Removal Experiment Reveals Limited Effects of a Behaviorally Dominant Species on Ant Assemblages

Abstract

Ant communities are thought to consist of a competitive hierarchy of interacting species, with an assemblage of subordinate species being structured by a dominant species. Mensurative and behavioral studies suggest a significant role for competition in structuring ant communities, although there are few experimental studies to support this contention. We examined the effect of the dominant ant Iridomyrmex purpureus on the ant fauna of sandstone outcrops in southeastern Australia. We conducted a mensurative survey using transect counts to compare ant assemblages at eight outcrops with, and eight without I. purpureus. Using cages, we then successfully excluded the dominant ant from four outcrops and compared assemblages at these exclusion sites with those at sites with and without I. purpureus and with procedural control sites over a period of 12 months. We conducted behavioral studies comparing I. purpureus with six other common species in terms of their abilities to locate, recruit to, and defend bait. While initial surveys indicated both positive and negative effects of I. purpureus on the activity of several subordinate species, exclusion of I. purpureus resulted only in changes in the abundance of other species of Iridomyrmex, which are behaviorally and ecologically similar to the dominant species. Iridomyrmex purpureus was faster at discovering bait than other species, but not always better at recruiting to the bait. It interfered with the foraging of all species tested; however, it displaced other Iridomyrmex from bait most often. While longer‐term studies may provide more definitive results, exclusion of the dominant species over one year had strong effects only on ecologically similar species. Despite its behavioral dominance and association with several other species in the mensurative surveys, I. purpureus was not shown to cause any other major changes in ant community structure.

Experimental evidence that human impacts drive fire ant invasions and ecological change

“…Under natural conditions, fire ants simply do not occur in this habitat; therefore, normally have little or no effect on the natural ant community (17,28). Previous manipulative experiments have also revealed that dominant ant species may lack widespread competitive effects, instead only negatively impacting ecological equivalents (24,26,27). In this system, native species such as P. morrisi and P. dentata have similar-sized workers, similar diets, large colonies, and high abundance (28), and can thus be described as ecologically similar to S. invicta.…”

“…Thus, the success of S. invicta and many other invasive ant species is also thought to result primarily from behavioral dominance and release from natural enemies that allows them to attain numerical superiority and competitively suppress co-occurring species (20,22,23). However, experimental demonstration of competition in ant communities is uncommon, especially for invasive species, and very few manipulative studies exist (24)(25)(26); to our knowledge, only King and Tschinkel (27) have previously manipulated entire populations of ants over multiple years.…”

Experimental evidence for mediation of competition by habitat succession

“…Ants may respond to increased favorability in a number of ways, including an increase in worker numbers (Gibb and Hochuli 2004), the formation of new nests, often through nest budding or relocation to more favorable environments (Rosengren 1986, Gibb andHochuli 2003), or increased alate production (Pontin 1969). This suggests that the two year time frame for this experiment is adequate to detect responses from ant assemblages.…”

Does the availability of arboreal honeydew determine the prevalence of ecologically dominant ants in restored habitats?

“…This group is behaviourally and numerically dominant in most ecosystems in Australia (e.g. Shattuck, 1999;Hoffmann and Andersen, 2003;Gibb and Hochuli, 2004). The definition makes no reference to the type of resources used by this group and no previous study has tested whether ants from this group are the predominant tenders of hemipterans in any ecosystems.…”