Community Ecology Processes, Models, and Applications

pollination. Similarly, in The Netherlands, a declin-
ing bee population has caused a decline in plants
exclusively pollinated by bees (Beismeijer et al.
2006).
Invasions by alien species have the potential to
disrupt mutualistic interactions within native com-
munities and can also modify successional trajec-
tories. Impacts of alien pollinators have yet to be
conclusively demonstrated. However, it is thought
that alien pollinators might either increase pollen
transfer among plants, thereby increasing plant fit-
ness, or might have very low visitation rates and
consequently might not (or negatively) affect repro-
ductive success (Traveset and Richardson 2006).
Alien pollinators are known to displace native pol-
linators. Specialist plants dependent on the dis-
placed pollinators are also likely to be displaced,
resulting in changes in plant species composition.
Alien pollinators might bring about changes in
community composition by preferentially visiting
plants that might not be preferred by native polli-
nators. The red-whiskered bulbul (Pycnonotus joco-
sus), an invasive species in Mauritius, visits the
flowers of the extremely rare endemicNesocodon
mauritianusmore frequently than do native birds
(Traveset and Richardson 2006).
Disruptions of mutualisms may accelerate the
decline and extinction of species in a community.
The loss of one species could cause the subsequent
loss of other species that are directly or indirectly
dependent upon them. Disruption of belowground
plant mutualists like mycorrhizas and symbiotic
bacterial associates can have significant effects on
aboveground plant communities (Klironomos 2002;
Stinsonet al. 2006; van der Puttenet al. 2007).Al-
liaria petiolata(garlic mustard), an invasive species,
disrupts arbuscular mycorrhizal associations (Stin-
sonet al. 2006). Garlic mustard may consequently
affect composition of mature forest communities by
favouring plants with low mycorrhizal dependency
and repressing the regeneration of canopy trees that
are dependent on arbuscular mycorrhizal (AM)
fungi symbiosis. While some invasions might alter
soil-borne mutualisms and consequently reorga-
nize recipient plant communities, mutualistic asso-
ciations might also favour invasions (Richardsonet
al. 2000). Although there is little evidence for domi-
nance and competitive exclusion of native species

by invasive plants through establishment of new

mutualisms, evidence is plentiful for promotion of

plant invasions caused by mutualistic interactions

with other soil biota (Simberloff and Von Holle

1999; Reinhart and Callaway 2006). Arbuscular my-

corrhizas can potentially colonize a broad range of

hosts although specificity may exist for growth re-

sponses, making it possible for invaders to use the

native mycorrhizae of a new region (Callawayet al.

2004).

13.3.3 Plant nutrition symbiosis

In the following sections we discuss two of nature’s

most important mutualistic interactions, the le-

gume–rhizobia and the plant–mycorrhiza sym-

bioses and how these important mutualisms affect

community organization. Both rhizobia and mycor-

rhizal fungi supply limiting nutrients to their plant

hosts and are especially important in nutrient-poor

ecosystems.

13.3.3.1 Legume–rhizobia symbioses

More than 15 000 species of legumes are involved in

symbioses with rhizobia (de Fariaet al. 1989). Plant

symbioses with nitrogen-fixing soil microorgan-

isms play an important role in organizing commu-

nity structures. Nitrogen-fixing organisms may be

free-living or form intimate associations and fix

atmospheric nitrogen in specialized structures

such as root nodules in many legumes. The le-

gume–rhizobia association involves a formal and

physiologically complex symbiosis. Rhizobia are

Gram-negative heterotrophic bacteria classified

within six genera–Allorhizobium,Azorhizobium,Bra-

dyrhizobium,Mesorhizobium,RhizobiumandSinorhi-

zobium– that interact with plants of the family

Leguminosae, leading to profound physical altera-

tions in both organisms (Pepper 2000).

Although quite an extensive literature exists on

the cross-talk between plants and rhizobia and the

subsequent formation of the symbiotic nodules, lit-

tle information has been gathered as to the fate of the

rhizobia after this point. However, this information

is critical to understanding both the evolutionary

and ecological functioning of the symbiosis. What

we do know is that, following the formation of no-

dules in the plant, the bacterial cells undergo a

MUTUALISMS AND COMMUNITY ORGANIZATION 185