Cooperation and Conflict between Populations

Resource-based theory was first tested by chemical engineers interested in commensalistic industrial fermentations: a commensalism in which the commensal does not benefit and a mixed interaction that can vary from obligate commensalism with competition through facultative commensalism with competition to pure competition, depending on the amount of the cross-fed resource that is externally supplied. The unstable node characteristic of obligate mutualisms, in which low-density communities collapse while high-density communities persist, has been detected experimentally. A mutualism involving a toxic cross-fed resource and competition for another cross-fed resource illustrates just how complex even a simple community can be. Several examples illustrate that production of benefits need not be costly to fitness. Containment enforces cooperation. Under natural circumstances, microbial interactions may be so diffuse that one would hesitate to call them mutualisms with so many actors at play. Experimental evolution of a costly mutualism highlights the importance of localizing interactions in space so that benefits can be targeted at reciprocating partners while distant “cheats” starve.