Changes in trophic niche—the pathways through which an organism obtains energy and nutrients—are a fundamental way in which organisms respond to environmental conditions. But the capacity for species to alter their trophic niches in response to global change remains largely unknown. We examine food webs in three long-term and large-scale experiments to test how resource availability and nutritional requirements interact to determine an organism’s trophic niche in the context of one of the largest global trends in land use—the rise in bioenergy production. We use carbon and nitrogen stable isotope analyses to characterize arthropod food webs across three bioenergy crops representing a gradient in plant resource richness (corn monocultures, fields dominated by native switchgrass, and restored prairie), and to quantify changes in the trophic niche of a generalist ant species across habitats. In doing so we measure the effects of basal resource richness on food chain length, niche breadth, and trophic position. We frame our results under two hypotheses that explain variation in trophic niche—the niche variation hypothesis which emphasizes the importance of resource diversity and ecological opportunity, and the optimal diet hypothesis which emphasizes energetic constraints and the availability of optimal resources.
Results/Conclusions
Increasing plant species richness lengthened food chains by 10 to 20% compared to monocultures. Niche breadths of generalist ants did not vary with resource richness, suggesting they were limited by optimal diet considerations rather than by ecological opportunity. Ants instead responded to changes in plant richness by shifting their trophic position. Ants in resource-poor monocultures were top predators, sharing a trophic position with obligately predatory spiders. In resource-rich environments, in contrast, the ants were omnivores, relying on a mix of animal prey and plant-based resources. The shift towards more predatory behavior in resource-poor environments is probably driven by the absence in monocultures of plant-based carbohydrates like honeydew and extra-floral nectar. The shorter, simpler, and more top-heavy food chains in corn fields may also mean that food webs in agricultural landscapes devoted to monocultures are less stable than those producing native perennial biofuels. Our results highlight several novel ecosystem impacts of alternate bioenergy landscapes, and suggest that niche breadth and trophic diversification depend more on the presence of optimal resources than on ecological opportunity alone.