Trading public goods stabilizes interspecific mutualism

► Interspecific mutualism raises a collective action problem: why cooperate? ► Punishment is not possible when one cannot discriminate cooperators from defectors. ► Assume that two groups of individuals trade non-linear goods. ► In this case mutualism is maintained by the exchange of these goods alone. ► Punishment is only necessary if benefits are linear (the N-person prisoner's dilemma).