Positive interactions: mutualism, commensalism, and symbiosis

Mutualism, commensalism, symbiosis. Plant–plant co-operation (facilitation, protection, oxygen, shading). Plant–fungus co-operation (mycorrhizae, lichens). Plant–animal co-operation: pollination by birds, seed dispersal by animals (tapir, ants, rodents), the fates of seeds, dodos. The costs of sexual reproduction. The value of sexuality. Animals defending plants. Symbiosis in animal guts: a consequence of cellulose. Models of mutualism. Conceptual obstacles in the study of mutualism. Introduction Definitions Mutualism is an interaction between two species or individuals that is beneficial to both. It is, in other words, a +/+ interaction, in contrast with competition, which is the reverse, a −/− interaction (Chapter 5). Mutualism is sometimes confused with symbiosis, but the term symbiosis denotes merely that two species are living together in close association. Close association is insufficient evidence for the occurrence of mutualism. Some of the problems and issues that arise in the study of mutualism are related to those that arose in the study of competition (Chapter 5), since the same problems can arise whether one is trying to assess positive or negative interactions among individuals. Further, just as competition can be asymmetric, where one individual experiences more negative effects than the other, so too can mutualism be asymmetric. Since it is, in fact, rather unlikely that both individuals in a mutualistic relationship will receive identical benefits from the association, most mutualistic interactions probably are asymmetric.