Black fly larvae facilitate community recovery in a mountain stream

Abstract

SUMMARY 1. Early colonising ecosystem engineers modify habitats and alter the abundance of basal resources following disturbances. These changes can have profound effects on ecosystem recovery via facilitative or inhibitory effects on subsequent colonists. 2. We quantified how black fly larvae, which can be pioneer species during secondary succession in streams, influence initial community recovery following a simulated drying disturbance. 3. Black fly larvae anchor themselves to the stream substratum with silk, and diatoms adhere to the silk of black flies. Therefore, we hypothesised that black flies speed community recovery following disturbances by increasing the accrual rate of basal resources with their silk. 4. We compared algal and detrital resource abundance and invertebrate community recovery on recently submerged cobbles across three treatments: increased black fly abundance, added black fly silk plus ambient black fly abundance and ambient black fly abundance (control). 5. After 24 h, the increased black fly treatment had more chlorophyll a, detritus and greater invertebrate abundance and richness, and replicates had more self-similar communities than the control treatment. 6. The added silk treatment responded similarly to the increased black fly treatment, supporting the hypothesis that black flies increase the rate of basal resource accrual with their silk, increasing the colonisation rates of other invertebrate species. 7. Our study suggests that black flies are akin to other organisms that facilitate recovery following disturbance (e.g. alders fixing nitrogen following glacial retreat). Further research is needed to determine the effect of black flies on long-term patterns of recovery and the applicability of our results to natural disturbances in streams.