Plants and animals can have large impacts on landscape morphodynamics, and a growing body of literature suggests that physical models that do not incorporate these biological impacts can generate qualitatively incorrect predictions of sediment transport. However, work to date has focused almost entirely on the impacts of single, usually dominant, species. In addition, including biological traits of organisms in geomorphic process models remains a challenge. Here, I ask whether multi-species communities have different impacts on sediment mobility compared to single-species systems due to competitive interactions and variation in traits among species.

Using a combination of modeling, laboratory, and field studies, I investigate how silk produced by two common species of net-spinning caddisfly larvae (Ceratopsyche oslari, Arctopsyche californica) influences the critical shear stress required to initiate sediment grain motion in streams where the caddisflies are alone in monoculture versus together in polyculture. Experimental results suggest that the presence of caddisfly silk when in monoculture can increase the force required to initiate sediment motion by more than a factor of two. When the species are together in polyculture, critical shear stress increases non-additively.

A novel theoretical model that accounts for the mechanical properties, geometry, and vertical distribution of insect silk, as well as interactions between different caddisfly species, on the threshold of sediment motion helps explain how species-specific differences among silk net characteristics, such as tensile strength, can determine the magnitude of the biological effect on incipient sediment motion. Finally, field experiments suggest that non-additive effects of species polycultures on the threshold of sediment motion may be density-dependent and limited to streams in nature where densities are relatively high. Taken collectively, my findings indicate that stream insects can significantly increase the forces required to initiate sediment movement and that species interactions will be an important consideration for future studies that mechanistically link biology and geomorphology.