Influence of disturbance on insect communities in Pacific Northwest streams

Coniferous forests of the Pacific Northwest provide a unique setting for stream ecology research becauseof the great age of the forests and the important role of wood debris in structuring aquatic systems. Thecomposition 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 QuartzCreek, which had a 40 yr deciduous canopy. Of the 256 taxa identified, Mack Creek had the highestspecies richness (196) and evenness. The open site•had 191 taxa but high dominance of a few grazertaxa. The deciduous-canopy site had 165 taxa with abundant detritivores. Despite differences in den-sity, the biomass of emergence was similar at the three sites, ranging from 1.53 to 1.65 g M-2 yr
The importance of disturbance in structuring stream communities was demonstrated in phenomeno-logical studies after a debris torrent at Quartz Creek, and by monitoring stream recovery following theeruption of Mt. St. Helens in 1980. At Quartz Creek, the debris torrent eliminated the fauna from a 300 mreach, but there was rapid recovery. Emergence density in the same year was similar to that of the controlsite. The major shift in populations was a decrease in detritivores and moss associates and an increasein grazers, especially Baetis mayflies. •
At Ape Creek on Mt. St. Helens, over 200 taxa were recorded by 1987 but most occurred in very lowdensities. This site is reset by winter freshets and by infilling with glacial fines in the summer so the faunacontinues to be dominated by weedy, or early successional species. At Clearwater Creek, the presenceof wood debris as a stable substrate and limited inputs of fine sediment after 1980 have hastenedpopulation recovery. A decade after the eruption this site can be characterized as being in the mid-stagesof succession with high insect productivity from an algal-'cased food web. With further growth of theriparian vegetation I predict a shift towards .a detritus-based food web and fauna more similar to MackCreek than it is at present.