Factors Influencing the Distribution and Abundance of Macroinvertebrates on Stones in Sagehen Creek, California

DOCTORATE DISSERTATION:This dissertation considers factors that determine the distribution and abundance of macroinvertebrates on stones in Sagehen Creek, CA. Monthly samples of macroinvertebrates were taken from April to October, 1992. The species-abundance data did not fit a lognormal distribution for April and September, suggesting non-equilibrium in the community during these months. Diversity was highest in the spring and fall, lowest during the summer. Changes in diversity were due to changes in evenness rather than richness. There was a significant negative correlation between evenness on individual stones and stone size in the spring and fall. Competitive monopolization by few taxa on large, stable stones during high discharge can explain this trend. Species-area relationships for macroinvertebrate taxa on stones varied significantly over the sampling period. Random placement was rejected as a mechanism giving rise to the species-area relationships. The summer samples had the lowest degree of random placement, which can be explained by the importance of biotic interactions during low, stable discharge during the summer. There was a significant positive relationship between the slopes for each period and discharge, which can be explained by an undersaturation of species on small stones due to increased tumbling during high discharge. The colonization of macroinvertebrates on to artificial substrates was examined during 1993. Three factors were considered as possible influences on colonization: pupal cases of the caddisfly Anagapetus, substrate size, and density of the green alga Cladophora. Richness, diversity, and evenness were analyzed as response variables. Diversity was measured with the Shannon index, evenness with the J and E5 indices. The size treatment had a significant positive effect on richness and the J index. The case treatment had a significant positive effect on richness. Cladophora had a significant negative effect on the diversity and a positive effect on the J index. A multivariate general linear model with size, cases, and Cladophora density as factors indicated a significant main effect of cases and Cladophora on individual macroinvertebrate taxa densities. The combined main effects of substrate size, Anagapetus pupal cases, and Cladophora have dynamic influences on this macroinvertebrate fauna.