Experimental test of the effects of prolonged flooding practices on animal communities in rice paddy fields

Although environmentally friendly rice farming is expected to contribute to biodiversity conservation in paddy fields, scientific testing of the impact of individual management practices in rice cultivation is still in its early stages. This study used field experiments to quantify the effects of winter flooding and summer, mid-season drainage (a short-term soil drying in the middle of the rice growing season) on animal communities in paddy fields. We used a 2×2 factorial design with the factors winter flooding (±) and mid-season drainage treatment (±), crossed, and three replicate paddy fields in each combination (total n = 12). Our field experiments showed that winter flooding had little effect on the taxonomic richness and abundance of freshwater insects and spiders, but it increased the number of egg masses in amphibians such as the montane brown frog Rana ornativentris and the Japanese black salamander Hynobius nigrescens. In contrast, mid-season drainage negatively affected the taxonomic richness and abundance of freshwater insects, and the abundance of spiders. In particular, mortality usually occurred in species whose larval stage overlapped the period of mid-season drainage. Winter flooding promotes the biodiversity of animal communities in paddy fields. For amphibians that breed in the early spring, winter flooding provides breeding habitat. Mid-season drying had negative effects on animal biodiversity. Delaying or ceasing mid-season drainage would improve reproductive opportunities for freshwater insects and maintain species diversity. Diversification of winter flooding and winter drying across a rice paddy landscape and prolonged summer flooding practice may improve conservation outcomes.