Do pesticide residues have enduring negative effect on macroinvertebrates and vertebrates in fallow rice paddies?

Rice is one of the most important staple food in the world, with irrigated rice paddies largely converted from natural wetlands. The effectiveness of rice fields in help preserve species depends partially on management practices, including the usage of pesticides. However, related studies have focused predominately on the cultivation period, leaving the effects of soil pesticide residues on aquatic invertebrates during the fallow periods little explored; other animals, such as waterbirds, also rely on aquatic invertebrates in flooded fallow fields for their survival. We therefore investigated vertebrates and macroinvertebrates (terrestrial and aquatic) on rice stands and in flooded water during cultivation and fallow periods in organic and conventional rice fields in Taiwan. Association of environmental factors with terrestrial and aquatic organisms was also analyzed. In total, 32,880 individuals of 144 invertebrate families and 381 individuals of 15 vertebrate families were recorded after nine samplings each for six organic and six conventional rice fields. Family richness and abundance of all invertebrates (terrestrial and aquatic) were higher in organic than in conventional fields during the cultivation period, but were similar between the two agricultural practices during the fallow period. Richness and abundance of terrestrial invertebrates in both organic and conventional fields increased with the progression of rice cultivation, so did the differences between the two practices. Richness of aquatic invertebrates was mostly constant across the sampling period, while abundance increased but differences decreased during the fallow period. Richness and abundance of terrestrial invertebrates were positively associated with ambient temperature and height of rice stand. Abundance of aquatic invertebrates were positively associated with pH value and amount of dissolved oxygen but negatively associated with water temperature. Richness and abundance of all vertebrates and each of the constituting groups (fish, amphibian, reptile, bird, and migratory waterbird) were statistically similar between the two practices although abundance of migratory waterbirds in organic fields were two times those in conventional fields during the fallow period. Our study suggested accumulated effects of pesticides on suppressing terrestrial invertebrates during the cultivation period, but diminishing effects of pesticide residues on repressing aquatic invertebrates during the fallow period. This comprehensive study provided a holistic picture on macroinvertebrate and vertebrate fauna, as well as ramifications of pesticide usage, in a representative Southeast Asia rice paddy ecosystem. Further study should compare rice fields with natural wetlands to better assess how to capitalize on agroecosystems for biodiversity conservation.