Environmental Fluctuations Promote Host Reward Strategies That Maintain Partner Diversity in Multispecies Mutualisms

AbstractIn multispecies mutualisms, hosts might be expected to reward only the highest-quality partner in order to maximize benefits and prevent the proliferation of cheaters. In a fluctuating environment, however, partner quality is likely to vary over time, and the maintenance of low-quality partners has been shown to be beneficial in some environmental regimes. Here, we present a model of a simple tree-fungal mutualism with two distinct environmental conditions and a host that can employ reward strategies with varying degrees of preference for higher-quality fungi. We find that in many environmental regimes, the most successful strategy for the host is one that actively maintains equal densities of the two fungal partners, in spite of their immediate differences in quality. This conservative bet-hedging strategy leads to reduced variance in the tree's carbon resources and high resilience to environmental perturbation. An alternative reward strategy, which supports only the highest-quality partner at a time, is most successful under some conditions when fluctuations in the environment are infrequent. Longer periods of environmental stasis thus increase the risk to the tree of losing fungal partner diversity. This theoretical work identifies a mechanism by which biodiversity may be actively maintained in multispecies mutualisms but that may be disrupted as environmental conditions change.