Net spinning caddisflies as stream ecosystem engineers:: the influence of Hydropsyche on benthic substrate stability

1. Organisms that physically modify or create habitat (ecosystem engineers) can have a profound influence on community and ecosystem dynamics. 2. Here evidence is presented that one of the most abundant and widely distributed lotic insects could act as an ecosystem engineer in streams by increasing the stability of benthic substrates during flooding. 3. Natural densities of larval net spinning caddisflies (Hydropsychidae) were established in stream channels that had standardized physical properties. The mobility of three particle sizes were measured during simulated flooding and the fraction of particles eroded compared with that of control streams. 4. Larvae increased the initial velocity required to erode sediments by 10-30%. At velocities sufficient to scour 87% of particles from control channels, 57-100% remained stable in channels colonized by larvae. 5. Assuming larvae have similar effects in natural streams, caddisflies could be expected to increase the recurrence interval of a substrate scouring flood from 1.67 year to 2.41 year, corresponding to a 17% decrease in the probability of bed scour per year. 6. Our study suggests these insects could play an important role in generating the spatial 'refuges' that moderate the resistance of lotic communities to flooding. It is argued that, as has occurred in marine systems, a better understanding of how freshwater organisms engineer their physical environment has much potential to complement our historical focus on the abiotic forces that constrain populations and communities.