If I were one of those ants, i'd put up a fight to leave too, they hit the jackpot - lol. Here is some information I found to help you out:
For most species, control is only necessary in the garden if the population gets out of hand. Garden varieties do not actually eat plants or actively destroy any plant matter. Their main source of food is sugar, usually found in nectar, sap, or the honeydew excrement from aphids. In some cases, garden ants can actually be a beneficial insect as they will kill off more destructive pests like caterpillars. They are also natural soil aerators through their extensive tunneling systems. As food for beneficial animals such as birds, lizards, and toads, they will also encourage wildlife to flourish in your garden.
Two types of Ants to look out for: Carpenter Ants, and Fire Ants. Basically it is saying that as long as the nest is not excessive, then they will benefit the plant by aerating the soil. but if they happen to get out of hand, and build their nest at the base of the roots, you are in for trouble.
Best of luck!
Left on Green
www.leftongreen.org