Intensity and Importance of Abiotic Control and Inferred Competition on Biomass Distribution Patterns of Simuliidae and Hydropsychidae in Southern Québec Streams

Abstract

Biomass of Simuliidae larvae was measured on individual rocks in four surveys of southern Québec streams to quantify the intensity and importance of relationships between biomass of black flies and microhabitat features (depth, current velocity, rock size), site properties (distance from the lake, seston quality), and biomass of Hydropsychidae. Microhabitat features and site properties accounted for 14-47% of the variability in biomass of hydropsychids and simuliids on individual rocks, and between 30 and 67% of variability in mean biomass across sites. Addition of biomass of potential competitors did not significantly improve the proportion of variability accounted for by the multiple regression models. Within most sampling sites, biomasses of simuliids and of hydropsychids were positively correlated. However, analysis of microdistribution patterns across sites suggests a small but significant negative partial correlation between hydropsychids and simuliids in summer and a positive partial correlation in winter. Analysis of covariance revealed strong site effects that could not be explained by microhabitat features or biomass of potential competitors. These results suggest that hydropsychids have a negative effect on biomass of simuliids in summer, but that biomass of simuliids in different sites is more influenced by microhabitat features and site properties than by the biomass of hydropsychids. Reanalysis of published work on interactions between simuliids and caddisflies suggests that intensity of competition is relatively constant, but that its importance is overestimated by controlled experiments. Quantitative models describing the distribution of filter feeders across sites should include better correlates of site suitability rather than considering abundance of competing groups.