FACTORS DETERMINING POPULATION-DENSITY AND SIZE DISTRIBUTION OF A FRESH-WATER SNAIL IN STREAMS - EFFECTS OF SPATIAL SCALE

Eleven spring-fed streams located in central Oklahoma were sampled over a two-year period to evaluate which abiotic variables were most important in determining the abundance and size distributions of the freshwater snail Physella virgata virgata. To determine if spatial scale and sampling resolution affected the conclusions derived from stepwise multiple regression and principal components analyses, data were analyzed in three ways. An among-stream assessment included data collapsed over time and space for each stream. Within-stream analyses were also conducted on each stream separately, and a between-habitat (pools versus riffles) analysis was performed for each stream separately. At the largest scale, crayfish (Orconectes virilis) abundance and algal biomass were most strongly associated with both the size and abundance of snails among streams. Within-stream analyses suggested that flow rate and substrate particle size were most important in predicting within-stream abundance patterns of snails. Significant differences in both snail size and abundance were found between pools and riffles, with higher abundances and larger snails being found in pools. Variables associated with microhabitat selection of snails also differed between pools and riffles, with flow and substrate particle size being most influential in riffles, while algal biomass and macrophyte densities were most important in pools. Our findings suggest that spatial scale and sampling regime are critical in determining benthic invertebrate size and abundance patterns.