Prevailing wisdom holds that imported fire ants marched across the southern United States on the virtue of their fierce nature and superior competitive ability. The fire ant conquest of the south reads like a tale of bravery and intrigue, but according to Walt Tschinkel and Josh King it is also not true. They have a must-read study in PNAS this week detailing a tight set of field experiments that turns the conventional wisdom upside-down.
King and Tschinkel disturbed various patches of native Florida pine forest by mowing or plowing, later adding fire ants to some of the patches and to undisturbed control plots. The results after three years of observation are striking. Intact forest patches did not host fire ants even where they were released, while disturbances alone reduced native ant numbers even in the absence of fire ants. In other words, human-mediated disturbance by itself can explain the retreat of native species, without the fire ants so much as laying a tarsus on the turf. Not only that, but fire ants only thrived in the disturbed patches. If these findings hold up (and this is an big *if*; the study pertains to one type of habitat and comprises only the less virulent monogyne ants) they’ll be a great example of how correlation should not be taken for causation: just because native ants aren’t as abundant in the presence of fire ants does not mean the fire ants caused the decline.
The uncomfortable implication is that the real pest may not be the fire ant all, but ourselves. Native species decline because we’ve altered their habitat. The fire ants are merely along for the ride, symptoms of the problem. In light of this interpretation, King and Tschinkel eschew the idea that imported fire ants are an invasive species, preferring the perhaps more accurate term of “disturbance specialists”.
I’ll give one criticism of the paper. In passing, King and Tschinkel discuss the Argentine Ant as another species that might be more of a disturbance specialist. Here I disagree. It is true that disturbance favors the Argentine Ant, but extensive field studies in California by Deborah Gordon, David Holway and others have documented the spread of Argentine ants through relatively pristine coastal and riparian habitats. It seems clear from experimental studies that the concurrent decline in native species does indeed follow from the invasion itself, so we’re still best off evaluating the ecology of each species on a case-by-case basis.
postscript: The New York Times covers the story