Ontogenetic trajectories of direct and indirect defenses of myrmecophytic plants colonized either by mutualistic or opportunistic ant species.

Abstract

Myrmecophytic plants are expected to produce greater direct defenses when young and switch towards indirect defenses once they reach the size and vigor to produce enough rewards for their ant mutualists. The presence of opportunistic ant species, however, is likely to promote the variation in these ontogenetic trajectories. When plants do not obtain benefits from ants, they cannot rely on this indirect defense. Hence, the expression of direct defenses is expected to remain constant or even increase during the development of plants colonized by opportunistic ants, whereas a reduction in resource allocation to indirect defenses should be observed. To assess if myrmecophytic plants adjust their ontogenetic trajectories in defense as a function of the colonizing ant species, we estimated direct and indirect defenses at four ontogenetic stages of the myrmecophytic plant Vachellia hindsii colonized by either mutualistic or opportunistic ant partners. We report that cyanogenic potential decreased while leaf thickness and the production of sugar in extrafloral nectaries increased along plant development. The magnitude of these ontogenetic changes, however, varied as a function of the identity of the colonizing ants. As expected, when colonized by opportunistic ants, plants produced more direct defenses and reduced the production of rewards. We suggest that facultative changes in the expression of ontogenetic trajectories in direct and indirect defenses could be a mechanism to reduce the fitness costs associated with opportunistic interactions.