Disturbance and Variation in Competition Between Two Stream Insects

Abstract

This study examined temporal and spatial variability in competition as well as the relative importance of competition and natural disturbance to Hydropsyche oslari and Simulium virgatum in a southern California stream. These two filter—feeding insects dominated the community studied, accounting for 99% of all individuals. I maintained four field treatments every 2 wk for a year: (1) removal of Simulium; (2) removal of Hydropsyche; (3) removal of both taxa and subsequent scrubbing of the rock surfaces; and (4) unmanipulated controls. Competition from Hydropsyche oslari restricted the spatial distribution of Simulium virgatum in all seasons except late winter in upstream but not in downstream sections of the study site. Downstream abundance of Simulium was probably limited by food availability, not competition from Hydropsyche. During the winter, when floods occurred and densities were low, competition between these insects had no significant impact on spatial distributions. Although competition was seasonally variable, it occurred frequently and was not rare. Although disturbance mediated competition, competition was more important in determining the abundance of Simulium in 1983—1984, a drought year. Disturbance caused a 13% increase, while competition caused a 34% decline in the simuliid population over the year. It was not possible, however, to rank the importance of competition and disturbance when considering both species together. The relative importances were taxon specific. The abilities of Simulium and Hydropsyche to recover quickly after disturbance were inversely related to their ability to withstand competition. In addition, the interaction between the two factors was extremely important in determining the relative abundance of both taxa through the year. Competition and disturbance both played an important role in regulating this community, at times accounting for up to 60% reductions in the numbers of individuals present in the stream. The relative importance of competition and disturbance in Refugio Creek should vary from year to year depending on the timing and intensity of winter storms.