Vicious invasive ants form gigantic supercolonies

The next time you’re outside, look down. Odds are, you might spot one of Earth’s estimated 20 quadrillion ants scurrying around. Depending on where you are, you may detect an Argentine ant. It might not look like much, measuring on average just 1/16th of an inch long. But research suggests these little, highly successful invasive critters are having an outsized effect on their environment.

It shouldn’t be surprising that ants are an important part of many ecosystems. Depending on the species, they can be hunters that keep other insects’ populations in check, scavengers that help in decomposition, and pollinators that aid in flower reproduction. Ants can also play a key role in dispersing plant seeds: many feed on tasty (to them) oils on the seed coating, but leave the germinating part of the seeds intact, allowing new plants to sprout. 

That balance is a delicate one. Even a seemingly minute switch, such as an invading army of a new species of ants coming in and replacing them through a brutal war writ small, can lead to cascading effects. While it’s been long known that Argentine ants are altering landscapes around the world, a recent study, published in Functional Ecology, is shedding light on exactly how those aftereffects occur. 

Invasion of the Argentine ants

For the study, researchers from the United Kingdom’s Royal Botanical Gardens, Kew studied several areas of a nature reserve in South Africa’s Cape Floral Region, an area known for a high degree of biodiversity and an outsize number of endemic flora species. Some areas had been found to have an Argentine ant presence, while the invasive ants were absent from others. Importantly, in areas where Argentine ants had moved in, they had tended to completely displace the native species. 

“What we wanted to contribute with this study was to really understand, when you have more than one invasive plant species and it’s a species that is co-evolved to to have a relationship with ants, how does that affect their reproduction?” said Adam Devensh, a research fellow at the gardens who worked on the study. 

Twice a day for each week, the scientists would place seeds from both invasive and local species of plants in predetermined spots in each of those sectors. After three hours, they went back and collected any seeds that hadn’t been collected, tallying them up. 

They then compared the rate of seed dispersal for both native and invasive plant species in each of the sectors, comparing the numbers between areas with Argentine ants, and areas with none. 

Devenesh and his colleagues weren’t surprised by what they found. While the native species had long been adapted to their environment, Argentine ants have different dietary preferences and, therefore, different behavior. In areas where the Argentine ants had made themselves at home, they found fewer seeds in total were getting spread around, which makes sense, since Argentine ants aren’t known to feed on seeds as a primary food source. But the seeds they did feed on were overwhelmingly from invasive plants, such as acacia trees: the ants were three times more likely to pick those up than seeds from native plants. 

“In the time we studied, there was less dispersal, and that was just in terms of the volume of seeds dispersed,” said Devenesh. “There’s also a lot of evidence in the literature to suggest, for example, that the Argentine ants can’t disperse in the same distance. So there’s definitely less dispersal happening.”

The large and seemingly small impact of the ants

From the vantage point of a human, this might seem like small potatoes (or bird feed, if you will). Indeed, as Devensh acknowledged, the actual impact of Argentine ants on human beings are “quite benign.”

“They don’t really impact you,” he added. “They don’t sting. They’re too small to bite you. And so to most people, they would seem the same as a black garden ant. They’re quite inconspicuous.”

However, shrink down to seed level, and the impact grows. Seeds turn into plants. If enough invasive plants grow, soon enough, the whole ecosystem can be thrown into chaos. 

“At an ecological level, they’re quite detrimental,” said Devensh, “because they can just overwhelm the system.”

While the system changes might eventually affect other creatures in the local food web, other implications can be even more immediate, and more devastating. Acacias tend to form dense clusters, which can spread fire more quickly. 

“In many ways in our system, acacias are probably more detrimental than the ants, by altering fire regimes,” said Devensh.

The findings are all the more alarming because of how pervasive Argentine ants have become. As their name suggests, the ants were originally discovered in that country, but less than a century and a half since the first invasive colonies were found outside Argentina, they’ve become truly global. The ants are now found in Mediterranean climate zones on all six continents, from South America to Japan, to Hawaii and California, to New Zealand, to Portugal, Italy, and Spain. 

While their arrival in these disparate locales was made possible by human-made transportation, they have some innate characteristics that are responsible for their success. According to the Center for Invasive Species Research, the ants have no known predators that would help control their spread. They also get a boost from the fact that Argentine ants don’t attack other Argentine ants. Rather than competing with each other, different colonies have essentially combined to form gigantic supercolonies. 

“Super colonies, which lack genetic diversity, results in less competition between Argentine ant colonies, so that enables them to spend more time foraging and a lot less time acting in aggressive behavior to defend territories, like they normally would be doing in Argentina,” said Mark Hoddle, a professor of the University of California, Riverside who specializes in invasive pests. 

The ants’ other big advantage

“Argentine ants are just super efficient,” added Hoddle. “They’re very good at building up in large numbers, very good at finding resources and then essentially overwhelming that resource with incredibly efficient foraging behavior.”

While Argentine ants have shown a remarkable ability to infiltrate and thrive on a planetary level, they do have their vulnerabilities. Hoddle said he has seen some success running experiments in citrus orchards that aimed to target the colonies with relatively low-toxic substances. 

“Liquid baiting programs are extremely effective at controlling Argentine ants,” he said. “The way that those control programs arrive is that you put out a sugar water resource for them, 25 percent sucrose solution is ideal, they love that. Then you put an ultra-small amount of insecticide. They drink that material, they take it back to the nest. At least in infested citrus orchards, within a day, those colonies are collapsing and foraging worker ant numbers are falling significantly.”

Devenesh said he hopes that other researchers will take a cue from his research. According to the U.S. Department of Agriculture, there are more than 6,700 invasive species in the United States alone. High profile examples include the spotted lanternfly, which has spread to 18 states and is so pervasive in New York, that residents are encouraged to squash them on sight. While the primary effects of such invasions can be obvious, as some species are notorious pests, their downstream effects on the environment can often be more opaque. 

“The bigger picture is more that we need to perhaps expand how we look at the effects of invasive species, beyond looking at the direct effects,” said Devenesh. “We tend to focus very much on the direct effects, and we tend to not study the sort of indirect effects.  I think, in a way, that means we might be underestimating the impacts of these species if we just take them at face value, to some extent.”

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