Agriculture Reference
In-Depth Information
Usually only one or the other of these interferences
takes place in a particular interaction, but they can occur
together in some interactions, as discussed below. When
conceptualized with this framework, an interaction
between two or more organisms is comprised of an impact
on the environment (an addition or a removal) perpetrated
by one organism (and in some cases an additional impact
created by the other organism), followed by a response on
the part of both organisms to the resulting changes in the
environment. Note that the “environment” is not necessar-
ily external to the interacting organisms — it can include
the tissues or body of either or both organisms. Types of
removal and addition interferences are described in greater
detail below and then summarized in Table 11.2.
earlier chapters in this topic have described the condi-
tions under which resources may become limiting and
thus set the stage for competition.
Viewing competition as removal interference provides
an alternative way of understanding what is commonly
thought of as competition for space. Under this frame-
work, “space” is seen as a complex mixture of resources
that is impacted by the removal effects of the organisms
that occupy that space; thus, organisms are in competition
over the resources of the space, not the space itself.
Competition between individuals of the same species
—
intraspecific competition
— can be quite intense since
the needs of the interacting individuals are so similar.
Monoculture agriculture has invested much energy in
determining how densely crops can be planted without
competition between individual plants negatively affecting
production.
Competition between individuals of different species,
called
interspecific competition
, can also be important
when levels of resources are not sufficient to meet the
needs of both. The mechanisms of the interaction involve
either removal of a resource or its direct protection or
sequestration by an organism (e.g., when an animal
defends a territory and its resources). In either case, the
resource is the primary focus of the interaction.
Competition is a very important concept in ecology,
but it also has a history of controversy and discussion. On
the one hand, interspecific competition is a cornerstone of
evolutionary ecology. Competition is considered the
engine of natural selection and a force, which all organ-
isms must contend within their struggle to survive and
R
EMOVAL
I
NTERFERENCES
When one organism removes something from the environ-
ment as part of its life activities or interaction with other
organisms, it can affect other organisms. This type of
interference is generally negative for one or more mem-
bers of the interaction, but it can have positive effects as
well. There are several types of removal interferences in
agroecosystems.
Competition
Only a shift of emphasis is needed to understand com-
petition as removal interference. Competition occurs
when two organisms are removing a resource from the
environment — light, nitrogen, or water — that is not
abundant enough to meet the needs of both. Many of the
TABLE 11.2
Summary of Interference Interactions
Creator of
Interference (A)
Receiver(s) of
Interference (B)
Type and Identity
of Interference
Location of
Interference
Effect on A
a
Effect on B
a
Competition
Roles
interchangeable
Roles
interchangeable
Removal of
resources
Shared habitat
-
-
Parasitism
Parasite
Host
Removal of
nutrients
Body of host
+
-
Herbivory
Herbivore
Consumee
Removal of
biomass
Body of
consumee;
shared habitat
+
- or +
Epiphytism
Host
Epiphyte
Addition of habitat
surface
Body of host
0
+
Proto-cooperation
Roles
interchangeable
Roles
interchangeable
Addition of
material or
structure
Shared habitat or
body of A/B
+ (0)
+ (0)
Mutualism
Roles
interchangeable
Roles
interchangeable
Addition of
material or
structure
Shared habitat or
body of A/B
+ (-)
+ (-)
Allelopathy
Allelopathic plant
Potential habitat
associates
Addition of active
compound
Habitat of
organism A
+ or 0
+, -, or 0
a
Symbols in parenthesis refer to the effect when the organisms are not interacting.
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