Stabilizing Mutualisms Threatened by Exploiters: New Insights from Ant-Plant Research

Mutualisms are commonly threatened by parasites and cheaters: species that exploit the host-derived resources without providing an adequate service. Here, we summarize mechanisms for the stabilization of obligate defensive ant-plant mutualisms, a typical element of tropical lowland forests. Host plants exert partner choice and can sanction non-defending ants by shedding the domatia that serve as nesting space or ceasing the production of ant rewards. Hosts can also restrict the exploitation of the ant rewards by means of specific biochemical traits that decrease their quality for non-adapted generalist exploiters and, thus, convert them into exclusive rewards. Reward provisioning can even shift the competitive balance between mutualists and exploiters in favor of the mutualists. In turn, plant-ants show adaptations in their colony structure and changes in their digestive capacities that enhance their efficiency in the use of the host-derived resources. Founding queens use plant odors for host choice behavior, and ants not supplied with adequate amounts of EFN decrease their defensive service and thereby exert partner sanctions. Theoretical models and empirical research into mutualisms usually focus on actions that are taken by the host. Using ant-plants as model systems, we are now discovering the importance of contributions that come from the symbiont. This discovery indicates the potential for multiple reciprocal interactions between phenotypically plastic hosts and symbionts, which contribute significantly to what is still considered a miracle: the stability of mutualisms in the presence of exploiters.