pollinators. For instance, non-rewarding orchids
are known to benefit from displaced pollinators by
being located close to rewarding flowers (Johnson
et al. 2003; Ghazoul 2006). Generalizations in polli-
nation mutualisms facilitate the pollination of sev-
eral coexisting species and contribute to coexistence
and diversity. The plant–pollinator interaction con-
tinuum can range from full specialist to full gener-
alist. Such pollinator functional diversity enhances
diversity in plant communities (Fontaine et al.
2006).
13.3.2 Plant–protector mutualism
Mutualisms involving protection may have the ef-
fect of decreasing species diversity in a community.
An estimated 20–30% of all grass species host endo-
phytic symbionts (Leuchtmann 1992).Neotypho-
dium coenophialum, a fungal endophyte of the
invasive grass speciesLolium arundinaceum,pro-
tects the host from herbivores and pathogens
through the production of toxic alkaloids (Finkes
et al. 2006).Neotyphodium coenophialumalso alters
the detritivore composition in the rhizosphere, sug-
gesting possible influences on ecosystem processes
like decomposition and nutrient turnover (Lemons
et al. 2005). Because of the unpalatability rendered
by the endophyte, the competitive ability ofLolium
arundinaceumis increased and this grass is able to
displace other competing species from the commu-
nity, thereby bringing about a reduction in species
diversity. In this way, a mutualist associated with a
dominant plant species enhances the competitive
ability of its host, leading to lower biomass of the
competing species and reduction in the productivi-
ty of the community (Rudgerset al. 2005). The
strong influence of fungal endophytes on plants
and herbivores may cascade to other trophic levels.
TheLolium–Neotyphodiummutualism has been re-
ported to influence composition of spider commu-
nities by causing reductions in prey populations
and changes in plant assemblages that affected
web building (Rudgers and Clay 2005; Finkeset al.
2006).
Another example of plant-protection metabolism
is that several plant species form mutualistic asso-
ciations with ants. The ants protect the plants
against herbivores while the plants provide food
and/or housing in return. For example, the
whistling acacia (Acacia drepanolobium) has modi-
fied thorns called stipular spines that house sting-
ing ants. The ants deter herbivores by swarming out
of their nests and attacking an intruder at the smal-
lest movement. Without the protection by ants,
plants like the whistling acacia might be eliminated
by herbivores such as giraffes, for whom thorns are
no great deterrent. However, recent experiments in
which large herbivores were excluded from feeding
onAcaciatrees demonstrated that, in the absence
of defoliation threats from large herbivores,Acacia
depranolobiumreduced investments in extrafloral
nectaries and modified stipular thorns for feeding
and housing symbiotic ants. The reduction in re-
wards to ants caused a shift in competitive domi-
nance within plant–ant community from nectar-
dependent symbiotic ant species to an antagonistic
ant species. This shift in ant community and the
resultant breakdown of ant–Acacia mutualism
could lead to potentially negative consequences
forAcaciagrowth and survival (Palmeret al. 2008).
In a few cases, protective ants might actually
castrate their hosts and reduce the plant’s fitness
(Stantonet al. 1999).Cordia alliodoraforms symbiotic
associations with ants for protection against insect
herbivores. Ant associates such asAzteca pittieri
gave significant benefits to the plants by protecting
them against insect herbivores, while other associ-
ates such asCephalotes setulifer(which do not pro-
vide any benefits) actually formed a drain on the
plant resources (Tillberg 2004).
An example of perturbation in ecological pro-
cesses demonstrates the important role mutualisms
have in community organization. Habitat fragmen-
tation results in disruptions in the foraging range of
pollinators and reductions in pollination services
(Kearnset al. 1998). This has implications for the
fitness and genetic profile of plant populations. In
fragmented plant populations, pollinators increase
geitonogamy by visiting a higher proportion of
flowers on individual plants, leading to increased
genetic drift and inbreeding depression (Kearnset
al. 1998). Shifts in pollinator assemblages can result
in parallel changes in plant communities. In Britain
a declining insect pollinator population caused a
parallel decline in insect-pollinated plants and an
increase in species reliant on abiotic factors for184 FUTURE DIRECTIONS