Questions: Can resource-resource trade mutualism offer a competitive advantage to plants? If so, what are the conditions that give mutualism an advantage especially with regard to the size of the neighborhood? Hypothesis: We hypothesized that mutualism could offer a competitive advantage if the benefits outweighed the costs. We also hypothesized that this competitive advantage could lead to coexistence between mutualist and non-mutualist strategies within the same population. We also hypothesize that local neighborhood size (the number of competitors at a given moment) would change this response, though the specific direction of change was unclear to us. Method: We created an evolutionary game theoretic model in which a plant could either be a mutualist or non-mutualist that incorporated nutrients freely available to the plant, nutrients obtained only through mutualism with microbes, the cost of producing roots, the cost of trade with microbes, and neighborhood size. We sought ESS solutions as defined by the Nash equilibrium criterion. Key Assumptions: 1) Costs and benefits are fixed for all plants. 2) Freely available nutrients are equally shared between all competing plants in a local neighborhood. 3) Microbially obtained nutrients are shared equally between mutualistic plants in the local neighborhood. Conclusion: We found that mutualism could offer a competitive advantage if the net benefit was positive. Coexistence between mutualist strategies in our model happens because of competition between mutualists over the microbially available nutrient. Coexistence was more likely with greater neighborhood size but at the expense of mutualist fixation.

Understanding drivers of species coexistence is a central challenge in ecology. Coexistence cannot be observed directly, and while species co-occurrence in time and space is necessary for coexistence, it is not sufficient to prove coexistence. Species exclusion from a region is potentially observable, but can take decades to occur, and still might occur stochastically. Thus, ecologists generally use theory to identify indirect observations that are indicative of mechanisms driving coexistence or exclusion. Various methods have been developed to indirectly infer coexistence, each of which requires different data, and none of which are usually conclusive on their own. Here, we demonstrate agreement using three different approaches examining coexistence of multiple hardwood species. First, in an experimental planting of three mature tree species we found no relationship between productivity and species diversity, which could be due to a lack of niche differences among species. Second, we used modern coexistence theory to calculate niche and fitness differences for each pair of species, which confirmed the lack of niche differences among species, and showed high fitness differences that could create a neutral distribution of species in nature. Third, we used the United States Department of Agriculture Forest Inventory and Analysis data to examine co-occurrence patterns of our species across thousands of natural forest stands and found that indeed, these three species were distributed randomly throughout the USA. Given that these independent methods agree, we take this as strong evidence about a lack of coexistence.