Modernization of drainage systems decreases gray-faced buzzard occurrence by reducing frog densities in paddy-dominated landscapes

Modernization of drainage systems is suspected of causing a deterioration in the biodiversity of paddy-dominated landscapes that are substitutes for natural wetlands. Here, we focused on the gray-faced buzzard (Butastur indicus), a top predator that inhabits agricultural landscapes in East Asia. We evaluated the impacts of drainage system modernization on buzzards by measuring reductions in paddy-dwelling frogs, a major prey for buzzards that are common in rice fields. Field surveys were conducted in yatsu valleys (“yatsu” refers to small, narrow valleys in which rice cultivation often dominates the valley bottom) in Shimane prefecture, western Japan. We found that the distribution of buzzards was positively correlated with the densities of black-spotted pond frogs (Pelophylax nigromaculatus) and forest green tree frogs (Rhacophorus arboreus), although the relationship between buzzards and pond frogs was stronger. In addition, pond frog density was negatively correlated with modern drainage systems, whereas tree frog density did not show such a pattern. Therefore, modern drainage systems may reduce the buzzard population indirectly by reducing pond frog densities.