Inland Ants Crave Salt, and Hurricanes May Help

John Roach
for National Geographic News
October 28, 2008
Salt-deprived animals and insects living far inland from some coasts may benefit if global warming increases hurricane intensity, a new study suggests.

Storms bring sodium—a necessary nutrient for almost all life-forms other than plants—from the seas farther inland, explained study lead author Michael Kaspari, an evolutionary biologist at the University of Oklahoma in Norman.

Kaspari specifically studies ants, which are considered indicators of ecosystem health.

Ant communities with a balanced diet, for example, more efficiently disperse seeds, munch leaves, eat fungi, and aerate the soil than do communities deficient in sodium.

(Related: "Insects Key to Rain Forest Diversity, Study Shows" [March 10, 2005].)

Kaspari's team found that ants in the Americas that live between about 6 and 60 miles (10 and 100 kilometers) from the coast are in a sodium sweet spot.

"That's where the activity of the ants from our study was the highest," he said.

Ants closer to the coast were oversaturated with salt—perhaps at risk of the ant equivalent of hypertension. Ants farther inland are salt starved—and thus operate in subprime form.

Stronger hurricanes, he noted, would bring more salt-saturated rainwater to the inland ants, providing a nutrient boost that might allow them to ramp up their activity.

Kaspari and colleagues report the findings in a paper online this week in the journal Proceedings of the National Academy of Sciences.

(The research was funded in part by a grant from the National Geographic Society's Committee for Research and Exploration. The National Geographic Society owns National Geographic News.)

Salt Tubes

Rainwater, Kaspari explained, has high salt content near the coast, but becomes more and more distilled as storms move farther inland.

As a result, his team hypothesized, that it would find ants with stronger salt cravings the farther inland they went.

To test the idea the researchers stuffed cotton swabs soaked in water with varying levels of salt and sugar solutions into tubes. The team placed the tubes at various distances from the coast throughout the Americas.

The scientists then counted the number of ants that came to each tube. Farther from the coast, ants increasingly preferred the saltier baits.

The 6- to 60-mile (10- to 100-kilometer) sweet spot reflects where ants most went for the sugar—a measure of ant activity.

The trend was more accentuated among ant species that eat plants contrasted with more carnivorous species.

"Meat is salty, whereas plants are not," Kaspari noted.

Seán Brady is an entomologist at the Smithsonian Institution in Washington, D.C. He said the research shows a "clear and consistent" effect of salt limitation that warrants further exploration.

Carbon Cycle

Study leader Kaspari is interested in the effect climate-induced changes in salt availability due to shifting rainfall patterns might have on ant activity.

"One half of the big carbon cycle—the rate at which carbon in plants is released back into the atmosphere—we suspect, is limited by the amount of sodium that is available in the system," he said.

This December, the team plans to visit the Amazon rain forest in Peru, which is salt starved, to test the idea that they can ramp up the ecosystem by spraying a salt solution.

"The notion that rainwater, and the salt in it, might all by itself determine whether or not the ecosystem is going at top speed or is actually inhibited in some way really intrigues us," Kaspari said.

Brady noted that increased salinity might also change ant community dynamics.

"Ants that are living in these salt-stressed environments are somehow adapted to that environment," he said. "And if you increase the salt content, that could change competitive interactions among species."

© 1996-2008 National Geographic Society. All rights reserved.