6 - The dispersal of seeds and fruits by ants

Summary

Many species of ants gather seeds. Harvester ants store them in underground granaries and consume them during the winter or dry season. These ants are granivores and the net interaction is usually predation. Other ants gather seeds and fruits distinguished by the presence of external tissues, collectively called elaiosomes, which attract ants and stimulate them to carry the entire seed or fruit back to the nest (Figure 9). There the elaiosomes are removed and typically fed to the larvae. The seeds are then discarded, both intact and viable, either in an abandoned gallery of the nest, or close to an entrance in a refuse pile together with other organic waste. Because the elaiosomes contain ant attractants, and as the seeds are not harmed, the interaction, known as myrmecochory, has long been assumed to be a mutualism. Until recently, however, the advantages of the interaction, especially for the plants, have remained undocumented. Recent studies have shed light on the problem, and there appear to be five current hypotheses on the selective advantage to plants of dispersal of seeds and fruits by ants.

The predator-avoidance hypothesis

Seed predators are so diverse and abundant that plant species must be under great selective pressure to avoid them. The essence of this hypothesis is that seeds released from the parent plant are quickly taken by ants to their nests, where they find refuge from predators. The ants are rewarded with nutritive elaiosome tissue and then rapidly lose interest in the seed, which either is not a part of their diet, or is unavailable because it is protected by a tough seed coat.