Ants as Gardeners

[A] A surprising number of plants depend on ants to move their seeds. Botanists have counted more than eleven thousand species that rely on the arrangement, on every continent except Antarctica, and it is clearly no accident. [B] Seeds of these species carry a small fatty growth called an elaiosome, which ants find worth the effort of carrying home. Once inside the nest, the elaiosome is fed to the larvae and the seed itself, which is of no use as food, is carried out to the refuse pile. [C]

The plant gains three things from this. The seed ends up buried, out of reach of the mice and birds that would otherwise eat it. It also ends up in the nest's waste heap, which is far richer in nitrogen than the surrounding soil. [D] And it ends up some distance from the parent plant, which reduces the competition a seedling faces from its own relatives. The ants, meanwhile, get a concentrated meal for a short walk. Neither party is doing the other a favour; each is simply doing well out of the other's habits. The relationship is old enough that neither could now easily do without the other.