Substrate Relationships, Spatial Distribution Patterns, and Sampling Variability in a Stream Caddisfly Population

Abstract

The influence of substrate composition on spatial distribution patterns of final instar larvae in a single-cohort population of the limnephilid caddisfly Dicosmoecus gilvipes (Hagen) was examined in a pool of the McCloud River, Shasta Co., California. Four distinct substrate habitats were identified; three strata, each composed of different stone sizes, had low variability (sorting coefficient = 1.0), whereas the 4th was composed of highly mixed stone sizes (SC = 1.7). Each stratum was divided into 0.093-m 2 quadrats and Surber samples were collected in all areas where depth was < 1 m. Sampling efficiency was optimized by incorporating the surface-dwelling behavior and large size of D. gilvipes larvae into sampling design. Considering the pool as a single habitat, D. gilvipes exhibited an aggregated spatial pattern fitting a negative binomial distribution, with large numbers of samples necessary to estimate accurate mean density. However, within each of the uniform substrate ribbons, the spatial pattern of D. gilvipes was non-aggregated and counts from the samples followed a Poisson series. Accordingly, sampling variability was low and fewer samples were necessary to estimate mean densities within each of these areas. In the unsorted substrate ribbon, an aggregated distribution was again exhibited, resulting in high sampling variability. Habitat stratification may reduce sampling variability within the constituent strata. Integration of life history features of study populations into sampling design may result in reduced sampling effort and greater accuracy.