Host discrimination in modular mutualisms: A theoretical framework for meta-populations of mutualists and cheaters
Discriminating hosts can stabilize mutualism by preferentially allocating resources in modules colonized by mutualists and/or terminating resource allocation into modules colonized by cheaters (sanctions). When symbiotic services are costly, cheaters tend to outcompete mutualists when they are mixed within the same modules. However, preferential allocation and sanctions potentially offset this advantage by giving mutualists a competitive advantage among modules. We developed a meta-population model of module level discrimination to determine how mixed infection undermines the stability of mutualism.
We find that as the number of partners per module increases, hosts must increase the amount of resources that are preferentially allocated in order to stabilize mutualism. Further, we demonstrate that the coexistence of mutualists and cheaters within a single host is not a general feature of discrimination with mixed infection; rather, it is possible only when preferential allocation is a saturating function of the number of mutualists in a module. Finally, we suggest how this model can be applied to various mutualisms.