Abstract
Plants engage in multiple root symbioses that offer varying degrees of benefit. We asked how variation in partner quality persists using a resource‐ratio model of population growth. We considered the plant's ability to preferentially allocate carbon to mutualists and competition for plant carbon between mutualist and nonmutualist symbionts. We treated carbon as two nutritionally interchangeable, but temporally separated, resources—carbon allocated indiscriminately for the construction of the symbiosis, and carbon preferentially allocated to the mutualist after symbiosis establishment and assessment. This approach demonstrated that coexistence of mutualists and nonmutualists is possible when fidelity of the plant to the mutualist and the cost of mutualism mediate resource competition. Furthermore, it allowed us to trace symbiont population dynamics given varying degrees of carbon allocation. Specifically, coexistence occurs at intermediate levels of preferential allocation. Our findings are consistent with previous empirical studies as well the application of biological market theory to plantroot symbioses.
Highlights
Mutualisms are interactions between species that confer benefit to both partners (Boucher, James, & Keeler, 1982)
When we consider competition for construction carbon and allocation carbon in isolation from one another, we find that the nonmutualist outcompetes the mutualist for construction carbon (C*N < C*M ) due to the cost of mutualism borne to the mutualist (Figure 1)
There is a longstanding paradox regarding how mutualistic interactions persist in the face of cheating, and a more recent restructuring of this paradox to ask how variation in partner quality persists in nature (Foster & Kokko, 2006; Heath & Stinchcombe, 2014)
Summary
Mutualisms are interactions between species that confer benefit to both partners (Boucher, James, & Keeler, 1982). We build from the framework of Bever (2015), which predicts the coexistence of a more beneficial and less beneficial root symbiont (designated as the mutualist (M) and nonmutualist (N), respectively), even in the face of considerable costs to the mutualist of provisioning the host plant with nutrients. These costs are overcome by incorporating physiological plasticity of plants to preferentially allocate carbon (C) to the mutualist.
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