credibly complex physiological mechanisms that
allow mutualisms to function. In reality, the deci-
sion to cooperate, and with whom to cooperate,
needs to be understood in the context of their
unique physiological constraints (Kiers and van
der Heijden 2006).
To gain a broad perspective of mutualisms at the
community level, it is important to understand how
biotic and abiotic factors influence transitions from
parasitism to mutualism (Johnsonet al. 1997; Saikko-
nenet al. 1998; Pellmyr and Leebens-Mack 1999).
Biotic factors, such as intraspecific competition, num-
ber of partners available or other alternatives avail-
able to a partner (Yamamura 1993; Lipsitchet al. 1995;
Herreet al. 1999) may determine the value conferred
by a given mutualistic strategy. If there are viable
alternatives, cooperationmay be destabilized. Abiotic
factors will play a role as well. For example, in the
acacia–ant mutualism, nutrient-rich habitat (created
by termites) plays a significant role for the coexistence
of different mutualist guilds (Palmer 2003). Such mu-
tualist guilds may function along a continuum of
cooperation and parasitism (Stantonet al. 1999).
Switching from parasitic to mutualistic lifestyles
may occur in interactions between plants and fungal
endophytes. The fungusColletotrichum magnacauses
anthracnose in cucurbit plants, but mutant isolates of
this fungus can exhibit mutualistic effects (Kogelet al.
2006). The recognition molecules involved in both
mutualistic and parasitic interactions appear to be
similar. However, it is not clear to what extent friend-
ly recognition overbalances unfriendly recognition.
The mutual interactions between fungal endophytes
and plants are an outcome of balance under environ-
mental, physiological and genetic control that results
in fitness benefits for both partners (Kogelet al. 2006).
Mutualisms are often characterized by an appar-
ent asymmetry (one partner extracts significantly
more benefits than the other) in benefits received
by the two sides of the interaction (Kawakita and
Kato 2004). For instance, many plant species are
pollinator generalists that recruit the services of
diverse potential pollinator species. Several pollina-
tors are also generalists that may visit many plants.
When the asymmetry becomes exacerbated, a
previously mutualistic interaction can become par-
asitic, and some interactions have been shown to
slide back and forth between the two (Hoeksema
and Bruna 2000). Plant–mycorrhiza associations
that are generally at the mutualistic end of the con-
tinuum can become parasitic depending on ecologi-
cal conditions, suggesting that mycorrhizas may
have begun as parasites dependent on host plant
carbon, and subsequently evolved into mutualists
by offsetting carbon costs to the plants by providing
net gains through uptake of other nutrients (John-
sonet al. 1997). Trade models have identified three
factors important in determining whether a poten-
tial partner becomes parasitic or remains mutualis-
tic under asymmetrical conditions: (1) relative
differences between the partners in their resource
acquisition abilities; (2) relative differences between
the partners in their resource requirements; and (3)
variation in the shape of resource acquisition trade-
offs (Hoeksema and Schwartz 2003). If the environ-
mental conditions change, such as an increase in the
productivity of the environment, interactions are
predicted to become less beneficial (Kierset al.
2002; Thrallet al. 2007).
There is much interest in the identity of the factors
that align the interests of cooperating partners so that
the relationship remains mutually beneficial and
evolutionarily stable (Herreet al. 1999; Westet al.
2002a; Sachset al. 2004; Moran 2007). Mode of trans-
mission may be important in aligning partner inter-
ests. Mutualisms established by vertical transmission
of symbionts from parent to offspring will tend to
favour cooperation. Conversely, high horizontal
transmission among different individuals of the
host and competition with other endosymbionts
within a host may push towards parasitism (Herre
et al. 1999), because the symbiosis has to be formed
anew each time the symbionts meet, providing
opportunities for parasites (or less beneficial sym-
bionts) to invade the system. Wilkinson and Sherrat
(2001), however, argue that, depending on the con-
ditions, horizontal transmission can also lead to suc-
cessful mutualisms. Other criteria for aligned interest
include (1) genotypic uniformity of symbionts within
individual hosts, (2) spatial structure of populations
(but see Westet al. 2001) leading to repeated interac-
tions and (3) restricted options outside the host
(Herreet al.1999;Sachset al.2004).Additionally,
interests may be aligned through control mechan-
isms, such as host-mediated punishment, in which
those that fail to cooperate suffer from sanctions,MUTUALISMS AND COMMUNITY ORGANIZATION 181