Plant diversity promotes species richness and community stability of arthropods in organic farming

Trophic cascades among plants, herbivores, and natural enemies can be affected by agriculture planting shifts from complex (polyculture) to simple (intensive). Yet, the information on regulation of trophic relations in organic agriculture is limited. To address this, we explore the impacts of plant species richness on composition of three functional groups of arthropods (pest, natural enemies, and neutral insects), as well as the richness and temporal stability of natural enemies. We tested four plant species richness levels and divided into 24 similar sized small field plots on an organic farm. We found 48 species of arthropods (29 species of pests, 9 species of predators, 4 species of parasitoids, and 6 species of neutral insects), with their abundance and species richness significantly increased with increasing plant species richness, suggesting a bottom-up effect. Total arthropod stability (pests, natural enemies, and neutral insects) increased with plant diversity, but individual species stability decreased for most arthropods, which resulted from enhanced structural diversity and reduced resource specificity in these plots. Plant species richness was significantly correlated with specialist and generalist pest and natural enemy species abundance as well as their stability. These findings suggest diverse organic farming to better sustain generalist predators with increased provision of food resources and breeding habitats.