Abstract
Many studies have shown that the riparian forest suppresses the light intensity, primary productivity and the abundance of stream invertebrates by canopy shading in headwater streams. However, most studies were conducted at the stream-reach scale, whereas the light intensity seems to vary at smaller scales. We conducted a field survey to reveal the spatial pattern in the light intensity at consecutive riffles and the response of stream scraper assemblages to the light gradient, in a Japanese headwater stream. Canopy opening varied considerably (4-38%) throughout the study segment. Some survey points located in stream reaches without riparian forest showed greater degree of canopy opening. Difference in the abundance and species composition of canopy trees and understories may be responsible for generating the variation in the canopy opening. Analyses using a generalized linear model revealed that canopy opening significantly and predominantly explained the variance in density, biomass and taxon richness of stream scrapers. These invertebrate parameters were high at survey points with greater degree of canopy opening. The density and the biomass of many dominant scraper species also increased with canopy opening. These results imply that the quantity and the diversity of scrapers responded to the light gradient at the spatial scale smaller than the stream-reach. We suggest that the variation in the light intensity at a small spatial scale, which might have been even treated as an error, is an important determinant of the spatial distribution of stream scraper assemblages in headwater streams.
