ecologically similar species (Boucheret al. 1982).
However, ultimately, partners can achieve stable
mutualisms through the enforcement of coopera-
tive behaviour by actively rewarding cooperation
and punishing cheating (Westet al. 2002a). Mutual-
isms will remain stable if benefits for both partners
exceed costs; otherwise the interaction is predicted
to shift to parasitism (Hollandet al. 2004).
13.2.1 Mutualism can also develop without evolution
While an extensive body of literature describes the
coevolution of mutualists (Anstettet al. 1997; Kato
et al. 2003; Machadoet al. 2005; Mehdiabadiet al.
2006; Moran 2006), the different traits of different
species can give rise to mutualistic associationsde
novoeven between organisms that are not coevolved.
The association may subsequently be stabilized
through natural selection acting on each species
(Moran 2007). For example, invasive ant species
may provide some dispersal services to native plant
species that previously relied on native mutualistic
ant species (Lach 2003). The facultative nature of
many mutualisms also indicates that their formation
is not always a result of coevolution. An established
mutualistic association is open to parasitic exploita-
tion by either partner or a third species that might
profit from the benefits provided while not give any-
thing in return (Boucheret al. 1982). Several birds and
small animals forage for nectar onButea monosperma
(Fig. 13.2), a leguminous tree from India. While the
purple sunbird (Nectarinia asiatica) and the three
striped squirrel (Funambulus tristriatus)providerecip-
rocal pollination services, the rose ringed parakeet
(Psittacula longicauda) is a cheater that benefits by
foraging on the flowers but provides no reciprocal
benefits to the plant (Tandonet al. 2003). These exam-
ples show that mutualistic interactions are complex
and are subject to continuous evaluation and selec-
tion by the environment.
13.3 Mutualisms in community organization
Mutualisms provide partner species with novel
options for adjusting to changing physical and biotic
environments. They can be pivotal in affecting the
organization, structure and function of communities.
Mutualisms commonly support the key species that
define entire ecosystems and can play important
roles in moving energy and nutrients across ecosys-
tem borders (Hayet al. 2004). Mutualists, through
affecting the fitness of their partners, can have a
strong influence on community organization. Mod-
ern angiosperms are estimated to comprise250 000
species and an estimated 70–90% of these recruit
animal mutualists for pollination services (Heywood
1993; Kearnset al. 1998; Fontaineet al.2006).Upto
80% of all terrestrial plants are believed to form sym-
biotic associations with mycorrhizas for nutrient ac-
quisition and mycorrhizas are believed to have
hastened the invasion of land by plants (Malloch
et al. 1980; Simonet al. 1993; Smith and Read 1997).
Below we will discuss a number of mutualisms that
are known to influence plants and plant community
composition. We will start with plant–pollination
and plant–protector mutualisms. In the two
subsequent sections we will then discuss two promi-
nent examples of mutualistic interactions, the le-
gume–rhizobia and plant–mycorrhiza interactions.13.3.1 Plant–pollinator interactions
Plant–pollinator interactions have been hypothe-
sized to coevolve towards an increasing degree of
specialization (Stebbins 1970). However, recent em-
pirical studies are demonstrating a scenario in
which most plants exploit a wide and diverse
range of pollinators (Sahli and Conner 2006;
Gomezet al. 2007). Generalization may benefit pol-
linators foraging on several plant species as they
can acquire multiple resources, such as pollen, nec-
tar, mates or prey, each provided at different plants
(Ghazoul 2006). Increased diversity may be one
consequence of plant and pollinator seeking gener-
alization in mutualist partners. Generalizations
provide mutualist partners with ‘competitor-free
space’ (Ghazoul 2006). Pollinators displaced from
preferred flowers by aggressive competitors have
the option of returning to their preferred flowers if
they temporarily relocate to alternative species, not
visited by the competitively dominant pollinators.
By providing a competitor-free space, less reward-
ing flowers benefit from hosting the displacedMUTUALISMS AND COMMUNITY ORGANIZATION 183