INDIVIDUAL AND POPULATION MOVEMENTS OF ADULT STREAM INSECTS.

R.A. Briers, H.M. Cariss, and J.H.R. Gee. Institute of Biological Sciences, University of Wales, Aberystwyth, SY23 3DA, UK

Stream ecologists are becoming increasingly aware of the potential importance of the adult dispersal in determining population dynamics and distribution of stream insects, but empirical knowledge of the dispersal process remains poor. We studied the dispersal of adult stream insects, chiefly stoneflies and caddisflies, at three low order upland streams in Mid-Wales, UK, which have contrasting catchment land-uses: open moorland, mature coniferous plantation forestry and 15-year re-growth of plantation forestry. Malaise traps were employed to determine the extent of dispersal of the adult population away from the stream channel and novel harmonic radar techniques used to study the activity and dispersal behaviour of individual adult caddisflies (Rhyacophilidae). Both techniques indicated that dispersal distances were generally short. The decline in abundance with distance from the stream channel was best described by an inverse power function with 90% of individuals dispersing less than 11m. There was no evidence of catchment vegetation influencing the extent of dispersal, but flight activity of both taxa was affected by weather conditions, principally air temperature and wind speed. Harmonic radar studies indicated that there was significant flight activity of adult caddisflies during the day and males were generally more active than females.