Preferential allocation of benefits and resource competition among recipients allows coexistence of symbionts within hosts

Functionally variable symbionts commonly co-occur including within the roots of individual plants, in spite of arguments from simple models of the stability of mutualism that predict competitive exclusion among symbionts. We explore this paradox by evaluating the dynamics generated by symbiont competition for plant resources, and the plant’s preferential allocation to the most beneficial symbiont, using a system of differential equations representing the densities of mutualistic and non-mutualistic symbionts and the level of preferentially allocated and non-preferentially allocated resources for which the symbionts compete. We find that host preferential allocation and costs of mutualism generate resource specialization that makes the coexistence of beneficial and non-beneficial symbionts possible. Furthermore, coexistence becomes likely due to negative physiological feedbacks in host preferential allocation. We find that biologically realistic models of plant physiology and symbiont competition predict that the coexistence of beneficial and non-beneficial symbionts should be common in root symbioses, and that the density and relative abundance of mutualists should increase in proportion to the needs of the host.

Methods
The zipped folder is the repository of analyses: Preferential allocation of benefits and resource competition among recipients allows coexistence of symbionts within hosts

This theoretical study is carried out by solving a four-variable ordinary differential model that represents two carbon resources and two consumers (i.e. mutualist and non-mutualist symbionts on plant's root). No empirical dataset is used. Codes are written using open-source software (i.e. R, FORTRAN). Analytical derivations are given in the related manuscript with details in the appendix.