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 e
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1-May-2004