individuals can be interconnected to these hyphal
networks. Such networks not only connect indivi-
duals of the same plant species, but, owing to a lack
of specificity in many mycorrhizal associations, also
individuals of different plant species. Subsequently
these networks can act as symbiotic support sys-
tems for seedling establishment of different plant
species (van der Heijden 2004; Horton and van der
Heijden 2008). Some studies even suggest that car-
bon and nutrients can flow from one plant to anoth-
er through hyphal networks (Francis and Read
1994; Simardet al. 1997; Selosseet al. 2006; Whitfield
2007). However, it is still controversial whether
significant amounts of carbon and nutrients can be
transferred and some authors view carbon transfer
as essentially a fungal phenomenon wherein the
carbon stays inside the mycelium. For instance,
Fitteret al. (1998) and Pfefferet al. (2004) observed
that carbon transfer to a recipient plant through AM
fungal hyphae remains largely or entirely within
the mycorrhizal roots, even under conditions that
facilitate root to shoot migration. However, some
plants do obtain carbon from hyphal networks.
There are about 400 species of myco-heterotrophic
plants that are completely dependent on carbon
and nutrients that they receive from mycorrhizal
fungi that they parasitize (Bidartondoet al. 2002;
Selosseet al. 2006). Myco-heterotrophic plants lack
chlorophyll (Bjo ̈rkman 1960; Bidartondoet al.2002).
Myco-heterotrophic plant lineages have evolved in
11 families of plants in five orders (Leake 1994),
suggesting that there is a widespread potential for
carbon flow from mycorrhizal fungi to their ‘host’
plants. Furthermore, several other green plants
have a mixed strategy and are thought to acquire
carbon through photosynthesis and via fungal links
(Tedershooet al. 2007; but see Zimmeret al. 2007).
SomePyrolaspecies may obtain up to 50% of carbon
from fungal hyphae (Tedershooet al. 2007). More-
over, mycorrhizal fungi provide germinating
orchid seeds with carbon and nutrients (Cameron
et al. 2006).
About 250 different AM fungal species have been
described (Mortonet al. 1994). The actual number
of AM fungal species is probably much higher
because molecular techniques have shown that
about 60% of environmental sequences of AM
fungi do not match with AM fungi that have been
brought into culture (van der Heijdenet al. 2008).
The identity of AM fungi present in a plant com-
munity is important because plant species respond
differently to different AM fungi (van der Heijden
et al. 1998a; Maherali and Klironomos 2007) and the
diversity of AM fungal communities can affect
plant productivity, plant community composition
and plant diversity (van der Heijdenet al. 1998b,
2006b; Vogelsanget al. 2006). An intriguing ques-
tion in mycorrhizal ecology is how uncultured AM
fungi contribute to plant diversity and productivity
in natural communities. The fact that specialist
fungi are most affected by soil perturbations (Hel-
gason et al. 2007) might even suggest that our
knowledge of how AM fungi affect plant commu-
nities is far from complete, especially because this
knowledge is based on studies of easily culturable
generalist AM fungi. It is also important to mention
that AM fungal communities in the soil are
completely different from those in plant roots
(Hempelet al. 2007), opening up many new ques-
tions of how members of AM fungal communities
interact and support plant growth.13.4 Conclusions
In nature, organisms from every kingdom are
involved in some kind of interspecific mutualism
and the mechanisms through which mutualisms
influence community organization are likely to be
similar across different organismal groups from
terrestrial to marine habitats. We focused on mutu-
alisms involving plants, as they occupy a pivotal
position in community organization owing to their
role as primary energy producers. Mutualisms pro-
vide partner species with novel options for adjust-
ing to changing environment and biotic stresses.
They may play a critical role in regulating organi-
zation, structure and function of communities
through activities that regulate the acquisition of
resources and ameliorate stresses. Disruption of
established mutualisms by habitat fragmentation
or through biological invasions has caused cascad-
ing shifts in community composition. Mutualisms
may mediate the outcome of interspecificMUTUALISMS AND COMMUNITY ORGANIZATION 191