EFFECTS OF LOCALIZED DISTURBANCE ON MACROINVERTEBRATE COMMUNITY STRUCTURE IN RELATION TO MODE OF COLONIZATION AND SEASON

Relative importance of different dispersal modes (drift and crawling) on macroinvertebrate colonization within seasons under differing distu rbance regimes was tested by field experiment. Forty individual substr ata (tiles) were placed in a series of riffles in Mink Creek, Idaho, a third-order, Rocky Mountain stream. Twenty tiles were mounted on stee l bars to elevate them above the streambed and 20 were placed directly on the streambed to distinguish invertebrates colonizing by drift fro m those colonizing by crawling. After an initial colonization period o f 32 days, all invertebrates were removed from tiles every 2, 4 or 8 d ays or left undisturbed during the 16 day experimental period. Experim ents were conducted in the spring, summer and autumn of 1992. In the s pring experiment, invertebrate abundance was greater (524%) but divers ity (D) was lower (-42%) on elevated tiles as compared to unelevated t iles. No significant differences were found between elevated and unele vated tiles or disturbance frequencies in terms of abundance and diver sity in summer. Abundance decreased with increasing levels of disturba nce in autumn (-58%) (2-day disturbance to 8-day disturbance treatment s). No significant differences in diversity were found between disturb ance treatments or between tile elevations in the autumn. Relative imp ortance of drift and crawling as modes of colonization varied with sea son. Colonization through drift was important in spring, when water te mperature was low discharge was high, and algal resources were low. In summer and autumn, when water temperature was high, discharge was low , and algal resources were abundant, drifting and crawling taxa coloni zed equally rapidly.