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Invasive Species Week: The Argentine Ant

Alex Wild took this photo of Argentine ants where they belong: in Argentina. | Photo © Alex Wild, used with permission

Next week might be Shark Week, but the California Department of Fish and Wildllife has declared August 2-10 California Invasive Species Action Week, an event intended to boost public awareness of the problems some non-native species of plants, animals, and even microorganisms can cause when they're imported into the state. We're celebrating, if that's the right word, by featuring five of California's most prevalent invasive species every day this week. The blue gum eucalyptus tree was featured yesterday.

Deep thinkers in the environmental realm have long questioned the language we use to describe introduced species that create problems in their new homes. The word "invasive," they say, conjures up images of marauding armies deliberately trying to conquer territory. The terminology inflames emotions, they suggest, and gets in the way of evaluating each new species introduction rationally.

It's a compelling argument with some sense behind it. But you hardly ever hear it raised when the species being discussed is the Argentine ant, which fits the militaristic metaphor uncomfortably well.

First documented in California in 1907, Linepithema humile just might be the most successful invasive animal species in the state, at least in terms of sheer numbers. About three millimeters long and dark brown with a wedge-shaped head, the Argentine ant's one of the most common ant species in coastal towns and cities from Humboldt County to the border. The species has colonized much of the Sacramento Delta area as well.

The ant's simple presence in the state need not have been a serious ecological problem, were it not for an interesting quirk of the species' behavior. Many ant species spend a significant amount of time in conflict with neighboring ant colonies, even if those neighbors are of the same species. Ants use chemicals called pheromones to communicate, and members of neighboring ant colonies likely just smell wrong.

Argentine ants in California are radically different. They get along with their Argentine ant neighbors very well: so well, in fact, that an ant born into one colony can wander into a neighboring nest and be greeted as a member in good standing. That apparently holds true across the ants' entire range in California: take an Argentine ant from Escondido and drop it near an anthill in Arcata and the locals will groom it amicably and put it to work.

In other words, there is one colony, or supercolony, of Argentine ants in California, and it occupies a territory 560 miles long.

That lack of what the behaviorists call intraspecific aggression allows Argentine ants to live in much more dense populations than other ants. Which means that the ants can pretty much outcompete native ant species wherever they get established, either by scouring the entire area for food and water or by attacking other ant's colonies, having saved up all the energy that might have been spent fighting among themselves.

To overstate things only slightly: if you live in a city or town within a hundred miles of the California coast, and you have small ants in your house, they are Argentine ants.

Argentine ants | Photo © Alex Wild, used with permission

Readers who harbor no special love for ants of any species might well wonder what the big deal is. So one species of ant gets replaced by another; who cares? Here's the thing: California is home to at least 245 native species of ants, and that number would likely rise dramatically if entomologists were able to do a serious survey. Each of those native species plays certain ecological roles in the state's natural landscape. With some species, those roles can be fairly specialized.

Take for example the harvester ant Pogonomyrmex subnitidus, an important Southern California seed disperser for the beautiful native shrub Dendromecon rigida. Dendromecon's fruits explode when ripe, tossing seeds a short distance away from the parent shrub. Those seeds bear a fleshy covering called an elaiosome, which contains a bit of oil that Pogonomyrmex subnitidus apparently finds tempting. The ants carry the seeds to their colonies, consume the elaiosomes, then toss the naked seeds out into their middens, where some will germinate, possibly a considerable distance from the parent plant -- and that distance also lessens the likelihood of rodents or other seed eaters finding the seed before it germinates.

When Argentine ants move into an area occupied by Pogonomyrmex subnitidus, the harvester ants usually aren't there for long. That's partly because the Argentine ants are more efficient scavengers of the kinds of food the harvester ants like, and partly because Argentine ants will raid harvester ant nests, immobilize as many workers as they can, and eat the colony's larvae.

And then there's bad news for any local Dendromecon: Argentine ants don't disperse their seeds. And it's not just Dendromecon at risk. An assortment of native plants, from the miners' lettuce that freshens coastal hillsides after a wet winter (remember those?) to Trillium and Datura rely on similar arrangements with native ants to spread their seed. It's a relationship called "myrmecochory," in which plants grow seeds that have elaiosomes or other bait to tempt ants to carry the seeds away -- and Argentine ants just seem to have no interest.

Argentine ant invasions are also bad news for any Blainville's horned lizards living in the vicinity. The beleaguered lizards, formerly known as coast horned lizards, eat ants like Pogonoyrmex subnitidus and its cousins, or other harvester ants in the genus Messor, turning to other, smaller native ants when necessary. But a 2000 study showed that the lizards just flat out don't eat Argentine ants. Despite the fact that the lizards can and do eat other ants the same size as Argentine ants, there seems to be something about Linepithema that repels the lizards. Which means that conversion of native ant habitat to Argentine ant habitat deprives the lizards of their food. Some seem to try to eat other kinds of insects and small spiders, but that's less than ideal. And as researchers studying Stanford University's Jasper Ridge Biological Preserve have found that Argentine ant invasions wiped out many other arthropods in the area, that wouldn't leave much of an alternative diet for the lizards anyway.

Some native ants are holding their own. A 2011 paper revealed that the native "winter ant," Prenolepis imparis, persists near Stanford -- including at Jasper Ridge -- by resorting to chemical warfare. Prenolepis secretes an oily mix of complex hydrocarbons from its hind end; when Argentine ants are daubed with the liquid, as many as four fifths die.

There is good news: it turns out that Argentine ants mainly thrive in places where humans have disturbed existing habitat and provided a bit of extra water. Away from lakes, streams, and the marine layer, California turns out to be just a little drier than Argentine ants would prefer, and often a bit colder than they'd like away from our urban heat islands.

Which means that though Argentine ants are likely here to stay, we can mitigate their spread by deciding not to turn more of California into irrigated suburbs.

Piece of cake. Right?

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