Resource gradients may shape the distribution and abundance of ant colonies. This link, between food resources, ant biomass and species interactions, is of importance in the organization of ant assemblages. In this study we asked how variations in colony size as well as colony density could be correlated to primary productivity and competition intensity between colonies of seed-eating ant species. The study was conducted at six sites along a precipitation gradient (100-600mm) differing in primary productivity, in two consecutive years. In each site we chose three plots, in which we placed 20 baits. We measured abundance of ant species at baits, forager number and foragers' body size. Competition intensity was examined indirectly by measuring co-occurrence patterns of species at baits and by analyzing nests’ spatial distribution.
The results show that with increase in site productivity colony density decreases, but worker number per colony increases. Co-occurrence patterns between species indicate that interference is more intense at the low-productivity sites, while the analysis of nests’ spatial distribution showed that overdispersion is common in the low-productivity sites. These results suggest that competition may be an important factor in the organization of ant assemblages in this system. In the low-productivity sites, interference is important since species avoid encounters, while in the high-productivity sites, competition may limit colony density.