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.
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."
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