Native Toad Fights Back Against Yellow Crazy-Ant Invasion in Sulawesi

Susan Milius, Wired Science 8 Sep 10;

After so many sad tales of invasive species overwhelming hapless natives, scientists have found a native toad in Indonesia that’s fighting back.

The common Sulawesi toad turns out to be a prodigious eater of ants, even aggressive invading ones, says Thomas C. Wanger of the University of Göttingen in Germany and the University of Adelaide in Australia. On the island of Sulawesi, the Ingerophrynus celebensis toads readily feast on yellow crazy ants, which are colonizing the island as well as other tropical locations.

Yellow crazy ants get their name from their color and their zigzag scurrying, and they have crowded out native ants and disrupted ecosystems elsewhere. The invaders meet any foe aggressively, releasing noxious chemicals during battle. The Sulawesi toads eat them nonetheless, Wanger says.

During a week of toad abundance on Sulawesi farms, test plots hopping with toads had as little as one-third of the invasive-ant populations found on plots where fencing kept toads out, Wanger and his colleagues report in a paper released online the week of Sept. 6 in Proceedings of the Royal Society B.

The paper could be a first in suggesting that a native toad might control populations of invasive ants, says ecologist Stacy M. Philpott of the University of Toledo in Ohio. “It is a really neat finding,” she says.

Wanger and his colleagues established that the toads disproportionately prey on ants, based on the toads’ fecal samples. “You wouldn’t believe how smelly these things are,” he says. The samples revealed that some kind of ant accounted for three-quarters of the diet of the toads storming through the test region, even though ants didn’t represent a large proportion of the arthropods there. Native ants presumably have long coexisted on the island with the toads, but populations of invaders may be taking a hit, the researchers suggest.

The test plots lay in cacao plantations, and the researchers speculate that the toads’ taste for ants may turn out to be a boon for cacao pest control. About every three months, the toads leave their usual forest home and surge through the cacao plantations to breed in the water of neighboring rice fields.

Toads feasting on yellow crazy ants may help keep the invaders from crowding out the native ants in cacao plantations. Other researchers have shown that a rich diversity of native ants helps keep cacao pests and diseases in check.

The study’s suggestion that the Sulawesi toads ultimately help control pests in the cacao plantations is plausible, Philpott says. She and a colleague have surveyed the scientific literature on how the diversity of ants can affect diseases and pests in coffee and cacao plantations. One kind of native ant preys on insects that leave sticky lesions on cacao pods, for example, and fewer sticky spots means the pods attract fewer visits from flies tainted with a pathogen causing pod rot.

Making a case for toads as protectors of cacao might rouse new enthusiasm for protecting native amphibians, as Sulawesi residents have similar priorities to those in agricultural communities the world over. “They are not really interested in biodiversity conservation but in economic questions,” Wanger says.

A scientist who has studied the ecological damage from crazy ants, Dennis O’Dowd of Monash University in Melbourne, Australia, notes that the researchers now need to investigate the other links in their hypothesis, such as whether the toads’ periodic gorging on yellow crazy ants has a lasting effect on ant populations.

Nonetheless, he says, “I certainly like the sentiments in the paper, and I’m all for conservation of native amphibians.”