Altered nitrogen and precipitation along urban gradients affect harvester ants and seed sources

We investigated the effects of nitrogen deposition and precipitation on Messor pergandei (Mayr) harvester ants and plants to identify alterations in the desert food web in California. We measured ant colony attributes and shrub fruit densities, as well as nitrogen and carbon concentrations and stable isotopes, at 18 sites along a nitrogen deposition gradient. Ant nest density increased from low to high deposition sites; however, ant nest mound size and the density of abandoned nests decreased as deposition increased. Nest mound size was positively correlated with the size and age of the colony; therefore, these results suggest that colonization has been more frequent with increased inter-colony competition in areas of high deposition. Nitrogen and carbon isotope values of perennial plant leaves and seeds, annual plant seeds, and ants were significantly enriched in the heavy isotopes from low to high nitrogen deposition regions, indicating the possibility of plants assimilating different sources of both elements, including anthropogenically-produced compounds. Plant carbon isotope discrimination also differed with the decrease in precipitation across the gradient. Considering that deserts are limited by both nitrogen and precipitation, our results suggest that altered nitrogen inputs in conjunction with precipitation may result in cascading effects through trophic levels and drive arid ecosystem change.