Imported fire ants near the edge of their range: Disturbance and moisture determine prevalence and impact of an invasive social insect

Background/Question/Methods

Habitat disturbance and species invasions are interrelated processes in natural systems, making it difficult to isolate the primary cause of ecosystem degradation.  In the invasion of the southern United States by red imported fire ants (Solenopsis invicta), dramatic losses in diversity followed invasion of central Texas by S. invicta.  However, more recent manipulative studies in Florida revealed no effect on diversity following removal of S. invicta from a disturbed pasture but moderate loss of diversity associated with their artificial introduction into undisturbed habitat, and no invasion without disturbance.  These studies present conflicting evidence as to the importance of S. invicta in driving diversity loss and question its ability to invade undisturbed habitat. We examine the distribution, and abundance of a largely monogyne S. invicta population and its association with the co-occurring ant assemblage at a site in south Texas close to the aridity tolerance limit of S. invicta.  

Results/Conclusions

We document that in this environment water availability modulates S. invicta densities.  Further, soil disturbing habitat manipulations greatly increase S. invicta population densities.  However, S. invicta penetrates all habitats regardless of soil disturbance history.  In contrast, controlled burns depress S. invicta densities.  Native fire ants were absent or extremely rare in habitats where S. invicta prevailed, and co-occurrence data indicate that this results from replacement by S. invicta.  However, S. invicta impacted native ants as a whole less strongly.  Intriguingly, native ants responded distinctly to S. invicta in different environments.  In wet, soil-undisturbed environments, high S. invicta densities disrupt the spatial structure of the ant assemblage by increasing clumping and are associated with reduced species richness, while in dry-disturbed habitats sites with high S. invicta densities tend to be species rich.  Analyses of co-occurrence indicate that reduced species richness in wet-undisturbed sites arises from negative species interactions between native ants and S. invicta.  However, these same data suggest that high species richness of dense S. invicta sites in dry-disturbed environments does not result from facilitative interactions with S. invicta.  Monogyne S. invicta populations play different roles in different environments, driving ant diversity loss in some, but being largely symptomatic of habitat disturbance in others.