A model that explains diversity patterns of arbuscular mycorrhizas

Worldwide arbuscular mycorrhizal (AM) plants in grassland ecosystems associate with multiple AM symbionts; the literature suggests that the typical range varies from 7 to 11 AM symbionts. Using a model based on group selection, we examine the evolutionary traits that could be responsible for maintaining this situation. We assumed that from a pool of 160 potential symbionts some of them would deliver a profit for the plant while the others (defectors) would deliver comparable damage. The plant could control the number of simultaneous fungi it associated with but not their identities. Plants admitting higher numbers of symbionts can gain greater benefits but run a greater risk of associating with defectors. Based on simulation results we can demonstrate convergence toward an optimal number of fungal symbionts which occurs faster for plants that experience low survivorship and higher selection pressure. Additionally, for seedling survivorship rates close to those experienced in grassland biomes (i.e. around 50%) there appears to be a steady plateau with an optimum of 7 symbionts over a wide range of defection probabilities. This does not hold for other biomes with higher plant mortality such as woodlands in which there is an unsteady situation with highly likely modification of the number of simultaneous symbionts. This may hold important clues about to the origin of the ectomycorrhizal associations where only one symbiont is optimum. (C) 2012 Elsevier B.V. All rights reserved.