INFLUENCE OF DISTURBANCE ON INSECT COMMUNITIES IN PACIFIC-NORTHWEST STREAMS

Coniferous forests of the Pacific Northwest provide a unique setting for stream ecology research because of the great age of the forests and the important role of wood debris in structuring aquatic systems. The composition and diversity of the insect community in Mack Creek, a stream in a 450 yr conifer forest, was compared with that in Grasshopper Creek where it flowed through a recent clearcut, and at Quartz Creek, which had a 40 yr deciduous canopy. Of the 256 taxa identified, Mack Creek had the highest species richness (196) and evenness. The open site had 191 taxa but high dominance of a few grazer taxa. The deciduous-canopy site had 165 taxa with abundant detritivores. Despite differences in density, the biomass of emergence was similar at the three sites, ranging from 1.53 to 1.65 g m-2 yr-1. The importance of disturbance in structuring stream communities was demonstrated in phenomenological studies after a debris torrent at Quartz Creek, and by monitoring stream recovery following the eruption of Mt. St. Helens in 1980. At Quartz Creek, the debris torrent eliminated the fauna from a 300 m reach, but there was rapid recovery. Emergence density in the same year was similar to that of the control site. The major shift in populations was a decrease in detritivores and moss associates and an increase in grazers, especially Baetis mayflies. At Ape Creek on Mt. St. Helens, over 200 taxa were recorded by 1987 but most occurred in very low densities. This site is reset by winter freshets and by infilling with glacial fines in the summer so the fauna continues to be dominated by weedy, or early successional species. At Clearwater Creek, the presence of wood debris as a stable substrate and limited inputs of fine sediment after 1980 have hastened population recovery. A decade after the eruption this site can be characterized as being in the mid-stages of succession with high insect productivity from an algal-based food web. With further growth of the riparian vegetation I predict a shift towards a detritus-based food web and fauna more similar to Mack Creek than it is at present.