2 - Origins and early evolution of ant–plant mutualisms

Summary

In this chapter I first examine fossil ants and plants and primitive living ants and angiosperms to try to reconstruct the origin and early evolution of ant–plant interactions. The fossil record is fragmentary and even extant primitive species may be only remotely related to those that were involved in the first interactions. This discussion is therefore speculative.

Ants and plants in the Cretaceous

It is now generally agreed that the flowering plants (angiosperms) had spread across most of the land masses of the world and had diversified dramatically during the early part of the Cretaceous period. By mid-Cretaceous, about 100 million years before present, this plant group was dominant among terrestrial vegetation (Raven 1977; Doyle 1978). The many suggestions for the causes of this comparatively rapid ascent include changing physical, climatic, and geographical conditions (Axelrod 1970), the rise of major insect pollinator groups (Takhtajan 1969; Crepet 1979), the appearance of avian and mammalian seed-dispersal agents (Regal 1977), the proliferation of herbivores (Ehrlich & Raven 1964; Burger 1981), and the evolution of novel plant secondary compounds (Swain 1977, 1978). In this chapter, I argue that another factor, the ants, contributed significantly to the success and adaptive radiation of the flowering plants.

Given that the fossil record places the angiosperm rise to dominance in the early to mid-Cretaceous, it is appropriate to ask when the ants began to flourish. The order Hymenoptera, to which the ants belong, first appeared in the early Triassic, perhaps 100 million years before the angiosperm accession.