Timing of irrigation and crop rotation affects the community structures of aquatic animals in paddy fields

1. Irrigated paddy fields serve as alternative habitats for various aquatic animals originating from natural wetlands. When irrigation timing significantly diverges from the phenology of aquatic species, these species may fail to use the irrigated paddies as spawning grounds or nurseries for their larvae. Late irrigation can also harm species undergoing dormancy in the soil due to an extended dry phase. Despite these evident impacts, the relationship between irrigation timing and aquatic communities remains poorly understood.

2. I compared aquatic animal communities in 39 Japanese rice paddies managed under three agricultural regimes, focusing on irrigation timing and crop rotation: early-planting culture (monoculture of rice with early irrigation), normal-season culture (monoculture with intermediate irrigation), and wheat-rice double-cropping with late irrigation.

3. The composition of aquatic animal communities varied significantly among the agricultural regimes. Several species were identified as indicators of early-planting paddies due to synchronization between irrigation and their reproductive periods. Notably, the Tokyo Daruma pond frog (Pelophylax porosus porosus) was found exclusively in early-planting paddies. Indicator species of early planting and normal-season cultures included species with dormancy stages (e.g., resistant eggs or estivation), while late-irrigation paddies lacked species with similar traits. This suggests that delayed irrigation negatively affects the survival of aquatic species reliant on dormancy during nonirrigated periods.

4. Wheat-rice double-cropping paddies were distinguished by a higher abundance of larval chironomids and carnivorous aquatic insects. Incorporating wheat straw into the soil after harvest likely benefits detritivorous chironomid larvae by increasing organic matter supply. The resulting rise in detritus feeders may support predator populations and enhance aquatic species diversity. Additionally, delayed irrigation may benefit these insects by aligning with their late reproductive periods.

5. Synthesis and Applications

This study emphasized the importance of irrigation timing and crop rotation in shaping aquatic animal communities in paddy fields. Adjusting irrigation timing could support the conservation of target aquatic species. Furthermore, maintaining a mosaic of paddy fields with varied irrigation schedules and crop rotation practices can enhance regional biodiversity.