Species Interactions in Crop Communities - Agroecology: The Ecology of Sustainable Food Systems - page 205

15 Species Interactions in Crop

Communities

In ecological terms, a cropping system is a community
formed by a complex of interacting populations of crops,
weeds, microorganisms, insects, and, sometimes, other ani-
mals. The interactions among the populations of the crop
community, which arise from the different kinds of inter-
ference, give the community characteristics, called emer-
gent qualities, which exist only at the community level.
These emergent qualities cannot be fully explained in terms
of the properties of populations or individuals. In both
natural ecosystems and agroecosystems, community-level
phenomena are of critical importance in a system's stabil-
ity, productivity, and dynamic functioning.
Agricultural researchers, however, normally focus
their attention on the crop population of central impor-
tance in the farming system, rather than on the community
of which it is a part. Because of this reductionist approach,
they fail to understand cropping systems as communities,
and thereby lose the ability to take advantage of community-
level emergent qualities or to manipulate community
interactions to the benefit of the cropping system.
To be sure, conventional agriculture has been greatly
concerned with species interactions — in the sense that it
has focused on the detrimental effects on crop yields,
arising from the impacts of noncrop organisms such as
weeds, pest herbivores, and diseases on the crop environ-
ment. Research for many years has been directed toward
eliminating these detrimental effects. Noncrop organisms
are said to “compete” with the crop or have a yield-
reducing effect; they must therefore be eliminated from
the cropping system. At the same time, considerable
research has been done to determine the optimum densities
for each crop (usually planted as a monoculture) in order
to minimize intraspecific competition for resources and
thereby obtain maximum yields.
By striving to eliminate and minimize interactions, the
conventional approach tends to simplify the crop commu-
nity. In a sense, the ultimate goal is to reduce it to a single-
crop population growing in an otherwise sterile abiotic
environment.
In contrast, the agroecological approach to cropping
system management is to understand species interactions
in the context of the larger community. The agroecologist
recognizes the existence of beneficial species interactions,
understands how they arise from the impacts of interfer-
ence, and knows that a certain level of complexity is
desirable. By paying attention to the ecology of the crop

community, it is possible to create beneficial interactions
and emergent qualities that not only reduce the need for
external inputs, but also increase overall yields.

INTERFERENCE AT THE COMMUNITY LEVEL

The basis for understanding species interactions in the
context of community structure and function was
developed in Chapter 11. There, we discussed how
organism-organism interactions can be conceptualized as
interferences , in which an organism has some kind of
impact on its environment, and through this impact,
another organism is affected. We identified two types of
interferences: removal interferences, in which the environ-
mental impact consists of the removal of some resource
by one or both of the interacting organisms, and addition
interferences, in which one or both organisms adds some
substance or structure to the environment. Either kind of
interference can have beneficial, detrimental, or neutral
effects on neighboring organisms. As was discussed in
Chapter 11, the advantage of the interference approach is
that it allows a more complete understanding of the
mechanisms of interaction.
At the level of the community, the existence of many
populations means that many kinds of interferences may
be going on at the same time. These many interferences
may interact with and modify each other, creating complex
relationships among the members of the community.
Despite this complexity, we can understand both the indi-
vidual types of interference that exist between populations
and the overall effect of the complex of interferences on
the community as a whole because the interference con-
cept allows analysis of the mechanisms of interaction.
Some of the ways in which interferences may combine
to affect the crop community are described in Figure 15.1.
Direct removal of something from the environment leads
to interactions such as competition or herbivory, whereas
additions can lead to allelopathy or the production of food
for beneficial organisms in the crop community. Both
removal and addition interferences may go on simulta-
neously, leading to different types of interactions. Many
mutualisms, for example, arise from combined addi-
tion/removal interferences. Examples are pollination
(removal of nectar and addition of pollen) and biological
nitrogen fixation (addition of fixed nitrogen by the bacteria
and removal of nitrogen by the legume). Additionally,

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