Adding insult to injury in Texas grasslands: Crazy ant extirpates imported fire ant and further reduces native ant and arthropod diversity

Background/Question/Methods

Biological invasions reduce biological diversity by causing local extinction of native species, and by homogenizing local biotas, replacing regionally distinct species assemblages with assemblages comprised of cosmopolitan taxa.  Over the past 100 years, the Southeastern United States and Texas have been subject to invasion by a series of exotic, ecologically dominant ant species with each subsequent invader displacing its predecessor.  Central Texas is in the early stages of invasion by an ecologically dominant exotic ant species novel to the region: Nylanderia nr. pubens.  Using a combination of baits and pitfall traps, we sampled two areas of disturbed Gulf coastal prairie invaded by N. nr. pubens to evaluate the degree to which this invader impacts the diversity and abundance of co-occurring ants and other arthropods. 

Results/Conclusions

In invaded parts of these habitats, N. nr. pubens rapidly attains densities up to 2 orders of magnitude greater than the combined numerical abundance of all other ants in the system.  Overall ant biomass also increases in invaded habitat, implying that N. nr. pubens is exploiting resources in the system not fully utilized by the local ant community.  As N. nr. pubens spreads, the current ecologically dominant invader, the imported fire ant (Solenopsis invicta) is entirely eliminated from areas with high densities of N. nr. pubens.  Non-ant arthropod species richness and abundance are greatly reduced by the invasion with respect to imported fire ant dominated habitat and these impacts vary some with trophic category.  Further, N. nr. pubens significantly reduces both the abundance and species richness of the remainder of the ant assemblage and does so in a non-random manner.  Regionally distributed species are strongly negatively impacted but globally distributed species are largely unimpacted.  This difference appears to be driven by species with smaller body size being less strongly impacted and predominantly comprised of introduced taxa.  The result is a net biogeographic homogenization of the local ant assemblage, a process that may be general to systems where an invasive ant species enters habitat previously colonized by tramp taxa.  S. invicta impacts wildlife and the structure of arthropod assemblages and is currently nearly ubiquitous in non-forested habitats regionally.  Its displacement by N. nr. pubens will have important implications for the natural systems of this region.  The nature of the changes that ensue will depend upon key differences between the biology of this ant and that of imported fire ants.  Examples are discussed.