Plant nutrient status shapes predator and herbivore dynamics through plant-mediated pathways

Plant nutrient availability shapes interactions among plants, herbivores, and natural enemies, yet predicting how fertility shapes predator and herbivore dynamics remain challenging. In systems where omnivorous predators are common, nutrient inputs may act through multiple pathways, simultaneously altering plant quality, herbivore performance, and predator responses. Here, we combined greenhouse experiments with controlled laboratory bioassays to examine how NPK fertility regimes influence plant physiology, western flower thrips ( Frankliniella occidentalis ) population dynamics, and the performance of the omnivorous predator Orius insidiosus . Nutrient enrichment enhanced plant growth, chlorophyll content, and flower production, resulting in higher herbivore densities under fertilized conditions. Predator responses were strongly dependent on prey availability but were further shaped by plant nutritional context, with increased predator abundance and reproduction observed under fertilized conditions only when prey were present. Laboratory assays revealed that predator responses to fertility were process dependent. Survival and oviposition were influenced by interactive effects between prey availability and plant context, whereas nymph recruitment reflected independent contributions of both factors. These findings demonstrate that nutrient availability does not influence trophic interactions through a single pathway but instead simultaneously modifies plant traits and resource availability that shape both herbivore and predator dynamics. This work highlights the importance of considering plant-mediated and prey-mediated processes together when predicting ecological responses to nutrient inputs in managed ecosystems.