beta-diversity decline of aquatic insects at the microhabitat scale associated with agricultural land use

摘要

Several studies report the decline of beta diversity caused by agricultural impacts in river ecosystems. However, the susceptible scale of beta diversity to agricultural impacts and the indirect effects on beta diversity within hierarchically nested ecosystems are unclear. We first tested the hypothesis that beta diversity between microhabitats is significantly influenced by agricultural land use. We also examined the indirect effects of agricultural land use on alpha and beta diversities at the microhabitat scale. Twelve microhabitat samples (25 cm(2)) were collected at 27 reaches within Kitamihorobetsu River, Japan. All reaches were classified into three agricultural intensities based on pasture area (low, middle, and high), and their hierarchical diversities (microhabitat, reach, and catchment) were calculated using additive partitioning. Indirect effects were demonstrated by structural equation modelling using indirect and direct environmental gradients. The alpha and beta diversities at the microhabitat scale decreased significantly with agricultural intensity. Increasing pasture cover within catchments showed a negative correlation with habitat heterogeneity and water quality (habitat homogenization and water degradation) and a positive correlation with sand cover on streambeds (sedimentation of fine particles). The alpha diversity decreased with sedimentation of fine particles, and the beta diversity decreased with habitat homogenization and water degradation. Our findings suggest that species diversity of aquatic insects at the microhabitat scale would be susceptible to agricultural land use. Furthermore, we emphasize that the diversity index used to evaluate restoration projects should be carefully selected because influential abiotic factors were different between alpha and beta diversities.