The biogeography of omnivory: Do omnivores increase prey consumption relative to plants in sodium-poor environments?

The mechanisms driving the omnivore abundance and their diet content are not well understood. One hypothesis is sodium-limitation. Sodium bioaccumulates from plants to predators, thus, heterotrophs are abundant Na-sources. In Na-poor landlocked environments, consuming proportionally more prey may alleviate Na-limitation. We first tested the assumption that inland ants are Na-limited relative to coastal ants. We collected ants at ten paired lowland forests from Georgia to Maine by laying transects of randomly placed Na, sugar, and water baits. Second, we tested whether inland ants increase their prey, relative to plant, consumption. Using stable isotope analysis, we assessed the δ¹⁵N of conspecific ants between coastal and inland forests. Third, we tested the plasticity of Na-hunger and predation in a laboratory experiment. We collected Solenopsis invicta colonies and fed them Low, High, or Control Na diets. After five weeks, we measured discovery of and recruitment to crickets and sugar.

Inland ants visited Na baits 2-fold more than ants in coastal pairs. Ant activity (sugar bait usage) was consistent between paired forests. Data from stable isotope analysis suggests ants in Na-poor forests are more carnivorous than conspecific ants in Na-rich paired forests. Further, laboratory Solenopsis invicta fed Low Na diets discovered crickets 3.5-fold faster than Controls. In contrast, sugar discovery (ant activity) was consistent among diets. Our results suggest limited access to Na increases Na-hunger and carnivory in common omnivores. This in turn has ramifications on the rest of the food web and ultimately ecosystem function.