Factors affecting abundance and species composition of generalist predators (Tetragnatha spiders) in agricultural ditches adjacent to rice paddy fields

• Abundance and species composition of Tetragnatha spiders in ditches were examined.
• Tetragnatha extensa and T. maxillosa accounted for 95% of the total abundance.
• The abundances of the two species differently responded to environmental factors.
• Species composition varied depending on vegetation height and distance to forest.

Agricultural ditches adjacent to paddy fields are expected to provide an alternative habitat to the fields for Tetragnatha spiders, a major component of the natural enemies of crop pests in rice paddy ecosystems. To better understand the spatial distribution of these spiders in agroecosystems, it is essential to evaluate their abundance and species composition in such alternative habitats. Here, we examined the factors determining abundance and species composition of Tetragnatha spiders in agricultural ditches around rice paddy fields. Field surveys were conducted in transects set along 43 concrete ditches in the northern Kanto region, Japan, prior to the rice growing season when spiders are absent in paddy fields. Four species of Tetragnatha spiders were found, with two species (T. maxillosa and T. extensa) accounting for 95% of the total abundance. These two species showed different responses to local and landscape factors. The abundance of T. maxillosa increased with ditch width, whereas that of T. extensa was highest at intermediate width. Vegetation height had a positive effect on the abundance of T. maxillosa but a negative one on that of T. extensa. Distance to the nearest forest had no effect on T. extensa, but negatively influenced T. maxillosa, indicating that forest environments are beneficial to the latter. Reflecting these differences, the proportions of the two species in overall Tetragnatha abundance varied depending on vegetation height and distance to forest. Our findings indicate that the potential for immigration of Tetragnatha spiders from ditches to fields depends on local conditions and the positional relationship to forest.