| Several unique aspects of arbuscular mycorrhizae, which are ubiquitous mutualistic symbioses between plants and fungi, are reviewed. The implications of one aspect in particular, that simultaneous association with multiple partners is common in these systems and that such multiple partners may have non-additive effects, is explored with theoretical models and greenhouse experiments. The models show that synergism or antagonism between multiple plants or fungi in their effects on a shared partner can promote coexistence via feedbacks on partner frequency. They further show that, via spatial structuring of environmental quality, synergism between multiple partners can promote coexistence while antagonism can promote founder effects and favor generous partners over more exploitative ones. Of six pairwise combinations of fungi, four had non-additive effects on at least one aspect of the growth of one host species, and both synergistic and antagonistic effects were seen. Heterospecific host plants were additive in their effects on the growth of one fungus, and on the growth of a second fungus near their roots, but antagonistic in their effects on the growth of the latter fungus in root-free zones between the plants. Of the two, the fungal responses to multiple plants may be more similar to the interactions explored by the theoretical models than the plant responses to the fungi. The implications of each experiment and model for interpretation of one another's results are discussed. |
