Data from: Spatial structure and interspecific cooperation: theory and an empirical test using the mycorrhizal mutualism

Explaining mutualistic cooperation between species remains a major
challenge for evolutionary biology. Why cooperate if defection potentially
reaps greater benefits? It is commonly assumed that spatial structure
(limited dispersal) aligns the interests of mutualistic partners. But does
spatial structure consistently promote cooperation? Here, we formally
model the role of spatial structure in maintaining mutualism. We show
theoretically that spatial structure can actually disfavour cooperation by
limiting the suite of potential partners. The effect of spatial
structuring depends on the scale (fine or coarse level) at which hosts
reward their partners. We then test our predictions by using novel
molecular methods to track the abundance of competing, closely-related,
cooperative and less-cooperative arbuscular mycorrhizal (AM) fungal
symbionts on host roots over multiple generations. We find that when
spatial structure is reduced, by mixing soil, the relative success of the
more cooperative AM fungal species increases. This challenges previous
suggestions that high spatial structuring is critical for stabilizing
cooperation in the mycorrhizal mutualism. More generally our results show,
both theoretically and empirically, that contrary to expectations spatial
structuring can select against cooperation.