Harvester ant responses to atmospheric nitrogen deposition in southern California deserts

Background/Question/Methods

Nitrogen deposition via airborne pollution has been affecting soil biogeochemical processes and plant communities across the United States. A number of studies have been conducted in forested systems and arid systems near urban centers; however, these investigations typically end with soil and plant analysis.  Building upon previous research, which revealed an atmospheric nitrogen deposition gradient from the San Bernardino Mountains through Joshua Tree National Park, we investigated the effects of increased nitrogen on plants and harvester ants. We measured the nitrogen and carbon concentrations in Messor pergandei ants at ten sites along the deposition gradient in Joshua Tree National Park and surrounding desert. In addition, we inventoried ant colony attributes, including nest density, the number of abandoned nests entrances near the active colony, and the distance from the nest to the nearest shrub.  Combined, these measurements are predicted to change in areas with higher nitrogen deposition and vary along the deposition gradient from west to east across the park.  Though research has demonstrated effects of nitrogen deposition on biogeochemical processes, this study increases our understanding of the effects of elevated nitrogen on components of the nitrogen cycle beyond the soil environment.  

Results/Conclusions

Although there were significant differences in nitrogen (p=0.02) and carbon (p=0.05) concentrations, as well as C:N ratios (p=<0.001), in M. pergandei individuals among sites, no correlation was found between these carbon and nitrogen parameters and the nitrogen deposition gradient through Joshua Tree National Park (%N r²=0.02, %C r²=0.007, C:N r²=0.02).  Furthermore, all other ant nest characteristics measured, including ant nest density, number of abandoned nest entrances, and distances from nest to the nearest shrub, were different among the sites; however, these parameters also did not follow a trend along the gradient.  Ant nest density ranged from 38-40 nests/ha in some of the high N deposition sites to 10 in one of the low deposition sites; however, the correlation between ant nest density and nitrogen deposition remained low (r²=0.22).  Our results suggest the amount of nitrogen being deposited on these ecosystems is not enough to alter the distributions and ecological stoichiometry of these ants.  Compensatory nitrogen cycling processes in the soil may reduce the effects of increased nitrogen on plants and thus higher trophic levels.  Harvester ant responses might change after longer ecosystem exposure to elevated nitrogen; however, other abiotic and biotic factors are likely driving current ant distributions and stoichiometry.