Maintenance of Mutualistic Variation Within and Between Species

Background/Question/Methods



Mutualisms and mutualistic interactions present shared and unique properties at different scales, such as ecological and evolutionary collapse and nestedness. One well-studied aspect is the presence of variation in mutualistic interactions at the individual, population, and community scale depending on ecological, evolutionary, and environmental context. Here, I present theoretical analysis on the maintenance of mutualism variation within and between species. Within species, I analyze how competition between plants in a mutualistic plant-microbe relationship may promote or hinder variation in the strength of mutualistic interaction using a game-theoretic model. This model includes features like microbially-derived nutrients and a locally competitive neighborhood. Between species, I examine how coevolutionary niche dynamics affects variation in mutualistic association and the resulting community structure. Recent evidence from simulated mutualism communities has shown that an adaptive niche process can account for the structure of mutualistic communities, though with the assumptions of a randomly generated fixed niche position and the same niche breadth among species. I sought to determine how niche coevolution affected variation present in mutualistic communities using an evolutionary game theoretic consumer-resource model of mutualistic interactions between two guilds. Several in-silico mutualistic communities were generated whose properties were subsequently analyzed.



Results/Conclusions



Within a species, we found that coexistence between mutualist genotypes is possible due to competition between mutualists over the microbially obtained nutrient. Furthermore, a larger competitive neighborhood is more likely to result in coexistence at the expense of mutualism fixation. In this way, ecological competition promotes variation in strength of mutualistic interaction, especially when the mutualistic resources are shared like in the case of mycorrhizal fungi. Between species, mutualism often acted as a hinderance to within-guild diversification in our in-silico communities. Species would seek to affiliate with a single mutualist leading to collapse of variation, sometimes to a single mutualistic species pair. Despite this, some variation seemed to be maintained especially with regard to niche breadth. Species showed heterogeneity in niche breadth with a few generalized species and several specialized species. These distribution in niche breadth is related to the niche position where more central niche positions result in greater specialization and vice versa. This heterogeneity in degree distribution also resulted in properties like nestedness. Going forward, combining within and between species processes will allow us to explore the full potential variation in mutualistic interactions.