Amechanistic model linking insect (Hydropsychidae) silk nets to incipient sediment motion in gravel-bedded streams

Plants and animals affect stream morphodynamics across a range of scales, yet including biological traits of organisms in geomorphic process models remains a fundamental challenge. For example, laboratory experiments have shown that silk nets built by caddisfly larvae (Trichoptera: Hydropsychidae) can increase the shear stress required to initiate bed motion by more than a factor of 2. The contributions of specific biological traits are not well understood, however. Here we develop a theoretical model for the effects of insect nets on the threshold of sediment motion, τ*_crit, that accounts for the mechanical properties, geometry, and vertical distribution of insect silk, as well as interactions between insect species. To parameterize the model, we measure the tensile strength, diameter, and number of silk threads in nets built by two common species of caddisfly, Arctopsyche californica and Ceratopsyche oslari. We compare model predictions with new measurements of τ*_crit in experiments where we varied grain size and caddisfly species composition. The model is consistent with experimental results for single species, which show that the increase in τ*_crit above the abiotic control peaks at 40-70% for 10-22mm sediments and declines with increasing grain size. For the polyculture experiments, however, the model underpredicts the measured increase in τ*_crit when two caddisfly species are present in sediments of larger grain sizes. Overall, the model helps explain why the presence of caddisfly silk can substantially increase the forces needed to initiate sediment motion in gravel-bedded streams and also illustrates the challenge of parameterizing the behavior of multiple interacting species in a physical model.