Outcomes of competitive interactions after a natural increment of resources: the assemblage of aphid-tending ants in northern Patagonia

Abstract

Ant-aphid relationships provide excellent opportunities to study how changes in resource availability may affect the outcome of competitive interactions. Variations in soil fertility may affect host plant quality, with concomitant effects on aphid abundance and the amount/ quality of aphid honeydew. This may determine the intensity at which tending ants defend aphids against natural enemies and competing ants. In a shrub-steppe of northern Patagonia, aphid-infested thistles naturally grow on contrasting fertility substrates: organic waste piles of leafcutting ants (refuse dumps) and nutrient-poor steppe soils. Thistles growing on refuse dumps have much larger aphid colonies than thistles growing on steppe soils. We took advantage of the co-occurrence in the field of plants with contrasting aphid density to study the effect of natural variation in food availability (aphid density) on aphidtending ant species richness and agonistic interactions among them. Enhanced aphid density did not promote the coexistence of aphid-tending ant species. Although all ant species are potential colonizers of the study plants, thistles were often monopolized by a single ant species, regardless of aphid density. Field experiments showed that increased aphid density did not modify aggressiveness toward an intruder ant, nor the probability of coexistence between two rival ant species after the invasion of a host plant. We discuss several hypotheses to explain why increased resource availability does not necessarily reduce competitive interactions in ant-aphid relationships.