Fire ants in our area often form lines of mounds along a street curb. Today I noticed a different pattern. This pattern consisted of a dozen, or more, fire ant mounds spaced in a more-or-less equidistant distribution in a large mowed lawn. Space between mounds averaged about 20 feet. I assume that these mounds represent separate colonies? How does one account for the spacing? It does not appear to be random. Is there some mechanism by which some ants from a colony move to a new location in response to some stimulus? Is the adaptive advantage of this dispersal pattern related to food abundance? What initiates sub-colony movement: new queen, reduction in available food, some scent left by ants that builds up as they exploit a site? Have there been studies of fire ant behavior in relation to the resource base? If my observation is accurate, this behavior should maximize efficiency of resource use.
Your observation of regular spacing among fire ant colonies is astute and similar to patterns you will see in lawns, pastures, and roadsides across the southeastern US, Texas, California, eastern Australia, and now parts of China. This natural patterning is a characteristic of dense monogyne (single queen form) fire ant populations. Each mound is the home of an individual colony with one queen. The space between colonies is "territory" which is divided up among adjacent colonies, usually in a rank order, where the largest colonies (which often have the largest mounds, too) occupy the largest territories.
Fire ants, especially the red imported fire ant, Solenopsis invicta, are one of the relatively few ant species that maintain absolute territories; that is, they guard and defend an area around the colony (mound), not just the mound itself, from any and all threats. Most other ant species defend only the nest, foraging trails, and food resources, so fire ants are a little unusual because of the amount of effort they put into maintaining their territories. Defense of a territory is probably an evolved response to intraspecific competitive pressure, that is, the fact that neighboring fire ant colonies are really good at competing with one another. The safest way to prevent having all of your food eaten or, more likely, your whole family captured and killed by neighboring fire ant colonies is to maintain a territory, remaining vigilant and defensive to any intrusion into the territory.
Maintenance of the colony boundaries is dependent largely on the interactions among a small number of individuals that scout around the boundaries of the territories. They respond to contact with non-nestmate ants by running away (in small numbers or as individuals, fire ant workers are actually rather timid when away from the nest). Thus, in zones of frequent contact at the edge of territories there actually exists a "no ants land" where there are very few individuals because they are "repelled" from one another by regular contact with non-nestmates. If they do not encounter ants from neighboring colonies they will, collectively, forage further and further from their territory. This thermostatic "control" of colony boundaries ensures that if, say, a neighbor colony suddenly dies, then its territory will rapidly be subsumed into one or more neighboring colonies' territory. As you can imagine, this kind of territoriality requires a large number of workers to maintain.
To the best of our knowledge, fire ant colonies generally do not appear to be resource limited or pathogen/parasite limited to the extent that it would drive frequent colony movements across populations. Fire ants, like most ants, are omnivorous and will opportunistically feed upon a wide range of food items. Colonies may, however, frequently be protein limited, but this does not appear to reach starvation levels that prompt a "move or die" scenario resulting from the resource base within the territory being completely depleted. Nevertheless, fire ant diet is an active area of research, as is territory maintenance. The best, and most complete work on fire ant territoriality has been carried out by my good friend Dr. Walter Tschinkel at Florida State University. Google him sometime and take a look at some of his papers or his book, The Fire Ants, if you would like to learn more!
Joshua King (guest expert) & the AntAsk Team