Chapter 13
Mutualisms and community
organization
David Kothamasi, E. Toby Kiers and Marcel G.A. van der
Heijden13.1 Introduction
When the Titanic attempted to manoeuvre past the
infamous iceberg, the captains did not realize that
the majority of the challenge was hidden. Current
knowledge about species interactions in communi-
ty ecology is comparable to the visible tip of a huge
iceberg – the majority of species interactions remain
invisible and unknown. In the past, ecologists have
focused on negative interactions such as competi-
tion, predation and parasitism to explain the orga-
nization of communities because these interactions
could be integrated with Darwin’s theory of natural
selection with relative ease. Mutualisms, positive
interactions between two or more species that sup-
port each other’s fitness, were difficult to reconcile
with the struggle for existence implied by natural
selection. Wilkinson and Sherratt (2001) emphasize
that one reason for the under-representation of
mutualisms in ecology may be a historical absence
of useful models. For example, when testing cooper-
ative interactions, the classic Lotka–Volterra models
produce ‘silly results’ as a result of unconstrained
positive feedbacks. However, even Lotka–Volterra
models have unveiled some fundamental character-
istics of persistent mutualisms, especially in obligate
partnerships (Vandermeer and Boucher 1978).
Mutualistic interactions between species are
ubiquitous in ecosystems and involve organisms
from every kingdom. Habitats ranging from de-
serts, to tropical rainforests to coral reefs are domi-
nated by species that depend on mutualists
(Bronstein 2001). While these interactions have
been described as mathematically unstable, key
evolutionary events such as the origin of the eu-
karyotic cell, invasion of land by plants and the
radiation of the angiosperms are linked to mutual-
ism (Bronstein 2001; Bronsteinet al. 2006; Morris
and Blackwood 2007). Indeed, the complex situa-
tions that an organism is likely to encounter in an
ecosystem will often favour cooperation over com-
petition (Cohen 1998).
Mutualisms can enhance diversity and influence
community organization by mechanisms involving
habitat modifications, acquiring food sources, dis-
persal and protection. These interactions promote
coexistence of competing species through positive
feedbacks on abundances in a manner that is simi-
lar to the negative feedbacks of predation that me-
diate coexistence of competing prey species by
reducing the probability of competitive exclusion.
For instance, mutualistic arbuscular mycorrhizal
fungi increase the competitive ability of plants
that are otherwise inferior competitors (Schmitt
and Holbrook 2003). Disruptions of mutualistic in-
teractions cause dramatic declines in population
sizes of plants and even shifts in plant community
composition (Rieraet al. 2002). Threats to critical
mutualists, for instance pollinators (Memmott
et al. 2007), can potentially endanger the evolution-
ary persistence of the plants that depend on them
and consequently cause significant perturbations in
community function and organization.
We focus here on the role that mutualists play in
plant community organization. We begin by tracing
the evolution of mutualisms from initial conflicts179