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    December 2011
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Here today, gone tomorrow: the case of a disappearing invasive ant species in New Zealand

Although the cost of invasive species has not sparked nearly as much debate as has the possibility of global warming, the combination of the two has bred some interesting research. In New Zealand, for example, a group of scientists recently examined what effect warming temperatures would have on the invasive Argentine ant, and whether native ant populations could recover if invasive ant colonies disappeared.

Most of us don’t think of invasive species in terms of their disappearance. In fact, the term “invasive” implies a conquest of sorts, taking over an ecosystem from insect, animal or plant inhabitants that already live there. However, according to Dr. Meghan Cooling, an ecologist at the Centre for Biodiversity and Restoration Ecology in Wellington, New Zealand, invasive species are susceptible to population collapse, sometimes regardless of whether or not they are managed.

Argentine ant

Argentine ant

Native to South America, the Argentine ant (Linepithema humile) is listed as one of the world’s worst invasive species. Colonies are usually large, and the ant displaces competing ant colonies, including fire ants (NCSU Insect Note). Argentine ants have been in the United States since the late nineteenth century, and are a serious problem in the southeastern and southwestern U.S.

First observed in New Zealand in 1990, Argentine ants have spread through the country, largely through accidental introductions by travelers. Some scientists have speculated that the species’ survival depends on warm temperatures, and currently Argentine ants inhabit regions that have more temperate climates. The climate in New Zealand varies from north to south; northern New Zealand has almost a tropical climate, while temperatures drop further south (and farther from the equator). Dr. Cooling and her team concentrated their research in Auckland, which is near the northern tip of New Zealand.

Cooling and her colleagues wanted to know how invasive insect species such as the Argentine ant might respond to future warmer temperatures as climate change progresses, especially in areas with more temperate climates. First, they posed the question about the present persistence of Argentine ant colonies and how climate variables like temperature and rainfall contributed to colony survival or collapse. Second, how might the warming temperatures associated with climate change affect ant colony collapse? And after colonies have collapsed, can the ant species that they displaced repopulate the area?

The team used three types of ant communities in Auckland, New Zealand, in their experiment: communities with high populations of Argentine ants, communities where Argentine ant colonies had disappeared, and communities that had never been invaded with Argentine ants. In January and February 2011, the researchers observed 150 locations where ant colonies had been reported between 1990 and 2008. They used approved models to estimate survival and climate change. In addition, ant colonies in research areas were not managed by human intervention.

In the 150 locations observed, ant colonies disappeared from 60 of them. Of the colonies that remained, most were in areas with warm temperatures. Rainfall reduced ant colony survival.

Climate change model predictions indicated that Argentine ants may be more likely to survive in areas that currently were seeing colonies collapse. In fact, prediction maps locate ant colonies in New Zealand, based on a climate change model, much further south than colonies are currently surviving. However, Dr. Cooling states that areas where Argentine ant populations have a greater than 80 percent chance of surviving for 15 years increases only from 0.25 to 1.29 percent with warming temperatures.

The team observed that areas that had seen a collapse of Argentine ant colonies were now rich with diverse species of native ants; in fact, native ant species in areas where Argentine ant colonies had disappeared were indistinguishable from areas that had never had Argentine ants.

Controlling Argentine ants had been predicted to cost the country up to $68 million per year. Cooling says that New Zealand may be able to save that money if colonies continue to disappear on their own.

However, she admits, the evolution of Argentine ants in New Zealand is not necessarily a model to be applied to other species or countries.

“Other invasive species and climate change clearly contribute to the current global biodiversity crisis, and their costs may be substantial,” she writes. “Determining which species are susceptible and the mechanisms for these collapses should be a high priority for invasion biologists.”

Source: Cooling, M., Hartley, S., Sim, D.A., and Lester, P.J. “The widespread collapse of an invasive ant species: Argentine ants (Linepithema humile) in New Zealand.” Biology Letters, 30 Nov 2011, DOI: 10.1098/rsbl.2011.1014. Online.

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