In species-rich ecosystems, such as subtropical and tropical forests, higher trophic level interactions are key mediators of ecosystem functioning. Plant species loss may alter these interactions, but the effects of plant diversity might be modified by intraguild interactions, particularly among predators. We analyzed the relationships between spiders and ants, two dominant predatory arthropod taxa, on tree saplings across a gradient from medium to high woody plant species richness in a subtropical forest in Southeast China. Neither ant nor spider total biomass was significantly related to plant species richness. By contrast, the biomass distribution of web-building and hunting spiders changed and spider family richness increased in the presence of ants, resulting in more web builder-dominated assemblages. However, these relationships depended on the plant communities, and were stronger in plots with higher plant species richness. Our results indicate that in addition to potential effects of ants on hunting spiders in particular, ants could indirectly influence intraguild interactions within spider assemblages. The observed shifts in the spider assemblages with increasing ant presence and plant species richness may have functional consequences, as web-building and hunting spiders have distinct prey spectra. The relationships among ants, spiders, and plant species richness might contribute to explaining the non-significant relationship between the overall effects of predators and plant diversity previously observed in the same forest plots. Our findings thus give insight into the complexity of biotic interactions in such species-rich ecosystems.