Biotic Factors - Agroecology: The Ecology of Sustainable Food Systems - page 154

exuded from roots, or released from shed plant material
during decomposition. Even flowers, fruits, and seeds can
be sources of allelopathic toxins. There are also cases in
which products do not become toxic until they have been
altered once they are in the environment, either by normal
chemical degradation or by conversion to toxic com-
pounds by microorganisms.
In natural ecosystems, allelopathy may help explain
some important phenomena:

TABLE 11.3
Types of Interference

Direct (occurs in or on
the body of one or
both organisms)

Indirect (occurs in
the shared habitat
of the organisms)

Symmetrical (both
organisms create
interference)

Protocooperation Competition

Mutualism Protocooperation

Mutualism

Asymmetrical
(interference
created by one
organism)

Herbivory Allelopathy

• the dominance of a single species or group of

species over others;

Parasitism
Epiphytism

• successional change and species replacement,

or the maintenance of a deflected stage in the
successional process;

• reduced ecosystem productivity; and much less show how they may interact in ways that deter-

mine which species and how many individuals of each
are able to coexist in a specific habitat. Ultimately, the
combination of interference types is going to play an
important role in determining the structure and function of
the ecosystem.
It is easy to imagine how allelopathy and competition,
for example, can both play a part in a polyculture cropping
system. The members of the mixture are simultaneously
adding materials to and removing resources from the envi-
ronment, modifying the microclimatic conditions of that
environment at the same time, and interacting with each
other in ways that permit coexistence or favor mutualistic
interdependence. It is important, though, to understand the
mechanisms of each interaction, beginning with the
impacts of each species on the environment in which they
all occur. The ability of farmers to successfully manage
complex crop mixtures and rotations depends on the deve-
lopment of this understanding.

• unique patterning or distribution of plant spe-

cies in the environment.

In agroecosystems, allelopathy may play important
roles in biological control, the design of intercropping
systems, and crop rotation management. Examples are
presented below and in more detail in later chapters.

C OMPARISON OF T YPES OF I NTERFERENCE

Table 11.2 provides a brief summary of the most salient
characteristics of each type of interference. Study of this
table may reveal that the grouping of interferences into
addition interferences and removal interferences does
not exhaust the ways in which interferences can be
classified. Mutualism, for example, shares with compe-
tition the property of involving symmetrical roles; that
is, the organism creating the interference is simulta-
neously the organism receiving the interference created
by the other interacting organism (note that this sym-
metry does not necessarily extend to the results of the
interaction). As another example, parasitism and epi-
phytism both involve interferences that act directly on
one organism's body rather than on the external, phys-
ical environment. These observations suggest that inter-
ferences may be grouped as either direct or indirect, and
as either symmetrical or asymmetrical. Allelopathy, for
example, is asymmetrical and indirect. Table 11.3 shows
the typology resulting from such a classification. Most
forms of interference occupy only one cell in the matrix,
but protocooperation and mutualism can be either direct
or indirect.

ALLELOPATHIC MODIFICATION OF THE
ENVIRONMENT

Ecological research has placed the greatest emphasis on
competitive interactions. This has been especially true
in agronomy, where great efforts have been made to
understand what the conditions of the environment are
that limit optimal crop development, and what kinds of
inputs or technologies are needed to correct the situation
when something that the crop needs is missing or is in
short supply. Crop arrangements and densities have
been researched and developed to avoid the effects of
competition.
More recently, however, the role of allelopathy in
agroecosystems has begun to receive considerable
attention. The growing desire to replace synthetic chem-
ical inputs to agroecosystems with naturally produced
materials has spurred a burst in applied research on
allelopathy, especially in Europe and India (Kohli et al.,

I NTERFERENCES AT W ORK IN A GROECOSYSTEMS

In most multiple-species interactions, plants are removing
and adding things to the environment simultaneously. It is
very difficult to separate removal and addition interactions,