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When plants need ants’ help, bigger is better

03.05.2004

A Rhytidoponera aurata ant drags an acacia seed by its elaiosome, a nutritious ant snack attached to the seed.

Small ant invaders put plants at peril

Not surprisingly, tiny ants just can’t tote seeds as far as their bigger cousins.

Because seeds are more likely to survive and sprout if they’re farther from the mother plant, it’s best for plants to form seed-moving partnerships with heftier ants.

Now ecologists have shown how much poorer small ants are at moving seeds.

The research suggests that plants that depend on ants for heavy lifting may be in for tough times if small invasive species like Argentine or fire ants move into the neighborhood.

"Bigger isn’t just better than smaller, bigger is a lot better than smaller," said team leader Joshua Ness, an ecologist at the University of Arizona in Tucson, adding, "Most native ants are larger than invasive ants."

As invasive ants replace the native species, the average size of seed-moving ants declines. The change in the ant community can influence the plant community.

Ness and his colleagues examined 57 ant species from 24 sites across six continents and found that just a small increase in body length meant the ant was a whole lot better at carrying seeds far from the mother plant. The research article, "Ant Body Size Predicts Dispersal Distance of Ant-Adapted Seeds: Implications of Small-Ant Invasions," will be published in the May issue of the journal Ecology.

Ness’s coauthors are UA ecologist Judith L. Bronstein, Alan N. Andersen of the CSIRO Tropical Ecosystem Research Centre in Winnellie, Australia, and J. Nathaniel Holland, formerly of the University of Arizona and now at Rice University in Houston. The research was funded by the National Institutes of Health.

Ness and his colleagues study the beneficial partnerships between species. As non-native species move into new ecosystems, such mutualistic relationships can be disrupted if one partner is displaced by an invader.

Plants that rely on ants to disperse seeds generally produce a tough-coated seed that has a little ant snack, called an elaiosome, attached to it. A foraging ant picks up the seed and drags it off, often to an ant nest. The food reward gets eaten, and the discarded seed germinates in the rich soil of the colony’s trash bin.

In natural communities, such interactions between species can benefit both parties: the ant has a reliable source of food, and the plant’s seeds are dispersed far enough to reduce competition between the seedlings and their parent. The further the seeds are dispersed, the better chance the seedlings have of making it to adulthood.

Such ant-plant mutualisms are common and include such plants as sacred datura in the Southwest and violets and trilliums in deciduous forests in the eastern United States.

But how far the seed gets carried depends on the size of the ant.

Therein lies the rub.

As small invasive ants such as Argentine and fire ants move into ecosystems in the United States and throughout the world, plants are losing their traditional partners, because the invading ants displace native ants. The plants have to make do with having the new, smaller ants disperse seeds.

The smaller ants have a tendency to rob the seed of its elaiosome and then take off. They don’t lug seeds very far from the mother plants. Moreover, the small ants are more likely to leave seeds lying on the surface exposed to seed predators like rodents and other insects.

That doesn’t bode well for the plants, Ness said.

In the South African ecosystems called fynbos, many of the plants have ant-dispersed seeds. Argentine ants have invaded that ecosystem, resulting in fewer seedlings of native plants. The seedlings that do exist are not dispersed far from the parent plant.

"The invasive ants take the reward and don’t provide the level of service provided by the larger ant," Ness said.

His team’s current findings suggest that what’s happening in the fynbos is not unique to South Africa or to Argentine ants.

"We would expect the set of problems we see with Argentine ants in South Africa to occur in other habitats invaded by small ants," he said.

But knowing what problems are likely to occur can help design strategies to mitigate the effect of ant invasions, he said. For imperiled plant species, managers may need to help the plants disperse their seeds.

Ness’s next step is looking at other ways invasive ants differ from natives and how those differences change invaded communities.