You are visiting your local nursery, getting ready for spring. The Bay Area’s winter is over and you can almost imagine the Early Girl tomatoes fruiting profusely, azaleas and marigolds blushing wild on the porch. Along the future streets of summer, wild mustard and California poppies bloom. You load an assortment of potted plants into the back of the car. At home, you turn over the raised beds in anticipation of the coming growing season and unwittingly make room for a wily hitchhiker—the one-tenth-of-an-inch-long dark-brown Argentine ant.
In terrestrial habitats across the planet, a largely unseen battle plays out underfoot. Its protagonists: over 14,000 ant species fighting for food, shelter, and territory. (Although that number may be double due to undercounting.) In some regions the Argentine ant (Linepithema humile) has emerged an unlikely champion—from its home range around the Paraná River region in Northern Argentina, this ant has spread to six continents and numerous islands, including Hawaii. In numbers, it is probably the most successful invasive nonhuman creature in California.
The earliest record of the Argentine ant in the United States is from around 1890, in New Orleans. Likely brought by ships carrying sugar from their native Argentina, the ants then traversed 2,000 land miles to reach the Golden State sometime around 1905. The ants, accustomed to a Mediterranean climate, have settled all along California’s coastline, as well as most parts of the Sacramento Delta. As their numbers spread, they’ve competed for resources with the state’s native ant species. In all nine Bay Area counties, they flourish in urban, semi-urban, and agricultural areas, where they find a regular supply of water as well as shelter from excessive heat and rain. Without those, the climate is just a bit drier and wetter than the ants prefer.
Once established in your garden, the ants will build a shallow nest, something simple using the cavities in rotting wood or leaf litter, about 15 inches deep, and begin to expand their territory. In doing so, they will displace many of your friendly neighborhood ants. California has 281 species of ants, 255 of which are indigenous and of those 39 are endemic, according to a review conducted by Philip Ward in 2005; the numbers have likely grown since. Ward, a professor of entomology at UC Davis, estimates that Northern California is home to about 100 of these species, including the carpenter, acrobat, thief, acorn, black harvester, velvety, and winter ant.
The Argentine, despite its small size—both black harvester ants and carpenter ants can be up to a half-inch long—and without weapons like stingers, still triumphs. While most ant colonies support a single queen, Argentine ant nests have many, and they reproduce at the same time. “An Argentine ant nest could have millions of workers. They’re not the strongest ants, but they are strong in numbers,” says Merav Vonshak, an entomologist whose postdoctoral research at Stanford focused on interactions between the Argentine and local ant species along a gradient of human disturbance from the Santa Cruz Mountains foothills to the city of Palo Alto. “It doesn’t matter that many workers die while fighting other ants, while foraging, looking for food, because they keep producing more and more workers and queens.”
Argentine ants have been documented aggressively going after other, bigger species of ants. “Several Argentine ants will grab different legs of the ant and hold it in place, and then they’ll grab the antennae. And then that larger ant is pinned down,” Ward says. “Argentine ants are very good at dispatching native ants that have large workers—it’s not an issue for them.”
And for reasons that aren’t entirely clear, Argentine ants behave less aggressively toward other colonies of their own kind, a peculiarity that lets them grow unchecked in foreign lands. “Just like many nonnative ants, they kind of ignore other nests of the same species and mostly fight other species,” Vonshak says. “They share some resources, and they won’t attack each other. And that’s important.”
Once the invasive ants have ahold of shelter and food, they must ensure that it lasts. But because the Bay Area’s climate is just short of ideal, they will move from the garden into homes during certain times of the year. In 1998, an 18-month-long study of 69 households in the Silicon Valley found that everybody had Argentine ants in their house at pretty much the same time—in the winter, when it’s cold and wet outside, and in peak summer, when it’s hot and dry outside. And regardless of what people did to get rid of them, the ants left when the weather got better for them outside. The study also showed that using deterrents like pesticides, while adding polluting runoff to the Bay, has no effect on the ants.
This makes matters even more difficult for farmers and orchard owners who have Argentine ant infestations. The ants are attracted to California agricultural crops like grapes and citrus fruit because of the insect pests that eat these crops, like mealybugs and aphids. The ants protect and tend the pests in exchange for the sweet secretions from these plant-feeding insects, sometimes even moving them to better food sources or locations. Of course, the ants also enjoy the regular supply of water available at farms.
Likely a permanent part of our urban and irrigated lives, the Argentine ant invasion has an unexpected twist: it has been harder for the ants to penetrate largely natural areas in coastal Northern California, and the ants’ long-term impact on the native ecosystems in the Bay Area, while complicated, is potentially less dire. A study drawing on an almost 30-year monitoring survey at the outskirts of Stanford University campus showed that this invasive species may struggle to thrive in the Bay Area’s intact ecosystems as the climate changes.
The survey began in 1993 at Jasper Ridge Biological Preserve (JRBP), an almost 1,200 acre protected area in the eastern foothills of the Santa Cruz Mountains. Just seven miles away from the Stanford campus, on a low ridge of the San Francisco Peninsula, it supports an incredible diversity of Bay Area geology, topography, and flora—blessed as it is with oak grasslands, dry chaparral, moist redwood forests, mixed evergreen and riparian habitats, and serpentine grasslands. “Because you have all of these different topographies bumping up against one another, there’s a great diversity of ant species there,” says Matthew Bahls, docent and volunteer ant survey co-manager at the preserve. There are 31 documented species of ants at the preserve, including invasive ones.
Surrounded by the developing Bay Area, JRBP remains an island in a semi-urban sea. Formally turned into a biological preserve in 1973, it contains conserved ecosystems that offer researchers a point of comparison for similar ecosystems affected by humans.
“If you wanted to follow the mostly natural dynamics of a biological invasion—and I say mostly natural because Jasper Ridge is an island of conservation in a wildland-urban interface—where the dynamics would largely be controlled by interactions between invasive species and natural communities, Jasper Ridge is a good place. Because with 1,200 acres, it’s feasible in size, and it’s very diverse,” says Nona Chiariello, staff scientist at the preserve.
The survey studies an average of 288 sites at Jasper Ridge every six months, once in the spring and once in fall. At each GPS-marked site, researchers “spend five person minutes surveying within 20 meters of that point,” Bahls says about the methodology. “And we’re not counting individual ants; we’re just looking for the presence of one species or another.”
Over the decades, researchers have used the survey to explore questions about ant reproduction and colonization, in an effort to monitor the ants’ burgeoning numbers. And in its attempt to find these answers, the survey has likely become the longest-running data set on the spread of any invasive ant species. But when it began, graduate student Katy Human and her adviser Deborah Gordon, professor of biology in the School of Humanities and Sciences, were simply charting the incursion. “Katy and Deborah were interested in the extent to which Argentine ants had already invaded Jasper Ridge, and in setting up a baseline monitoring system to see how much the invasion would progress over time,” Chiariello says. “When they were starting out, I think it was their expectation that because this is an invasive species, it’s going to just take over the place.”
But it didn’t.
In 2021, a paper published in the journal Ecology showed that the ants had, counter to expectations, actually retreated from some areas they had occupied in previous years at Jasper Ridge, likely in response to the extreme drought in Northern California from 2012 to 2015. Because of their shallow nests, the ants may have been especially vulnerable to the higher summer temperatures in those four years.
While early survey data showed the Argentine ants spreading rapidly in the preserve, they reached a stable distribution around 2001, and only by observing the entire data set did researchers detect the seasonal expansion and contraction of the species.
“Over time, because the climate has changed, and there’s been a very severe drought, we’ve seen how the Argentine ants’ need for water has affected how far they can spread,” Gordon says. “They can’t do very well here unless they get more water from us.” The area occupied by Argentine ants shrank between 1994 and 2020—in the fall by 30 percent and the spring by 27 percent—while the distribution of the native winter ant (Prenolepis imparis) expanded by 70 percent in the spring surveys during the same period. The researchers also found that overall native ant diversity was higher after the drought.
The winter ants seem to be in competition with the Argentine ants. “It looks as though they can spread into areas where the Argentine ants are declining,” Gordon says. “We don’t know if they actually are pushing the Argentine ants out, or they’re just able to occupy areas that the Argentine ants are retreating from because they’re not getting enough water.”
The paper concludes that the increasing drought and high temperature extremes projected for Northern California could limit the spread of the Argentine ant here.
Gordon warns against feeling too optimistic about the seeming triumph of the native ants. “They have evolved in this dry climate, but they may not be suited to the most extreme climate change that may be coming,” she says.
Native ant species have evolved to fulfill specific ecological roles in the landscape, including pollination, dispersal of seeds, predating, and influencing the flow of nutrients in the soil. Because they play a critical role in most environments (barring the north and south poles), they are also good indicators of ecological health.
The ability of native ants to resist Argentine ants varies from species to species—most native ants cannot resist well and are wiped out by the Argentine ants. “In places where the Argentine ant is well established, native species tend to decline in abundance and sometimes become locally extinct,” Ward says. Gordon agrees. “Where the Argentine ants haven’t reached yet, we see a certain distribution of species, but in the places where they have arrived, we see fewer species and different combinations.”
When native ants are displaced, it can disrupt whole ecosystems and reduce the diversity of other arthropods in the region. For example: the velvety tree ant (Liometopum occidentale) is a tree-nesting species dominant in lowland areas of California not inhabited by Argentine ants, and an entire community of arthropods live bonded with this ant—bugs in the family Miridae that mimic the velvety tree ants and forage on trees occupied by them, specialized beetles that live in their nests, and even a parasitic phorid fly for which the arboreal ants are the only host. “So when the Argentine ants invade an area, they don’t just eliminate the velvety tree ant, they cause the disappearance of all those other associated arthropods,” Ward says.
In Southern California too, the Argentine ant has displaced the native California harvester ant (Pogonomyrmex californicus). This, in turn, may partially explain the reduced numbers of the Blainville’s horned lizards (Phrynosoma coronatum), which feed on the harvester ants.
The Argentine ant study perhaps offers a new lens for how to approach these changing ecological relationships. Initially it was tempting for researchers to assume that the creatures with the best weapons—the biggest, meanest, and most aggressive species—would win. But the Argentine ant has subverted that narrative. “I think that there’s a general question in invasion ecology, not just about Argentine ants, whether we can explain the success of the invasion just by looking at the characteristics of the invasive species,” Gordon says. “We continue to find that it’s not just a characteristic of the invader that explains what happens; it’s always about how they interact with the other species that they encounter.”
For the ants, their most advantageous encounter has been with us.