Bay Nature published an article about Argentine ants with an alarming title and disturbing accusations, such as:
- “…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.”
- “Argentine ants have been documented aggressively going after other, bigger species of ants.”
- “…most native ants cannot resist well and are wiped out by the Argentine ants.”
- “When native ants are displaced, it can disrupt whole ecosystems and reduce the diversity of other arthropods in the region.”
After establishing the Argentine ant’s credentials as a dangerous “invasive species,” the article abruptly changes directions by contradicting itself. According to a 30-year monitoring survey of Argentine ant populations at the 1,200 acre Jasper Ridge Biological Reserve, “…the ants had, counter to expectations, actually retreated from some areas they had occupied in previous years at Jasper Ridge…” Early survey data showed the Argentine ants spreading rapidly in the preserve. They reached a stable distribution around 2001, and have since declined.
The monitoring survey at Jasper Ridge speculates that the decline in Argentine ant populations was caused by drought. However, that theory is not consistent with similar declines in Argentine ant populations elsewhere, where and when there was no drought.
Jasper Ridge is just one of many places that have reported declining populations of Argentine ants over many years. In 2011, Scientists in New Zealand reported the disappearance of the Argentine ant from 40% of sites they populated in the past and their populations have shrunk significantly where they are still found. Native ants have “reinvaded” the areas vacated by the Argentine ant. The scientists reporting this finding “concluded the species naturally collapses after 10 to 20 years.”
In an unpublished communication in 2011 with an entomologist at UC Davis, I learned that Argentine ant populations in Davis were declining.
In 2008, a study of ants in San Francisco’s “natural areas” in city parks reported that the existence of non-native Argentine ants does not have a negative impact on populations of native ants. (1) They report that Argentine ants occupy the perimeter of the “natural areas” where native ants generally are not found. This observation contradicts the usual nativist claims that non-native plant and animal species have negative impacts on native species.
The “invasion” curve
Introduced species are often accused of being invasive and there is a range of explanations, including the bias against non-native species that assumes every non-native species will eventually becoming invasive. In some cases, a new species spreads aggressively because it is better adapted to disturbed conditions to which it has been introduced. The initial success of an introduced species is sometimes enabled by the absence of its predators in its new home. This is called “predator release,” which does not confer permanent protection to a new species that will eventually encounter new predators. These and likely other factors are probably operating simultaneously.
Like most so-called “invasions,” introduced plants and animals may briefly expand, but eventually most find their niche in the ecosystem without causing permanent harm to their neighbors. The assumption that introduced plants and animals threaten native species is usually unsupported by empirical evidence
Journalistic due diligence
If the author of the Bay Nature article about Argentine ants had searched research literature about Argentine ants, she could have learned that the negative tone of the article and its hyperbolic title were not justified. In the author’s defense, the demonization of non-native plants and animals is routine in mainstream media. The bad news about introduced species always precedes their eventual participation in ecosystems and the record is seldom corrected when they do, as in the case of Argentine ants. The reader can compensate for this journalistic bias by reserving judgment about non-native species until more is known about their fate.
- Kevin M. Clarke, et. al., “The influence of urban park characteristics on ant communities,” Urban Ecosyst, 11:317-334, 2008