Intraspecific competition can stabilize asymmetric nursery pollination mutualisms

A fundamental problem in ecology is to understand how mutualisms remain stable. The density-dependent regulations within interacting species potentially impact the persistence of these interspecific relationships. Yet few studies explore such intraspecific regulations’ role in stabilizing mutualisms. In addition, partner species often gain unequal benefits in mutualisms. To what extent such an interspecific asymmetry affects the stability of mutualisms is also poorly understood. We here developed a dynamic model for the asymmetric interaction between plants and their pollinators in nursery mutualisms, considering the intraspecific competition of each mutualist. We found that (i) a mutualism can be stabilized only if both mutualists are subject to the regulation of intraspecific competition; (ii) stabilizing the system also requires that the degree of asymmetry in benefits between mutualists must be limited to a range of ‘tolerance’, which narrows as intraspecific competition increases and even fades away with strong competition within both mutualistic species; (iii) when intraspecific competition within a species increases, the tolerant range is compressed from the side beneficial for it, with thus its partner species gaining relatively more benefit allocation; (iv) if the plant-pollinator interaction initiates from a small host plant population, these host plants must offer pollinators high levels of benefits, that can be subsequently reduced to favor plants once the mutualism has been successfully established. The agreement of empirical data to theoretical predictions suggests model reliability. These results highlight the role of intraspecific competition and the degree of benefit asymmetry between host plants and symbionts in stabilizing mutualisms.