From friend to foe and back: coevolutionary transitions in the mutualism-antagonism continuum.

Interspecific interactions evolve along a continuum ranging from mutualism to antagonism. Evolutionary theory so far has focussed mostly on parts of this continuum, notably on mechanisms that enable and stabilize mutualism. These mechanisms often involve partner discrimination, ensuring that interaction intensity is higher with more cooperative partners. However, the gradual trajectory of coevolutionary transitions between mutualism and antagonism remains unclear. Here, we model how discrimination ability in one partner coevolves with mutualistic service provided by the other and analyse the resulting evolutionary trajectories in the mutualism-antagonism continuum. We show that strong ecological change, such as a radical host shift or colonization of a new environment, can trigger transitions in both directions including back-and-forth transitions between antagonism and mutualism. Moreover, we find an evolutionary tipping point: a stable mutualism may break down to antagonism if the cost of either mutualistic service or discrimination ability gradually increases above a threshold beyond which this transition cannot be reversed by reducing costs again. Our study provides a new perspective on the evolution of biotic interactions and, hence, on the dynamic structure of ecological networks.