Dispersal of adult aquatic insects in catchments of differing land use

1. There have been widespread changes in land use in the uplands of the UK but the implications for dispersal of adult stages of aquatic invertebrates are poorly known. We estimated the lateral dispersal of adult aquatic insects (Plecoptera, Trichoptera, Ephemeroptera) in seven small, upland streams draining catchments under three categories of land use (coniferous plantation forest, cleared forest, moorland). 2. Malaise traps were set out in transects perpendicular to each stream. More than 29 000 adult insects were taken, distributed among 15 species of stoneflies, 40 species of caddisflies and eight taxa of mayflies. Overall species diversity and equitability were highest in the moorland catchments, and few species were numerous in all catchments. 3. Nearly all the mayflies were taken in the moorland catchments, where caddisflies were also most abundant. Fewest stoneflies were taken in the forested catchments. 4. The vast majority of insects were taken either directly over, or very close to, the stream channel. Half the stoneflies were taken within 18 m of the channel, while 90% had travelled less than 60 m. Caddisflies and mayflies travelled even shorter distances. Although there were differences in lateral dispersal between some catchments, there was no overall effect of land use. 5. The overall sex ratio in stoneflies and mayflies in the riparian zone was close to 1 : 1 and lateral dispersal was similar between the sexes. Male mayflies outnumbered females in the riparian zone and males travelled further from the stream, on average, than females. In catches taken directly over the stream, female stoneflies outnumbered males. 6. Regardless of land use, the flight of mayflies and caddisflies was concentrated along the stream, rather than perpendicular to it. This was also true for two numerous stoneflies (Amphinemura sulcilcollis and Protonemura meyeri) and for female stoneflies overall. 7. Synthesis and applications. The stream corridor, including the riparian strip extending 10-20 m on either side of the channel, is the main habitat for adult aquatic insects, and its management may affect the biodiversity of aquatic communities. The stream corridor is also revealed as the main 'highway' for adult dispersal. While there is no evidence from this study of an effect on interstream dispersal of land use elsewhere in the catchment, such an effect cannot yet be refuted because rare long-distance dispersal events are difficult to record.