Community structure and functional organization of aquatic insects in an agricultural mountain stream of Taiwan: 1985-1986 and 1995-1996

Changes in stream water and habitat quality of Chichiawan Stream, which flo ws through Wulin Farm in central Taiwan, were examined using community stru cture and functional organization of aquatic insects at 4 sites in 1985-198 6 and 1995-1996. Long-term records of water chemistry for the study area in dicate that water quality in 1995-1996 had not degraded as compared with da ta in 1987-1988. It was found that there were significant differences in th e number of taxa and number of individuals per sample unit for samples at a ll sites combined between 1985-1986 and 1995-1996. In general, the number o f taxa and number of individuals per sample unit were higher in 1985-1986 t han in 1995-1996 at the 4 sites. Higher relative abundances of Baetis spp., Rhithrogena ampla, Cincticostella fusca, and Uenoa taiwanensis were found in 1985-1986 compared to 1995-1996, suggesting that the substrate quality o f the stream had deteriorated at sites located in agricultural areas. Simil ar results were found between the taxonomic and functional feeding group an alyses when the percentage similarity analysis was used. The functional org anization and community composition of aquatic insects at sites 1 and 2 in 1995-1996 were similar to those at site 4 in 1985-1986. Site 4 is located d ownstream of the confluence between Chichiawan Stream and Yousheng Stream w here the stream watershed has been developed for agricultural land use. Pri ncipal component analysis (PCA) indicated that, iii addition to the substra te quality of the stream, water temperature, dissolved oxygen, conductivity , and ammonia were the most important physicochemical variables shaping the aquatic insect community structure in the study stream reach. The study si tes in agricultural areas had poorer stream water and habitat quality. The raw cropping of orchards and vegetable farms greatly increased soil erosion and suspended solids inputs to the stream which may have been harmful to t he aquatic insect communities.