvial entrainment of sand. Plain Language Summary The power of water allows rivers to shape the landscape, transporting sediment downstream, creating landforms and habitat. Rivers are teeming with life, which can also affect sediment transport. Caddisfly larvae are aquatic insects, many of which have adapted to underwater life by building structures (e.g., cases) from sediment and silk. By combining many sand particles into single cases, caddisfly may affect the river energy required to move this sand. We compared the hydraulic force required to move caddisfly cases versus the loose sand grains they were built from. Most caddisfly cases are tube shaped and construction of these cases made the incorporated sand easier to transport. As a result, case construction