Preliminary observations on spatial distribution patterns of stream caddisfly populations

Abstract

Spatial heterogeneity may influence sampling variability of stream caddisfly populations. The mean number of Cheumatopsyche pettiti (BANKS) larvae/Surber square foot sample in a riffle of uniform depth and substrate size (Rock Creek, Carroll County, Indiana, USA) was calculated for sample sizes ranging from 2 to 52 with 30 replicates for each sample size. With a sample size of 2, means ranged from 1.5 to 14.5 larvae/square foot, a departure from the population mean of 6.2. Hyporheic distributions, resource orientation, and population age structure may influence the negative binomial distribution pattern of C. pettiti. Larvae of Dicosmoecus giluipes (HAGEN) in the McCloud River, Shasta County, California, USA, exhibited non-aggregated distribution patterns in areas of uniform substrate size and aggregated patterns in areas of mixed substrate size. A reduction in sampling variability may reflect the differences in microenvironmental variation between uniform and mixed substrate areas. Spatial distribution patterns may change temporally, e.g. the value of k for a population of Ceraclea ancylus (VORHIES) in Brashears Creek, Spencer County, Kentucky, USA ranged from 0.12 to 0.23 during larval development but increased to 0.39 during pupation. Both taxonomic and biometric considerations are necessary in designing ecological studies.