Ontogenetic variation in the benefits of ants to their host plants in an obligate mutualism

Background/Question/Methods

Ontogeny can alter species interactions. When individuals from two different species form intimate associations (e.g., mutualism), the ontogenic stage of both participants can influence the outcome and direction of the interaction. In ant-plant interactions, studies usually focus on turnover in ant species over the life of the host plants, rather than examining how the interaction between a single plant and a single ant species changes with plant and ant colony ontogeny. Here I describe the change in benefits conferred by the ant Myrmelachista flavocotea (Formicidae: Formicinae) on its obligate host plants, Ocotea atirrensis and O. dendrodaphne (Lauraceae), as a function of ontogeny across a chronosequence of plant and ant colony development. I hypothesized that larger colonies on large Ocotea would confer greater benefits to their host plants resulting in reduced herbivory compared to smaller colonies. To test this hypothesis, I determined colony aggression by measuring ants’ response to a foreign object (segment of wire) placed on the apex of the mainstem. For each Ocotea (n = 250), I measured plant height, branch lengths, number of leaves, leaf area, leaf toughness and thickness, and mean herbivore damage. I collected 30 intact colonies to determine the number of ants per colony. 

Results/Conclusions

Ontogenic stage of Myrmelachista colonies and Ocotea host plants had a clear influence on the outcome of the interaction between ants and plants. Myrmelachista queens colonize Ocotea seedlings and retain an association with their host plant until either the queen or the plant dies. All Ocotea trees >20 cm are inhabited by M. flavocotea. Colony demography was clearly linked to the ontogeny of their host plant. As Ocotea grow, ants excavate pith in new growth, expanding their nest space and colony size. Colony size was positively correlated with tree size. There was a significant hump-shaped distribution to ant aggression, with ants on trees of intermediate size the most aggressive. This modal pattern of ant defense was matched with a concomitant decrease in herbivore damage on plants of intermediate size. Patterns of host plant defense were influenced by ontogenic stage of the ant colony. Small colonies on small trees lacked the workers to defend host plant. Large colonies on large Ocotea trees appear to lack the per capita densities needed to adequately protect their host plants. The benefits conferred by Myrmelachista on their host plants were strongest on trees with rapid growth and rapid colony expansion.