Abstract
Most mutualisms in nature involve interactions between multispecies mutualist guilds and multiple partner species. While mechanisms such as niche partitioning can explain part of this diversity, the presence of low-quality partners, which produce relatively low returns on investment compared with other guild members, is not well understood. Here, we consider a novel explanation for this persistence: that low-quality partners are actively maintained by their hosts as part of a growth-maximizing strategy, even in the presence of higher-quality alternatives. We use a model inspired by the interaction between host trees and ectomycorrhizal fungi to demonstrate that when the environment is variable, trees maintain low-quality fungal partners that they would not otherwise maintain in constant environments. This active investment, which emerges as a response to saturating returns on investment in higher-quality partners, could contribute to the maintenance of diversity in multispecies mutualisms.
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