Winter-flooded rice cultivation (WFRC) is a firming system to keep paddies flooded after harvest. It has been conventionally believed that WFRC contributes to conservation of biodiversity of paddies, a major component of biodiversity of wetland in East Asia. However, little is known quantitatively to what extent WFRC affects species composition and abundance of aquatic animals in paddies. We experimentally tested the conservation effects of WFRC on aquatic animals in paddies by manipulating timing of flooding in experimental paddies in Shiga, Japan. We set up three paddies under different timing of flooding; early (start to flood in March), middle (April), and late (May), respectively, and collected aquatic animals in each site using dipnet during April to August, 2011.

The manipulation of the timing of flooding resulted in a mass emergence of Diptera within a short period shortly after the flooding in the early treatment, and also in the middle treatment though to a lesser extent, but no such emergence in the late treatment. Abundance and species diversity of aquatic insects, in particular Odonata, Hemiptera and Coleoptera, exhibited the descending order with the early, middle and late treatments, whereas no substantial effect was observed for tadpoles. In total, the early flooding treatment promoted biodiversity of paddies, in particular aquatic insects.