Community Ecology Processes, Models, and Applications

leading to decreased fitness (Denison 2000; Westet al.
2002a, b; Kierset al. 2003). Moreover, the exchange of
luxury resources may stabilize many mutualistic as-
sociations. For instance, in mycorrhizal associations
plants may trade surplus carbon for excess fungal
nutrients, such as phosphorus or nitrogen (Kiers
and van der Heijden 2006).
The predicament faced by any pair of species
engaged in new mutualisms is how to initiate coop-
eration from a previously non-cooperative state
and then maintain a stable interaction. This is the
classic prisoner’s dilemma problem: although indi-
viduals can benefit from mutual cooperation, each
one can do even better by exploiting the coopera-
tive efforts of others (Axelrod and Hamilton
1981). The payoff to a mutualist is in terms of the
effect on its fitness. Regardless of what the other
partner does, the selfish choice of cheating yields

a higher payoff than cooperation, if there are no

punishment mechanisms in place. However, if

both the partners cheat, costs for both partners

are more than if both had cooperated (Axelrod

and Hamilton 1981). If the interaction is iterated

repeatedly, then the best strategy can be to cooper-

ate with other individuals who cooperate too

(Nowak and Sigmund 1992). The investment deci-

sion of a mutualist depends on the payoff received

in the previous iteration. Biologically, this means

that healthy organisms have more to offer their

partners and the amount invested evolves as muta-

tions periodically arise (Doebeli and Knowlton

1998). Successful mutualists should be better com-

petitors (if they are rewarded for their mutualistic

behaviour), and eventually establish in the commu-

nity as the now combined abilities of the partners in

acquiring resources will reduce competition from

Figure 13.2Butea monosperma, an important legume from a deciduous forest ecosystem of India, is a keystone
mutualist forming mutualistic associations with some animal species for pollinator services.Butea monospermaalso
forms belowground mutualisms with rhizobia and arbuscular mycorrhizal fungi. Inset, a rose ringed parakeet (Psittacula
longicauda) foraging on the flowers ofButea monosperma; the parakeet is a nectar robber and does not provide any
reciprocal benefit to the plant. See plate 4.

182 FUTURE DIRECTIONS