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Omnivory enhances growth and diminishes mortality in a marsh-inhabiting grasshopper.
Goeriz, Rachel*,1, Fox, Mark1, Denno, Robert1, 1 University of Maryland, College Park, MD
ABSTRACT- The mismatch in C:N stochiometry between insect herbivores and their host plants strongly suggests that herbivores are nitrogen limited. Omnivory has evolved in some herbivore lineages, and the occasional consumption of nitrogen-rich prey may help offset the inherent penalty many herbivores pay for consuming only plant food. Thus, dietary supplementation through the consumption of prey items may increase fitness. We tested the consequences of diet on the growth and mortality of an omnivorous grasshopper, the tettigoniid Conocephalus spartinae, that feeds on Spartina cordgrass as well on the herbivores (Prokelisia planthoppers) associated with this marsh dominant. Grasshoppers were fed either Spartina or a combination of Spartina and planthoppers. Fed a cordgrass diet only, grasshoppers grew slowly and survived poorly compared to when they were offered a mixed diet of cordgrass and planthoppers. The disappearance of planthoppers from treatments confirms that grasshoppers attack and consume planthopper prey. In the field, grasshoppers occur most abundantly in low-marsh habitats where cordgrass is most nutritious and planthoppers abound. Moreover, the frequency of invertebrate parts in the gut contents of Conocephalus is positively related to the density of planthoppers across habitats. These data suggest that by tracking planthopper populations and selectively colonizing nutritious Spartina, grasshoppers can more easily meet their nitrogen demands. Thus, dietary supplementation with prey and the occurrence of omnivory may have been promoted in certain groups of "herbivores" as means of increasing nitrogen intake in a carbon-dominated world.
Key words: Conocephalus, omnivory