Dispersion and factors influencing drift of predatory stonefly nymphs (Plecoptera: Perlidae)

Natural dispersion patterns of several predatory stone-fly nymphs (Plecoptera: Perlidae) and associated prey items were examined. Dispersion was documented on natural snag material retrieved from the research site and similarly constructed into snag bundles as the experimental habitat units. Nymph dispersion was determined to be uniform and the associated prey items were significantly clumped. I considered two factors which may influence the drift of predatory stonefly nymphs--hunger state of the nymphs and initial nymph density. The previously constructed snag bundles were the experimental units from which drift response of satiated and starved nymphs; and high, medium, and low initial density snags were documented. There was no significant difference in drift response between starved and satiated nymphs (p = 0.3131). However, drift response varied significantly among days (p = 0.0001). An increase in drift occurred with increasing density between low, medium, and high initial density snags. Behavioral observations were conducted in an artificial stream mesocosm to determine if density-related drift was due to active or passive means, varied with time of day (day or night), and whether it was, in part, due to behavior modifications. Modifications investigated were differences in amount of time spent in movement, number of encounters, and types of competition encounters or modifications in the number of encounters. All drift phenomena observed were active detachment and not the immediate result of an intraspecific encounter. Differences in movement and number of encounters on the snag were documented for day and night. There was an increase in movement with increasing density (medium and high) with respect to the number of encounters. There was an increase in movement with increasing density during night-time runs and a decrease in movement with increasing density during day-time runs. Also, the type of interactions were much more aggressive during day-time runs.