The nutritional geography of ants: Gradients of sodium and sugar limitation across North American grasslands

Abstract

Sugar and sodium are essential nutrients to above- and below-ground consumers. Unlike other properties of ecological communities such as abundance and richness, we know relatively little about nutritional geography—the sources and supply rates of nutrients, and how and why they vary across communities and ecosystems. Towards a remedy, we present a suite of hypotheses for how sodium and sugary exudate availability should vary for a common omnivore—the ants—and test them using a survey of 53 North American grasslands. We do so by running transects of salt and sugar baits and inferring the magnitude of environmental supplies as the inverse of their use as exogenous baits. We then use estimates of potential drivers of the availability of salt and sugary exudates—plant and soil nutrients, and bioclimatic variables—to test the best predictors of sodium and salt use by ant communities. Beyond a baseline of ant activity, salt use increased as an inverse of the amount of sodium in an ecosystem's plant tissue, but not its soils. Plant sodium varied by two orders of magnitude in grasslands across 16° latitude. This suggests that plant exudates are an important source of sodium for grassland consumers. The three drivers that best predict exogenous sugar use by ants all point to factors constraining sugar production: net above-ground productivity, how far the community is into that year's growing season (both reflecting the rates of photosynthesis) and, intriguingly, the potassium content of plant tissue, which is likely linked to exudate production via plant turgor. These data suggest that ants and other consumers across a range of grasslands and climate vary significantly in the demand and supply of sugar and salt. This nutritional geography ultimately arises from gradients of climate and biogeochemistry with implications for the geography of plant–consumer interactions.