EUROCORES_TECT: Mutualisms, Contracts, Space and Dispersal

In biology, a mutualism is an interaction between species that results in increased fitness for both partners. Although mutualisms are ubiquitous in nature, there is currently no general theoretical framework that explains the evolution of interspecific mutualisms. We propose to use two theoretical approaches to develop such a framework: 1) contract theory from economics and 2) spatial games. Using the first approach, we will investigate how the evolution of "natural contracts" can distribute the benefits of mutualism among partners. A major difference between natural and human contracts is that there is no legal system to enforce contracts made between species in the natural world. So, we will draw and expand upon the economic theory of self-enforcing contracts to investigate how mutualisms persist in the face of potential exploitation by cheaters (organisms that reap the benefits of mutualism but do not reciprocate). Using the second approach, we will extend current models of evolutionary games in spatially structured environments to investigate how local dispersal and spatial heterogeneity stabilise mutualistic interactions over evolutionary time. We will parameterize the models we develop using data from several empirical systems, particularly ant-lycaenid, ant-plant, and bacteria-plasmid mutualisms. In so doing, we aim to describe general mechanisms that promote and maintain cooperation in diverse biological systems.

Many species interacting in nature are mutualistic, providing mutual help to each other. Such mutualisms are inherently vulnerable to cheating and free-riding, often resulting in one species exploiting another. The range of mechanisms counteracting such detrimental outcomes is not yet well understood, and the purpose of this project has been to shed light on the variety of ways in which cooperation can be prevented from turning into exploitation.In this vein, eight different cooperation-promoting mechanisms have been examined: strategy diversity, voluntary participation, social exclusion, institutional incentives, peer-to-peer rewarding and punishing, observation-based reputations, non-random interactions, and wealth heterogeneity. For each of these mechanisms, the scientific innovations enabled by this project are being documented in one or more publications in international peer-reviewed journals. Here we highlight the most interesting three groups of findings.Strategy diversity. Agents can escape exploitation by being reactive: in this case, their investments into their current interactions diminish when they are confronted with cheating or free-riding. Our research has shown how such reactivity leads to boom-bust cycles, in which agents vacillate between investing and not investing into interactions, thereby reducing total social welfare. On this basis, we have identified four mechanisms that help preclude boom-bust cycles: a social planner can achieve this by promoting increased reactivity in the investments of agents, diversity in their investment strategies, modularity within the population of agents, and/or heterogeneity between different modules. In populations with spatial structure, local adaptations along the boom-bust cycle lead to spatial mosaics akin to those observed in nature for the geographic distributions of mutualistic species. We have also identified a fundamentally new mechanism for how spatial or social structures can stabilize cooperation, through the emergence of insulating boundary layers that surround mutually adapted agents.Institutional incentives. Social dilemmas can be overcome by positive or negative incentives. Even though an incentive-providing institution may protect a cooperative society from invasion by free-riders, it cannot always convert a society of free-riders to cooperation: in the latter case, cooperators must be sufficiently numerous initially to avoid a collapse to full defection. This means that a society of free-riders can be caught in a social trap, with the incentive-providing institution being unable to provide an escape, except at a high, possibly prohibitive cost. Our research has shown that combining institutional incentives with voluntary participation fundamentally improves the efficiency of such incentives. In particular, voluntary participation allows institutions punishing free-riders to overcome the social dilemma at a much lower cost, and to promote a globally stable regime of cooperation. This removes the social trap. We have also demonstrated that punishing provides a lighter touch than rewarding, guaranteeing full cooperation at considerably lower cost, and that starting with rewarding and then switching to punishing is more effective and more efficient than either rewarding or punishing can be alone.Wealth heterogeneity. Although social structure is known to promote cooperation, by locally exposing selfish agents to their own deeds, studies to date assumed that all agents have access to the same level of resources. This is clearly unrealistic. In our research overcoming this basic limitation we have found that cooperation can be promoted when some agents have access to more resources than others. Cooperation can then emerge even in populations in which the temptation to defect is so strong that agents would act fully selfishly if their resources were distributed uniformly. We also show that resource heterogeneity can hinder cooperation once the temptation to defect is significantly lowered.