Response of aquatic insects along gradients of agricultural development and flood magnitude in northern Japanese streams

Agricultural activities have increased environmental homogenisation in stream ecosystems. These alterations reduce the availability of flow refugia during flooding and increase the effects of flood disturbances on aquatic insects. Thus, we examined the effects of the agricultural development (percentage of pasture cover within the catchment) and flood magnitude (ratio of shear stress at high flow to that at low flow) on the resistance indices measured by relative changes in taxon richness or abundance between pre- and post-flood (all insects, five orders and 31 dominant taxa) at 27 sites in the Kitamihorobetsu River, northern Japan. The resistance index of taxon richness decreased with increasing agricultural development, whereas that of the abundance of all insects decreased synergistically with increasing agricultural development and flood magnitude. Among 31 dominant taxa, the resistance indices of 20 taxa, generally belonging to Ephemeroptera, Plecoptera and Trichoptera, exhibited stronger negative relationships with agricultural development than with flood magnitude. By contrast, three Diptera taxa exhibited week negative relationships with agricultural development. These results showed that the interactive effect between agricultural development and flood magnitude was taxon dependent, but agricultural development could be detrimental to the resistance of most of the studied taxa, especially Ephemeroptera, Plecoptera and Trichoptera taxa. Additionally, agricultural developments in our study watersheds was relatively low (< 18% pasture cover), and nevertheless, apparent interacting effects with natural disturbance was detected. This implies that limited agricultural development along the river line can lower the resistance of instream insects to natural disturbances