Our observational study indicated that Tetragnatha spp. spiders, horizontal web weavers, suppress a mird bug density and its damage level in organic paddy fields (Takada et al. 2012). However, the molecular gut content analysis showed that only a small proportion of Tetragnatha spiders fed on the bugs (Kobayashi et al. 2011). In order to understand how the cascading effect by Tetragnatha, which is underestimated by the gut content analysis, is enhanced, two following hypotheses focusing trait-mediate effects and food web structure in organic paddy fields were examined.

The first hypothesis is that Tetragnatha spiders facilitated bug predation by wolf spiders through trait-mediated effects, in which their horizontal webs forced the bugs onto or near the ground and thereby into the hunting zones of wolf spiders. The second hypothesis is that the cascading effects of Tetragnatha spiders were enhanced by an abundant weed, Monochoria vaginalis var. plantaginea, through increasing chironomids, alternative important prey for the spiders. To examine this “weeds-chironomids-Tetragnatha spiders-mirids” link, we conducted field observations in 44 organic paddy fields of which data were analyzed using a Bayesian model.