A mechanistic model linking insect (Hydropsychidae) silk nets to incipient sediment motion in gravel‐bedded streams

Abstract

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 10-22 mm 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.

Resilience of aquatic net‐spinning caddisfly silk structures to common global stressors

“…Measurements of net architecture were used to calculate silk thread strength as r ¼ F C NT A where r is thread tensile strength, F C is the force of the full sample (including multiple threads), N T is the number of threads in tension, and A is the cross-sectional area of a thread (Albertson et al, 2014b). We sampled as many nets as we were able to successfully harvest from each flume without breaking the silk structure during the sampling effort (c. 30 nets per flume).…”

Comment on “A mechanistic model linking insect (Hydropsychidae) silk nets to incipient sediment motion in gravel-bedded streams” by L. K. Albertson et al.

“…Figure 9c of Albertson et al . [] displays the calibration curve for the 22 mm surface grain size as well as the estimated critical shear stress for each species composition. The linear trend line appears to fit the calibration data reasonably well.…”

“…The extrapolation method employed by Albertson et al . [] may have provided accurate estimates provided that the relationship between boundary shear stress and pump motor dial speed is linear (as they assumed when developing the calibration curves). Albertson et al .…”

“…Albertson et al . [] present an intriguing paper. They developed a theoretical model which incorporates the impact of caddisfly silk nets on critical shear stress.…”

Foraging Fish as Zoogeomorphic Agents: An Assessment of Fish Impacts at Patch, Barform, and Reach Scales

“…Fish and aquatic macroinvertebrates are geomorphological agents that can modify the character and, therefore, stability of river bed sediments (Albertson & Allen, ; Rice et al, ; Statzner, ). Previous work has mostly used ex situ experimentation in flumes, mesocosms, and other artificial channels to demonstrate the ability of stream fauna to alter sediment transport by changing bed material grain‐size distributions, microtopography, and fabrics (Johnson et al, ; Pledger et al, ; Statzner, Peltret, et al, ; Stazner & Peltret, ) or by increasing sediment cohesion through the addition of biogenic materials, including trichopteran silk (Albertson et al, ; Cardinale et al, ; Johnson et al, ; Statzner et al, ). With the exception of investigations of salmonid redd building effects on gravel transport (Field‐Dodgson, ; Hassan et al, ; Kondolf et al, ; Montgomery et al, ) and crayfish effects on suspended sediment loads (Rice et al, , ), in situ experiments and field assessments of fluvial zoogeomorphic impacts are rare.…”