Disturbance, biological legacies and community development in stream mesocosms.

Disturbances reduce the biota in stream ecosystems, and leave biological legacies, including remnant species, which potentially influence post-disturbance community development but are poorly understood. We investigated whether three remnant species, the snail Radix peregra, the mayfly Serratella ignita and the freshwater shrimp Gammarus pulex, affected community development in mesocosms that mimicked disturbed habitat patches in streams. Following 21 days of colonisation, we found that the occurrence of legacy effects depended on the identity of the remnant species. Radix had the strongest effect. By bulldozing epilithon, the snails acted as ecological engineers that promoted settlement of filter feeders (Simuliidae) and invertebrate predators (especially Pentaneura and Aphelocheirus) and strongly deterred settlement of non-predatory chironomids (e.g. Heterotrissocladius and Microtendipes). Gammarus increased in density (by 665%) where remnant, probably through rapid reproduction. Baetis and Pentaneura were scarce, and Asellus absent, in remnant Gammarus treatments, as a consequence of interference and/or predation by the amphipods. In contrast, Serratella tolerated the colonisation of immigrant species and did not affect the structure of the developing benthic community. Despite the observed effects on the presence and abundance of benthos, remnant fauna had no significant effect on assemblage taxon richness, or that of any specific trophic group. The contrasting effects of remnant species on immigrant colonisation echoed differences in their life-history traits and foraging behaviours. Our results indicate that biota can generate spatial patchiness of epilithon and benthic invertebrates in stream ecosystems.