Capitalizing on movement for optimal defense in protection mutualisms

Background/Question/Methods

Movement affects the cost-benefit balance of many mutualisms by shaping the dynamics of the interaction itself. In protection mutualisms, the protective species defends the protected species from its natural enemies in exchange for food or shelter. In this talk, I will consider how movement by the protective species may be harnessed to maximize the fitness benefit of the mutualism to the protected species. In ant-plant protection mutualisms, a colony of ants defends a plant from herbivores, pathogens, or competition. Because the amount of resources that can be invested in defense is limited, selection is hypothesized to favor so-called ‘optimal defense,’ which prioritizes protection of the most valuable photosynthetic and/or reproductive tissues. Such tissues are also frequently especially vulnerable to attack from herbivores. I tested whether plants encourage protection of valuable and vulnerable tissues by measuring reward provisioning on tissues of different ages and reproductive values in my own studies of symbiotic ant-plant mutualisms and in examples from the literature. I further tested whether differential reward provisioning is associated with higher densities of patrolling ants and attack rates on herbivores.

Results/Conclusions

The density of direct and indirect food rewards to colonies of protective ants is frequently highest on new leaves, as predicted by the optimal defense hypothesis. Furthermore, in at least one symbiotic ant-plant system, trees produce ant-rewarding structures specifically associated with flower and seed production. This spatial patterning of reward provisioning within the plant harnesses ant movement by increasing rates of ant patrolling, which is associated with higher rates of herbivore attack. The resulting damage to new leaves and reproductive tissues is lower when ants are present than when they are absent. I will conclude by discussing interesting avenues for future research, including: (i) how similar the mechanisms that produce optimal defense by ant partners are to the mechanisms that can produce optimal defense by endogenous plant traits like chemical toxins; and (ii) whether the ability of ants to move actually presents a selective advantage of mutualistic ant defense over defense by endogenous plant traits in some environments.