Diet breadth mediates arthropod responses to nutrient availability across trophic levels

Nitrogen mediates a complex sprawl of ecological interactions, challenging our ability to predict energy flow in food webs. For example, diet breadth determines unique consequences of nitrogen for herbivorous insects, with specialists less closely tied to plant quality than generalist herbivores. Predator responses to nitrogen also vary along the lines of diet breadth; foliar-feeding omnivores respond to plant quality, while generalist predators may lack direct links to nutrient availability. Here, we manipulate a gradient of soil fertility in a field experiment on two model host plants: zucchini and tomato, measure responses of herbivores and predators that vary in diet breadth, and examine consequences for prey suppression. Diet breadth mediated responses to fertility across trophic levels, with specialist and generalist herbivores exhibiting unique responses to fertility on zucchini that varied across fertility concentrations. Foliar-feeding omnivores increased in abundance in response to fertility on both host plants, while generalist predators had a neutral response. The combined action of omnivores and generalist predators resulted in neutral prey removal across fertility treatments. Rather than promoting herbivore outbreaks, our results suggest that bottom-up effects of nutrient availability might come back around as top-down control via omnivore recruitment. By examining nuanced responses to nitrogen across trophic levels and within functional groups, we increase the resolution with which we can trace basal resources through trophic cascades.