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.