Spatial and temporal distribution of adult aquatic insects
Most quantitative ecological research on aquatic insects has addressed their immature stages. To understand their population dynamics, however, information on the adult stage is essential. This study aimed to obtain quantitative estimates of life cycle features, such as emergence, adult life span and fecundity, and to develop further the methods used. Field studies on adult insects revealed patterns in spatial and temporal distribution and the effect of land use on lateral dispersal. Finally, methods used to estimate dispersal and gene flow were discussed. Studies on Leuctra nigra demonstrated that their emergence is associated with the quantity of emergent substratum locally available. Female L. nigra can be classified as "immature" and "mature", based on the maturity of their eggs, and this was used as an approximate age marker in studies of adult survival and spatial distribution, and to improve estimates of emergence by excluding mature females from catches in pyramidal traps. A female biased sex ratio observed in L. nigra stemmed from a discrepancy in the life span of the genders, with shorter-lived males, combined with a change in habitat by the females. Most adult aquatic insects stayed close to the stream. Thus, half of the stoneflies travelled less than 18 m lateral to the channel and the dispersal ranges of caddis flies and female of mayflies were even shorter. Comparison of the movement along the stream with the lateral dispersal suggests that most dispersal is in the 'stream corridor' itself. Little distinction was found between lateral dispersal in different catchment land uses. It is still too early, however, to conclude that dispersal is unaffected by land use. Temporal and spatial distribution of leuctrid stoneflies differed among neighbouring catchments, with Leuctra inermis and L. nigra dominating in moorland and catchments managed for forestry, respectively. Both physiochemical factors and biological interactions could explain this distribution.