Human disturbance and climate change negatively impact stream ecosystems. For example, walking paths, anthropogenic debris, and prolonged drought restrict water flow in local streams. Following the extended drought in Southern California, during which many streams essentially stopped flowing, the Santa Monica Mountains recently experienced torrential rainfall in 2017. These extreme weather patterns have resulted in rapid change in stream topography. Caddisflies of the genus Lepidostoma are commonly found in freshwater streams in the Santa Monica Mountains. These larval insects are important to the ecology of streams as food for fish and as leaf shredders and decomposers. Leaf shredding behavior also helps caddisflies in a variety of ways as they encase themselves in shredded leaf material. These cases facilitate respiration, enable individuals to remain cryptic, and serve as mechanical protection. Although case building is clearly advantageous, it remains unknown how flow rate specifically affects leafy caddisfly case building performance. We investigated the effect of water flow rates on the behavior of caddisflies of the genus Lepidostoma. We predicted that caddisfly case building performance decreases as flow rate increases. To test this hypothesis, we constructed a recirculating laboratory flume to simulate ecologically relevant flow rates observed in streams in the Santa Monica Mountains. We manipulated flow rate through the flume and supplied each uncased caddisfly with the streambed detritus necessary to build a case. Then we measured the percent mass of case accumulated after seven hours.
Results/Conclusions
Caddisfly case building performance declined as water flow increased (P = 0.004 for aggregate data on 4 groups; P = 0.098 for individual data on 11 caddisflies). The caddisflies at the highest experimental flow rate (157 ml/s) were unable to build a functional case, accumulating a minuscule load of detritus that on average amounted to only 13% of their body mass. These results demonstrate how the behavior of caddisfly larvae is influenced by local stream conditions, which themselves are altered by more expansive regional forces, such as human disturbance and climate change. More broadly, this study highlights the impact that substantial changes in stream flow rate can have on crucial members of the freshwater benthic macroinvertebrate community.