Hydrologic influences, disturbance, and intraspecific competition in a stream caddisfly population

In an earlier study, exploitative intraspecific competition was demonstrated for middle instars of the caddisfly Helicopsyche borealis in Big Sulphur Creek, a stream in northern coastal California. This population is univoltine with early instars first appearing in June, growing into late instars the following spring, and emerging as adults in May. Competition occurs during the summer dry season (June-September), a period of low-flow conditions without storms or increases in stream discharge. Beginning in autumn, however, larval densities decline because of wet-season storms and high discharge. Using multiple regression, we found a strong relationship between spring (i.e., post wet season) density of late-instar H. borealis and various hydrologic parameters of the preceding wet season over a 4-yr period, 1985-1988. Of eight hydrologic parameters used as indices of disturbance, total wet-season rainfall and peak discharge together accounted for 42% (P < .0001) of the variation in H. borealis density. In contrast, densities of middle instars of the next generation in summer were unrelated either to the preceding wet-season hydrologic regime of late-instar densities of the previous generation. Field experiments conducted in enclosures using larval densities of H. borealis that correspond to harsh, moderate, and mild wet-season hydrologic regimes (low, intermediate, and high densities, respectively) indicated that: (1) increases in larval density had a strong negative effect on algal biomass (as chlorophyll a); (2) larval density was negatively correlated with the proportion of larvae that pupated, and with pupal size; and (3) the high density (mild hydrologic regime) treatment produced smaller adult males and females, and lower fecundities than intermediate and low densities (moderate and harsh regimes, respectively). 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.