Edinburgh Napier University

  Populations of benthic invertebrates in neighbouring streams are isolated from each other by intervening terrestrial habitat. The adult stages of stream insects that are capable of flight may disperse between streams, although little is known of the extent of inter-stream dispersal, or the degree to which movement is influenced by riparian vegetation. Double-headed Malaise traps were set at differing distances from the channel to measure the numbers of adult stoneflies moving towards or away from three upland streams draining adjacent catchments on the Plynlimon ridge in mid-Wales. Riparian vegetation differed between the streams: open sheep-grazed moorland, buffer strips left free from 15 year old plantation forestry, and mature conifers planted up to the stream bank. Adult abundance was broadly consistent with benthic larval abundance. In three of the five most abundant species (Amphinemura sulcicollis, Leuctra fusca and Siphonoperla torrentium) abundance varied inversely with the amount of forestry in the riparian zone. In the others (L. inermis and L. nigra) numbers caught were unrelated to forestry. Relationships between adult abundance and distance from the channel were best described by inverse power functions. Numbers of adults declined sharply with distance from the stream; 90 % of adults were caught within 11 m of the stream channel. The rate of decline of adult numbers with distance did not differ with riparian vegetation. Inter-stream differences in the numbers caught at particular distances were due to differences in overall abundance. There were no differences in the number of males or females caught at different distances from the stream. At all times more stoneflies were caught in the side of the trap facing the stream than that facing away, although the difference was greatest in the middle of the flight period and least at the beginning and end. Overall, the majority of adult stoneflies did not disperse a significant distance from the stream channel, suggesting that only a very limited number of individuals are likely to disperse between streams.