Species Interactions in Crop Communities - Agroecology: The Ecology of Sustainable Food Systems - page 207

It is possible, too, that many domesticated species
have undergone directed selection for coexistence by
being grown most commonly in polycultures for many
thousands of years. In this context, the plants would have
coevolved, each developing adaptations for coexistence.
(The traditional corn-bean-squash polyculture discussed
later in this chapter is a possible example.)
Mixed populations are able to coexist due to many
different mechanisms, such as resource partitioning, niche
diversification, or specific physiological, behavioral, or
genetic changes that reduce direct competition and allow
for its avoidance. Understanding the mechanisms of inter-
ference that make coexistence possible could form an
important foundation for the design of multiple crop com-
munities.
In agroecosystems, combining species with slightly dif-
ferent physiological characteristics or resource needs is an
important way of allowing for the coexistence of species in
a multiple cropping community. Such an approach to
designing the cropping community has much greater poten-
tial than trying to maintain single-species dominance in a
monocultural field where considerable human intervention
is needed to keep out potentially competing noncrop weeds
or herbivorous pest insects. Successful mixed crop commu-
nities around the world offer fruitful ground for research on
how avoidance of competition, or coexistence, plays an
important ecological role in cropping systems.

fixing bacteria in this relationship cannot func-
tion outside of the nodules formed on the plant
roots. This mutualism is the cornerstone of
many of the most sustainable farming systems
around the world.

• Exhabitational mutualisms: The organisms

involved are relatively independent physically,
but interact directly. An example is the relation-
ship between a flowering plant and its pollinat-
ing insect. Many crop plants are unable to
produce fertile seed without pollination from
bees, and the bees depend on the crop plants
for their main source of food in the form of
nectar or pollen.

• Indirect mutualisms: The interactions among a

set of species modify the environment in which
they all live to the benefit of the mixture. An
example is a polyculture agroecosystem. A tall
crop species can modify conditions of the
microclimate to the benefit of associated crop
species, and the presence of several crops
attracts a range of beneficial arthropods that
facilitate the biological management of poten-
tial pests. Unlike the first two types of mutual-
isms, indirect mutualisms involve more than
two species. Indirect mutualisms can also
include both inhabitational and exhabitational
mutualisms.

MUTUALISMS

Some mutualisms are obligate for all involved mem-
bers, while in others, only one of the members may require
the relationship. In other cases, called facultative mutual-
isms, all members of the mutualism may be able to survive
quite well without the interaction, but definitely do better
when in relationship. Often, the mutualism functions not
so much because of some stimulus or direct benefit to the
organisms involved, but because it helps the species avoid
some negative impact or impacts.
The expansion of the theory of mutualism in ecology
has begun to find ready application in the development of
more diverse cropping communities in which mutualistic
relationships can occur. Making these relationships an
integral part of crop communities is key to establishing
sustainable systems that require fewer external inputs and
less human intervention.
By contributing beneficial interactions, mutualisms in
agroecosystems increase the resistance of the entire sys-
tem to the negative impacts of pests, diseases, and weeds.
At the same time, the efficiency of energy capture, nutrient
uptake, and recycling in the system may be improved.
Whenever mutualistic relationships can be incorporated
into the organization of the cropping community, sustain-
ability is much easier to achieve and maintain.

Species with a mutualistic relationship are not only able
to coexist, but are dependent on each other for optimal
development. Mutualisms are likely the result of coexist-
ing species continuing in the same evolutionary direction,
coevolving adaptations for achieving mutual benefit
through some kind of close association. Ecologists now
know that mutualistic relationships among organisms of
different species are relatively common in complex natural
communities, creating intricate interdependencies among
community members. Their prevalence is another factor
explaining the observed complexity and diversity of
many communities and their food webs. The same coevo-
lutionary process has undoubtedly also occurred during
domestication in agriculture, either by deliberate human
selection or coincidentally in the context of multiple cropping
systems.
The types of mutualisms that are most commonly
recognized include the following:

• Inhabitational mutualisms : One mutualist lives

wholly or partly inside the other. A classic
example is the relationship between Rhizobium
bacteria and leguminous plants. The nitrogen-