Ants in the pants drive away birds

Supercolonies of invasive insects spoil avian picnics on Christmas Island

Dubbed “yellow crazy ants” by people, an invasive ant drives birds crazy too.

BERRY PICKING A native bird called the Christmas Island white-eye (left) doesn’t feed on as many berries as usual when invasive yellow crazy ants swarm over plants. Researchers measured this anti-fruit effect by setting out artificial berries and checking for pecks by a thrush (top) and white-eye (bottom). From left: Tony Patisser; Naomi Davis

CRAZY Named the yellow crazy ant because of its frantic behavior when disturbed, this species is spreading widely in the tropics. ABC Natural History Unit

CRAB WALK On Christmas Island, forest-dwelling red crabs migrate annually toward the shore. Once abundant and a major force in engineering the forest, crabs are now dwindling in numbers where invading yellow crazy ants swarm and kill them. Peter Green

Nicknamed for their wild scurrying, the ants keep birds from eating and dispersing fruit, says ecologist Dennis O’Dowd of Monash University in Melbourne, Australia. On Christmas Island, an Australian territory in the Indian Ocean, hordes of yellow crazy ants (Anoplolepis gracilipes) pester birds landing on plants. Experiments that kept ants off plant stems in invaded zones more than doubled the chances that a fruit would be bitten by a bird, O’Dowd and his colleagues report online September 15 in Biology Letters.

O’Dowd and others have been chronicling the ants’ impact on the native creatures of Christmas Island since the 1980s, but this paper is the team’s first to test ants’ effect on fruit eating. Ant impacts “ramify through a system in such extraordinary ways,” O’Dowd says.

“What never ceases to impress me is that something so small can affect vertebrates so directly,” says Dennis Hansen of Stanford University, who has studied invasive ants bedeviling geckos on Mauritius.

On Christmas Island, yellow crazy ants have caused so many changes that O’Dowd and coauthor Peter Green of La Trobe University, also in Melbourne, have declared an “invasional meltdown” of the original ecosystem. Ants probably hitchhiked onto the island sometime during the early 20th century and really boomed during the 1990s. Supercolonies now cover swaths of the island in such density that ants in a feeding frenzy can clear a forest of its once-important red crab species — swarming over the crabs, biting them and spraying them with formic acid.

Ants climb onto birds too. In ant-dense zones, O’Dowd sees birds stomp and ruffle their feathers as if maddened by crawling ants. To see if this nuisance affects the birds’ feeding, coauthor Naomi Davis, now at University of Melbourne, set out arrays of artificial fruits crafted from nontoxic modeling clay. In forests not yet invaded by ants, fruits showed more than twice as many peck marks as fake fruits in ant zones.

When Davis put an ant-trapping goo around the base of dowels holding her fake fruit in the ant zones, she found the goo’s presence more than doubled the number of pecks from two native fruit-eating bird species. Both white-eyes and island thrushes were more willing to try the fruit when ants weren’t raging over the plants.

Fruit eating could affect bird nutrition and population size and could also change the distribution of seeds and seedlings, O’Dowd says. He hesitates to predict just what those effects might be on Christmas Island, though, since ants are making such complex changes.

Elsewhere, other ant invasions might be driving birds off fruit too, says community ecologist Lori Lach of the University of Western Australia in Perth. Lach, who studies what invasive ants are doing to world ecosystems, warns that disruptive ants are moving far and wide. 

Once highlighted mostly as a tropical phenomenon, ant invasions now menace temperate regions, including areas of Maine, too, Lach says. Invaders have left seeds stranded without their native ant dispersers, lizards starving without palatable meals and flowers languishing for want of pollinating geckos. To explain the scope of the problem, Lach says, “the first thing is to get people to realize how important ants are.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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