Insect dispersal does not limit the biological recovery of streams from acidification

1. The ecological recovery of streams from large-scale perturbations, such as acidification, requires aquatic insects to disperse between catchments. While adults can usually fly, dispersal is seldom observed directly. Catches of insects in transects of traps perpendicular from streams suggest that lateral adult dispersal is limited. This paper evaluates whether this could explain limited biological recovery in streams recovering chemically from acidification. 2. At the replicate Llyn Brianne experimental catchments (Wales), Malaise traps (2000) and benthic sampling (1985-2005) were used to appraise inter-catchment dispersal in acid-sensitive Ephemeroptera, Plecoptera and Trichoptera. The results provide direct evidence for inter-catchment dispersal: eight species from three Orders were caught as adults alongside acid streams where larvae never occurred in 21 years' benthic sampling. 3. These data refute the hypothesis that limited dispersal per se explains delayed biological recovery from acidification in Welsh streams. Other factors affecting colonization (e.g. 'propagule pressure', mating or oviposition behaviour) and persistence (e.g. continued acid episodes) must be involved, with the first of these possibilities still poorly understood. These data add to a growing body of literature illustrating insect dispersal between catchments, and they have wider relevance to the recovery and restoration of river ecosystems following basin-scale impacts. Copyright (c) 2006 John Wiley & Sons, Ltd.