Adventures Among Ants - Mark W. Moffett [138]
The Argentine ant’s relationship to North Americans traces back to early trade out of Buenos Aires. The Plymouth Rock for these ants was likely a dock in New Orleans, the city where they were first recorded in 1891, in all probability arriving aboard a ship loaded with coffee. The first sightings of the species in California occurred circa 1907, in the San Francisco Bay area and around Los Angeles. The origin of these populations is not known, but since the Panama Canal had yet to open, it seems likely that they came from the southeastern United States rather than from Argentina, possibly stowing away in a railroad car.14
Slowly but surely, Argentine ants spread through southern California. Though the invaders never made off with our children, as some tabloid newspapers in the 1980s implied they might, the ants have become a part of the landscape, and they continue to expand onto land as yet uninhabited by their species. Even the Very Large Colony is still growing. Where moist areas are continuous, a colony can diffuse outward as the ants migrate to previously untenanted sites, an incremental process of budding new nests that remain part of the same colony in this case. Such migrations occasionally take place across uninhabitable sites such as narrow roads, but the Lake Hodges Colony is split by two freeways, a more formidable obstacle. With other species, a queen on her mating flight could have traversed such a barrier to form a new colony on the other side, but the Argentine queen doesn’t have that option. Either the Lake Hodges Colony is older than the freeways, or the colony was accidentally brought to the other side via human commerce—jump dispersal.
Argentine ants eating a piranha on the banks of the Paraná River in northern Argentina.
Jump dispersal has been essential for the Argentine ant’s success at domination, as demonstrated within California. The spread of the species by budding nests is slow—up to 150 meters a year. If the Very Large Colony had expanded only by this method, it would have grown to just a tenth its current size. However, given the volume of cargo we haul not just across the oceans but between neighborhoods, modern-day humans must aid and abet ant stowaways in reaching thousands of new sites every day.
The Argentine workers are less crowded on their home turf, most likely because they contend with more numerous, and more dense, colonies of their own species as well as colonies of other equally aggressive ants. No doubt the competition forces colonies to divert labor and resources from foraging and colony growth, keeping the size and the density of the worker populations under control and allowing other species to settle and expand. Elsewhere in the world, less skilled fighters fail, and so the Argentine ant infests new territory in prodigious numbers.
When the ants first arrived in New Orleans and spread across the Southeast, their numbers were astronomical from the start.15 In general, tramp species are expected to have few limits on their populations, thanks to what’s known as ecological release: they are free not only from their competitors but also from the predators and parasites that bedevil any organism in its native range. Even though an Argentine supercolony’s death toll is astonishing by human standards of warfare, mortality remains low overall because the vast majority of workers live far from borderland clashes. Once an Argentine ant colony lays claim to a plot of ground, the workers there may never experience conflict with their own or any other kind of ant. And so their populations shoot sky high.16
CONTROLLING THE LANDSCAPE
The success of the Argentine ant is facilitated by its fluid lifestyle.