Solenopsis invicta - invasive or just disturbed?
Prevailing wisdom holds that imported fire ants marched across the southern United States on the virtue of their fierce nature and superior competitive ability.Â 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.Â Â 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.Â The results after three years of observation are striking.Â 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.Â 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. Â 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.Â Â Native species decline because we've altered their habitat.Â The fire ants are merely along for the ride, symptoms of the problem.Â 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.Â In passing, King and Tschinkel discuss the Argentine Ant as another species that might be more of a disturbance specialist.Â Here I disagree.Â 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.Â 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:Â The New York Times covers the story
King, J. R., and Tschinkel, W. R. 2008. Experimental evidence that human impacts drive fire ant invasions and ecological change. PNAS, online early. doi
Great post. Really fascinating. BTW, I've dropped the 'e' off of your last name on my 'Blog of the Week" post. Sorry about that!
Great Post, it's yet another reason for people to minimize the size of their lawns and get more native vegetation in there.
I'm not from the south and don't have fire ants here in NJ (that I know of) but I've herd stories of them being aggressive towards native ant colonies. Raids actually going up trees to take on Camponotus in dead tree limbs an so on. This isn't very common perhaps?
Also couldn't it be said that a disturbance species could become invasive. All the disturbed environments we create seem to become invasive ant factories. I've herd/read from a number of sources that only 5% of the forest land in the US is reserved for park and wildlife, so that's 95% open for anyone to log and develop, and we've already removed 70% of that. The odds only favor ants that can live with us.
Love the blog and thanks for mentioning our work! Just a quick response to your thoughts as I appreciate your perspective...
I included Argentine ants in our discussion of our results specifically to get people talking about other important invasive ants and thinking about perhaps doing similar experiments. I would caution against comparing any of the experimental work that has been done with Argentine ants with our work simply because they do not include population level manipulations of (thousands) of colonies over multiple years. This is not to disparage the work that has been done, but to suggest that doing similar experiments might be prudent before making exceptions for other invasive ants. In particular, I would argue that some of the work by Bolger and the fragmentation study by Andy Suarez suggests that Argentine ants do benefit from disturbance. Scientific discourse - catch the fever!
Howdy Josh --
I have a little hesitation to accept your and Walt's results as general because of the "one type of habitat" argument posed by Alex. It might be general for upland habitats, but fire ants invade wet habitats such as bayheads in the Florida scrub and "high" ground in wetlands (here, often preferring rotten logs and stumps) throughout the Southeast, without any human interference nor any soil perturbation other than periodic inundation. One wonders how they're doing in the Everglades, especially with recent drying trends.
Flooding, of course is the type of disturbance with which they and perhaps all of the other South American fire ant species are so closely associated in their homeland. One of the densest natural populations of monogyne fire ants I've ever seen was on the edges of the equivalent of the Everglades' hammocks (forested islands surrounded by lower, floody, treeless ground) in the Brazilian pantanal. Perhaps you'll reply that none of this contradicts your findings, since disturbance (even though not human-mediated) is still involved, but it at least adds to the picture a bit.
James and Alex,
Regarding the issue of fire ants occupying habitats other than lawns and pastures... sure that is true. James, you have certainly touched on the main type of the "undisturbed" habitats that fire ants invade - those that are naturally disturbed and often poor in other ant species (we have a set of experiments from this type of habitat in the western Apalach. which will be the third installment from my post-doc). One of the key issues with these habitats and any of the habitats that fire ants (and I would argue the VAST majority of other exotic ants) invade is that they are characterized by frequent disturbance. One must also be skeptical of the concept of "undisturbed" when discussing any of these habitats in the U.S. or unfortunately, the rest of the world as Watts et al. (a recent Science article) showed that it is not possible to get further than 35 km from a road of some sort and when one focuses on the eastern US, that number drops dramatically (and that is just roads!).
Finally, I might also something in regard to all of the "ifs" that so readily appear around articles that go against the grain of established thinking. I think that it is quite reasonable to suggest that a large, long-term, population-level, experimental study might be a sound choice as a starting point for evaluating what is really going on with the ecology of invasive ants (and plants - this is where this type of experiment with invasives started!), when we don't have any other, similar experiments with which to compare it. As Walter is so fond of saying... Talk is cheap, show me some data (and of course he is referring to experimental data)!
PS James, how's the Polyergus treating you?