While ants don't directly eat much plant material, they are great farmers of insects that do suck on plants, as everyone probably knows. Scale, aphids, mealybugs, etc are carefully tended and spread by ants. They put aphids and mealies on the roots as well, sometimes with no above-ground indications. Ants are a gardener's enemy. Apologies to E. O. Wilson, but ants are almost always up to mischief when they occur in a garden.
Lou