Biogeosciences News

Ant Populations Destabilize Under Warming

In forest experiments in which artificial warming mimicked future climate conditions, heat-tolerant ants thrived, leaving other populations unstable.

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In the forest, competition for high-quality nesting spots forces ant species to move around regularly, perpetually looking for better homes. But a 5-year-long research endeavor suggests that in a warming climate, ants that thrive at higher temperatures may come out on top at the expense of other ant species’ well-being and that of the forest itself.

“This was really a chance to peek under the hood” of how climate change affects the way different species interact with each other, said Sarah Diamond, a biologist at Case Western Reserve University in Cleveland, Ohio, and lead author of a 26 October paper published in Science Advances.

Diamond noted that many attempts to model how species will respond to climate change ignore indirect consequences of climate change on interactions among species. To fill in this lack of data, she and her colleagues studied how various ant species interacted under artificially induced warming meant to simulate future climate change.

Artificial Warming

An artificially warming segment of the forest floor.
An artificially warming segment of the forest floor, fenced off by Diamond and her team. The large plastic tubes have holes in the bottom that pump warm air into the enclosure. Buried under a shallow layer of leaves lie nest boxes teeming with ants. Flags indicate locations of nest boxes and other study sites. Credit: Lauren Nichols

The team built enclosures full of ant nest boxes in two university-managed forests—the warmer, more southern Duke Forest in North Carolina and the cooler, more northern Harvard Forest in Massachusetts—to observe how the ant communities might be affected.

Heating systems with large tubes pumping air throughout each enclosure created warming expected with future climate change. Throughout the experiment, the researchers incrementally raised the temperatures between 1.5°C and 5.5°C above ambient temperatures, in line with forecasts by the Intergovernmental Panel on Climate Change. As controls, some of the experimental enclosures had ambient air pumped in, and some of the enclosures had no air pumped in at all.

The researchers checked once a month on the nest boxes, which attracted more than 60 species of ants. Each time, they noted whether each nest box was occupied and which ant species occupied it. At the end of the experiment, the scientists developed statistical models of the ebb and flow of different ant colonies among the nest boxes.

In a stable ant community, Diamond explained, colonies of various ant species constantly move in and out of nests with little or no vacancy of nests between residents—this is the equilibrium state. In the warm enclosures, however, the researchers noticed that the pattern tended to break down as temperatures rose. No longer were species moving to and fro as often as they had been. Instead, those species of ants that thrive at high temperatures, called thermophilic ants, were staying longer and longer in the nest boxes, Diamond said.

This disruption of the normal species interactions reduced the number of nest boxes available for other ant species, Diamond continued. Whether the thermophilic ants displaced other species by physically forcing them out or simply moved into vacated nests remains a mystery.

Ecosystem Consequences

Disrupting this equilibrium state—the ants’ constant movement from nest to nest—could affect resiliency of ant populations to environmental disturbance, Diamond said.

In the forest, ant colonies outnumber places to nest, and colonies without nests may die. In a warming world, heat-loving ants “will hold the high-quality nest sites for longer periods of time.” And if the less thermophilic ants can’t find shelter, “they may die off.”

“With warming, you start getting one or a few species that dominate the assemblage and become much more persistent,” said Jessica Blois, a paleoecologist at the University of California, Merced, who wasn’t involved in the new study.  “While that’s good news for those particular species, the community as a whole is less resilient” because normal movement patterns break down.

But what does this potential disruption mean for the forest itself? In fact, some of the ant species replaced by the heat-loving species happen to nourish a healthy forest, Diamond said. They disperse seeds and decompose waste. Although the thermophilic ants also serve as waste disposers, Diamond said, they don’t disperse seeds the way the less thermophilic ants do.

Ecologist Sarah Gilman of Claremont McKenna College in Claremont, Calif., who also was not part of the ant experiment, said the forest study adds a “layer of complexity to how we think about species’ responses to climate change” and “documented a new consequence of temperature changes on communities.”

For now, Diamond and her team have mountains more data to sift through. She said the group plans to look at responses of the other organisms in each enclosure, including plants, insects, and even spiders.

—JoAnna Wendel (@JoAnnaScience), Staff Writer

Citation: Wendel, J. (2016), Ant populations destabilize under warming, Eos, 97, doi:10.1029/2016EO062537. Published on 04 November 2016.
© 2016. The authors. CC BY-NC-ND 3.0