Biotic Factors - Agroecology: The Ecology of Sustainable Food Systems - page 147

11 Biotic Factors

Chapter 4 through Chapter 10 have focused on how
individual plants are impacted by abiotic factors of the
environment, such as light, temperature, and mineral
nutrients. In this chapter, we will complete the picture
of how the environment impacts plants by exploring how
biotic factors of the environment — that is, conditions
created and modified by living organisms — affect indi-
vidual plants.
In agroecosystems, the farmer is, in a sense, the organ-
ism with the greatest impact on the environment in which
crops are grown. The farmer alters and adjusts conditions
of the physical as well as the biological environment to
meet the needs of the crop or crops. To do so sustainably,
the farmer must have an understanding of the biotic inter-
actions of the agroecosystem — how each member of the
community impacts the agricultural environment and
alters conditions for its neighbors.
To conceptualize biotic factors in ecological terms, we
must enter an area of overlap between autecology
and synecology. Even though we begin from the perspec-
tive of the individual organism confronting an environ-
ment made up of various factors, we must deal with
interactions between organisms when the factors we
are concerned with are biotic. Despite their synecological
origin, however, the concepts developed in this chapter
to describe these interactions can be applied in an
autecological way by considering interactions in
terms of their impact on each individual organism in the
agroecosystem.
There are two basic frameworks for conceptualizing
the interactions between organisms in a community or
ecosystem; each has its respective advantages. Tradition-
ally in ecology, interactions have been understood in terms
of the effects that two interacting organisms have on each
other. This framework is the basis for such foundational
concepts as competition and mutualism. In agroecology,
however, it is often more helpful to view interactions as
deriving from the impact that organisms have on their
shared environment. Organisms remove substances from,
alter, and even add substances to the areas they occupy,
in the process, changing the environmental conditions for
themselves and other organisms. Thus each biotic factor
that an individual organism faces can be understood as a
modification of the environment created by another organ-
ism. Both of these frameworks, or perspectives, are
explained in more detail below.

THE ORGANISM-ORGANISM PERSPECTIVE

A broadly accepted system for classifying interactions
between organisms was developed by Odum (1971). This
system has many useful applications and has served ecol-
ogists well in understanding the biotic environment. Inter-
actions between two organisms of different species are
seen as having a negative effect (-), a positive effect (+),
or a neutral effect (0) for each member in the interaction.
For example, in the interaction classified as mutualism,
both organisms are impacted positively (+ +). The degree
to which the interaction is positive or negative for each
organism depends on the level of interdependence and the
level of intensity of the interaction.
In this scheme, there is an important distinction
between situations in which both members of the mixture
are present together and the interaction is actually taking
place, and situations in which the two are separate, or
together and not interacting. In Table 11.1, the “not inter-
acting” column shows the results in this latter situation
and gives an indication of the degree of dependence or
need for interaction that each member may have developed
over evolutionary time.
The interaction that has probably received the greatest
attention, especially in the design of conventional agroec-
osystems, is competition (- -). Competition occurs in an
environment where resources are in limited supply for
both members of the relationship, and even though one
member of the mixture may end up dominating the other,
both do worse when they are interacting in this way than
if there had been no interaction at all. The organisms
interact by removing something from the environment that
they both need. Two crop varieties of the same species are
highly likely to compete in a resource-limited environment
— for example, a crop field with low nitrogen levels in
the soil.
When two organisms have become so dependent on
each other that they suffer when not in interaction, then
it can be said that the interaction is a mutualism (+ +).
Both organisms depend upon the way in which the other
modifies the environment for both. Some interactions
between legumes and Rhizobium bacteria, for example,
are thought to be mutualistic: neither organism does as
well alone as they do together.
When an interaction benefits both members, but nei-
ther is negatively impacted in the absence of interaction,

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