Spatio-temporal dynamics of generalist predators (Tetragnatha spider) in environmentally friendly paddy fields

Earlier studies showed environmentally friendly farming (EFF) increased the populations of various organisms in agricultural landscapes, but the mechanisms of these increases were not well studied, due to a lack of knowledge on their temporal dynamics. Here, we examined spatio-temporal dynamics of Tetragnatha spiders, natural enemies of rice pests, in EFF and conventional farming (CF). Field surveys were conducted in 15 paddy fields under CF and 18 under EFF during three seasons in Tochigi Prefecture, central Japan. The results showed that Tetragnatha spiders were more abundant in paddy fields than in ditches during the growing season, but this tendency was reversed during the non-crop season. Thus, complementary utilization of ditches and paddy fields during different seasons appeared to maintain Tetragnatha populations. Both Tetragnatha spider and flying insect abundance increased in paddy fields under only EFF when the surrounding forest cover was high. There was a significant positive correlation between the density of flying insects and the population growth rate of Tetragnatha spiders, suggesting a bottom-up effect of flying insects emerging from paddy fields. We concluded that the high potential productivity in paddy fields, coupled with the presence of ditches as important habitats, maintained the high abundance of Tetragnatha spiders in paddy-dominated landscapes.