Case Study: The March of the Argentine Ants

Some species by their mere presence indicate that an ecosystem is under threat. The spread of Argentine ants from South America to North America illustrates how one species directly and indirectly can endanger other species by taking advantage of a quirk in genetic diversity.

Argentine ants arrived in New Orleans in the late 1700s aboard coffee-carrying ships from South America. By 1820 the insects had spread throughout New Orleans, and within the next century the ants began an inexorable march northward beyond Louisiana. By 2000 they had spread into the following states: Louisiana, Mississippi, Alabama, Georgia, Florida, South Carolina, North Carolina, Tennessee, Arkansas, Texas, and Oklahoma. Argentine ants have now spread into the Pacific Islands, Europe, and southern Africa.

Like many successful invaders, Argentine ants display aggressive tendencies when they take over new territory. They fight and defeat native insects and soon overrun natural ant colonies, the hives of native stinging insects, and even birds’ nests. Genetic studies of these far-flung populations have turned up a surprising phenomenon that helps them as invaders: The ants have almost no genetic diversity. A single small colony, perhaps from a single ship in the port of New Orleans, may have provided the ancestors for all the generations that have now spread around the globe.

The Argentine ants passed through what is called a genetic bottleneck in which very little genetic diversity exists. Usually such bottlenecks confer a disadvantage on species, but the Argentine ants manage to use their relatedness as a distinct advantage. Normal ant colonies have organized social structures so that members work in a cooperative fashion, and when one colony invades the territory of another, the two families fight for the spoils. Argentine ants, however, act as one big family of close cousins. Because they all recognize each other as kin, they form huge cooperative colonies that overtake species normally able to repel attacks. When Argentine ants invade, they actually make biodiversity go backward because they eliminate all other various ant species and replace the territory with their one supercolony. The biologist Andrew Suarez noted in 2000 in the New York Times, “Some people say, ‘Big deal; you’re just replacing some ants with others.’ But the thing is, you’re displacing twenty species. And all the roles they play are wiped out: dispersing seeds, pollinating plants, providing food for other animals. All the functions are totally
lost when Argentine ants move in.” These ants wreak all this havoc by creating what is called a trophic cascade.

A trophic cascade occurs when a change in one species’ population leads to major disturbances of other unrelated species. California’s coast horned lizard and the similar Texas horned lizard both live a life evading their predators: birds, mammals, and snakes. As their defenses, the lizards rely on spiny armor and cryptic coloring to blend in with their desert habitat. Each lizard species also reproduces slowly, so they are not likely to develop adaptations fast enough to meet changes in environment. Enter the Argentine ants. Both lizards depend on native harvester ants
as their main food source, but when the Argentine variety decimates native ant colonies, the lizards cannot adapt to new food sources. When the lizards roam out of their natural habitat in search of harvesters, their camouflage does not match the surroundings and predators pounce. Meanwhile plants that depend on harvester ants to spread their seeds also decline. When the native vegetation declines, so too do insects that depend on the plant life. The birds that eat the insects disappear, and next the animals that prey on birds go hungry.

The biologist Ted J. Case of the University of California at San Diego has offered some words of comfort on the unusual ability of Argentine ants to form huge, homogeneous colonies. The ants’ relatedness, he explained to the New York Times, “may be useful in the short term, as it is for Argentine ants and some other invasive ants, but in the long run it appears to be an evolutionary dead end because social behavior could not evolve.” Meanwhile the Argentine ants demonstrate the intricate ways in which biodiversity can be threatened.