Perfecting the poop-life balance: Surplus manure fertility in insect food webs

Resource availability, particularly the availability of nitrogen, mediates all trophic interactions between microbes, plants, and animals. Surplus fertility can enhance plant growth and nutrition, making host plants generally more attractive to herbivores. However, responses to resource availability may vary among herbivores that vary in their diet breadth. The net effects of fertility on the third trophic level are even less understood, despite their implications for predator-prey interactions and, of particular importance in agroecosystems, biological control. Here, we present a field experiment to understand how resource availability cascades to the second and third trophic levels in organic agroecosystems. We manipulated a gradient of organic poultry manure fertilizer in a field of zucchini and tomatoes and measured responses of herbivores and predators. We predicted contrasting responses between specialist and generalist herbivores, positive responses to fertility in omnivorous predators, and responses driven by herbivore density in carnivorous predators. In line with our predictions, omnivores exhibited a positive response to fertility. However, in contrast with our predictions, fertility had generally positive effects on herbivore growth. Carnivore responses to fertility varied between host plants. Understanding how nitrogen availability cascades to the second and third trophic levels should help to inform organic farmers’ fertility decisions, considering its influences on bottom-up and top-down biological control.