A basic question about populations is: Why is population density higher in some places than in others?
When we look at two equal-sized quadrats (50m x 50m) one of them has ten times the colonies of the other. Is this due to differences in available food? suitable habitat? or is it due to limitations in the availability of new recruits to the site. Maybe more queens arrive at the top site leading to more juvenile ant colonies and eventually more adult ant colonies.
In fact, it is probably all of the above. Some locations receive far fewer dispersing queens than others. These are locations that have higher population density. For a discussion of recruitment limitation in this species check out the following papers: Cole, B.J. and D. C. Wiernasz. 2002. Recruitment limitation and population density in the harvester ant, Pogonomyrmex occidentalis. Ecology 83(5):1433-1442. [This is the pdf version of the published paper....It is a large file due to a color figure. The color figure is somewhat odd in this scanned version. It is much better as Figure 3 below. This link is for a more compact version with Figures 1, 2, 3 and 4.] and Billick, I., B. Cole, D.C. Wiernasz. 2004. The scale of recruitment limitation in the harvester ant, Pogonomyrmex occidentalis (Hymenoptera: Formicidae). Annals of the Entomological Society of America. 97 (4):738-742.
Dispersing queens emanate from the hilltop mating swarms, declining in density as they move outward. However, there are differences in vegetation that also influence the local density of the population.