Abstract
Mutualisms are cooperative interactions between members of different species, often involving the trade of resources. Here, we suggest that otherwise-cooperative mutualists might be able to gain a benefit from actively restricting their partners' ability to obtain resources directly, hampering the ability of the restricted partner to survive and/or reproduce without the help of the restricting mutualist. We show that (i) restriction can be favoured when it makes the resources of the restricting individual more valuable to their partner, and thus allows them to receive more favourable terms of trade; (ii) restriction maintains cooperation in conditions where cooperative behaviour would otherwise collapse; and (iii) restriction can lead to either an increase or decrease in a restricted individual's fitness. We discuss the applicability of this scenario to mutualisms such as those between plants and mycorrhizal fungi. These results identify a novel conflict in mutualisms as well as several public goods dilemmas, but also demonstrate how conflict can help maintain cooperation.
Highlights
Mutualisms are cooperative interactions between members of different species, often involving the trade of resources
In some cases, plants might not be able to completely cut their interactions with their fungal partners, which could lead to a fitness cost of interacting with fungi, termed mycorrhizal depression (Fig. 4)[7,28,33]. How relevant is this model to mutualisms in nature? Mycorrhizal fungi appear to inhibit plants’ direct phosphorous uptake pathway via molecular suppression mechanisms[33,34,35,36]
Further evidence for restriction in mutualisms comes from the ant–acacia mutualism
Summary
Mutualisms are cooperative interactions between members of different species, often involving the trade of resources. For example: mycorrhizal fungi supply their host plants with phosphorous and other nutrients in exchange for host carbon; ants defend trees in exchange for food and housing; and flowering plants provide insects and birds with food in exchange for pollination services[3,4,5] These mutualisms are based on cooperative trade, there is an underlying tension because each partner is favoured to maximize its benefit from the interaction, leading to conflict and the potential for exploitation. We found that mutualists can be selected to actively restrict their partners’ ability to directly obtain resources This prediction arises because restriction increases the partner’s comparative advantage when engaging in the mutualism, and improves the restricting individuals’ terms of trade. These results emphasize how mutualists will continue to be favoured to maximize their own fitness, even at a cost to their partner
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