Community Ecology Processes, Models, and Applications

that evolved through trade-offs into cooperation
that bestowed selective advantages on the mutual-
istic partners. We then discuss the role of mutual-
isms in community organization and conclude
by evaluating performance in two prominent exam-
ples of mutualistic interactions: the legume–
rhizobia and plant–mycorrhiza interactions.

13.2 Conflicts, cooperation and evolution of mutualisms

Cooperative behaviours that benefit both the actor
and the recipient(s) of the behaviour are termed
‘mutually beneficial’ (West et al. 2007b). Even
though interspecific mutualisms are often viewed
in the context of reciprocal exploitations, they none-
theless provide net benefits to each partner (Herreet
al. 1999). Mutualistic interactions may be direct or
indirect (Westet al. 2007a; Fig. 13.1). In direct mutu-
alisms, the cooperating species interact physically,
whereas in indirect mutualisms cooperating species

benefit from each other’s presence, but there is no

direct contact. Direct mutualisms can include sym-

biotic interactions, which are defined as intimate

interactions among different species. In symbiotic

interactions the benefits exchanged can be classified

in four different types: nutritional, supply of energy,

protection and transport mutualisms.

One useful approach to understanding mutual-

isms is via an economic framework that defines the

value of the benefits and costs exchanged by the

partners. A biological trading price determined by

the balance between supply and demand for the

benefits being exchanged locates the interaction

along the mutualism–parasitism continuum. When

a price is favourable for both partners the interac-

tion moves to the mutualistic end of the continuum,

but if the price is favourable for one species and not

for the other the interaction becomes parasitic

(Schwartz and Hoeksema 1998; Hoeksema and

Bruna 2000; Hoeksema and Schwartz 2006). Such

trade-based models are useful for conceptualizing

partnerships, but they tend to gloss over the in-

Greenboard

Genetic

cues

Prior

environment

Shared

environment

Indirect

(reputation-based)

reciprocity

Direct

reciprocity

Sanctions Reciprocity Policing

Fixed

enforcement

Non-enforced Enforced

Direct benefits

Environmental

cues

Kin

discrimination

Limited

dispersal

Cooperation

Reward Punishment

Conditional

enforcement

Indirect benefits

Figure 13.1A classification of the explanations for cooperation. Direct benefits explain mutually beneficial cooperation,
whereas indirect benefits explain altruistic cooperation. Within these two fundamental categories, the different
mechanisms can be classified in various ways. These possibilities are not mutually exclusive; for example, a single act of
cooperation could have both direct and indirect fitness benefits, and interactions with relatives could be maintained by
both limited dispersal and kin discrimination. Reproduced from Westet al. (2007a) with permission from Elsevier.

180 FUTURE DIRECTIONS