HYDROLOGIC INFLUENCES, DISTURBANCE, AND INTRASPECIFIC COMPETITION IN A STREAM CADDISFLY POPULATION

Exploitative intraspecific competition has been demonstrated for middle instars of the caddisfly Helicopsyche borealis in Big Sulphur Creek, N coastal California, during the summer dry season (June-September). Beginning in autumn, however, larval densities decline because of wet-season storms and high discharge. The authors found a strong relationship between spring (post wet season) density of late-instar H. borealis and various hydrologic parameters of the preceding wet season. Of eight hydrologic parameters used as indices of disturbance, total wet-season rainfall and peak discharge together accounted for 42% of the variation in H. borealis density. In most years, storms and associated disturbance reduce late-instar H. borealis density below levels of larval competition; this counteracts earlier competition among middle instars that occurs during low-flow conditions. In drought years, however, densities may not be reduced as severely by wet-season storms and competition may continue until emergence. In this case, competition can reduce individual fitness, but because of high adult densities during these mild-disturbance conditions the reproductive capacity of the population is unaffected. -from Authors