Adaptive partner recruitment can help maintain an intra-guild diversity in mutualistic systems.

Mutualisms between assemblages of multiple species or strains (guilds) are considered unstable because of positive feedback between the guilds. Previous studies suggest that negative inter-guild feedback due to asymmetry in the exchange of benefits between the guilds can stabilize them, but preferential association for more beneficial partners may reduce such asymmetry and strengthen the positive inter-guild feedback. Here I develop a replicator dynamics model for mutualistic systems between two host and two symbiont strains to investigate conditions that stabilize mutualisms when feedback between host-symbiont guilds is positive. I assume that one symbiont strain is mutualistic for one host strain but parasitic for the other, whereas the other symbiont strain is the opposite. Hosts recruit their symbionts from the environment and discriminately offer them resources (partner preference), and only mutualistic symbionts return benefits to their hosts. I show that the two host and symbiont strains can coexist under strong partner preference by hosts if they adaptively adjust the number of associating symbionts, even when the intra-host strain competition is not so strong. Interestingly, there can be a stable coexistence equilibrium also under weak partner preference, but it disappears under intermediate levels of partner preference.