The Agroecosystem Concept - Agroecology: The Ecology of Sustainable Food Systems - page 23

2 The Agroecosystem Concept

An agroecosystem is a site or integrated region of agricul-
tural production — a farm, for example — understood as
an ecosystem. The agroecosystem concept provides a
framework with which to analyze food production systems
as wholes, including their complex sets of inputs and
outputs and the interconnections of their component parts.
Because the concept of the agroecosystem is based on
ecological principles and our understanding of natural
ecosystems, the first topic of discussion in this chapter is
the ecosystem. We examine the structural aspects of eco-
systems — their parts and the relationships among the
parts — and then turn to their functional aspects — how
ecosystems work. Agroecosystems are then described in
terms of how they compare, structurally and functionally,
with natural ecosystems.
The principles and terms presented in this chapter will
be applicable to our discussion of agroecosystems
throughout this topic.

environment determine where it will live. The adaptations
of the banana plant, for example, restrict it to humid,
tropical environments with a particular set of conditions,
whereas a strawberry plant is adapted to a much more
temperate environment.
At the next level of organization is groups of individ-
uals of the same species. Such a group is known as a
population . The study of populations is called population
ecology. An understanding of population ecology
becomes important in determining the factors that control
population size and growth, especially in relation to the
capacity of the environment to support a particular popu-
lation over time. Agronomists have applied the principles
of population ecology in the experimentation that has led
to the highest-yielding density and arrangement of indi-
vidual crop species.
Populations of different species always occur together
in mixtures, creating the next level of organization, the
community . A community is an assemblage of various
species living together in a particular place and interacting
with each other. An important aspect of this level is how
the interactions of organisms affect the distribution and
abundance of the different species that make up a partic-
ular community. Competition between plants in a cropping
system or the predation of aphids by lady beetles are
examples of interactions at this level in an agroecosystem.
The study of the community level of organization is known
as community ecology.
The most inclusive level of organization of an ecosys-
tem is the ecosystem itself, which includes all of the
abiotic factors of the environment in addition to the com-
munities of organisms that occur in a specific area. An
intricate web of interactions goes on within the structure
of the ecosystem.
These four levels can be directly applied to agroecosys-
tems, as shown in Figure 2.1. Throughout this text, reference
will be made to these levels: individual crop plants (the organ-
ism level), populations of crop species or other organisms,
farm field communities, and whole agroecosystems.
An important characteristic of ecosystems is that at
each level of organization properties emerge that were not
present at the level below. These emergent properties are
the result of the interaction of the component “parts” of
that level of ecosystem organization. A population, for
example, is much more than a collection of individuals of
the same species, and has characteristics that cannot be

STRUCTURE OF NATURAL ECOSYSTEMS

An ecosystem can be defined as a functional system of
complementary relations between living organisms and
their environment, delimited by arbitrarily chosen bound-
aries, which in space and time appear to maintain a steady
yet dynamic equilibrium. An ecosystem thus has physical
parts with particular relationships — the structure of the
system — that together take part in dynamic processes —
the function of the system.
The most basic structural components of ecosystems
are biotic factors , living organisms that interact in the
environment, and abiotic factors , nonliving physical and
chemical components of the environment such as soil,
light, moisture, and temperature.

L EVELS OF O RGANIZATION

Ecosystems can be examined in terms of a hierarchy of
organization of their component parts, just as the human
body can be examined at the level of molecules, cells,
tissues, organs, or organ systems. At the simplest level is
the individual organism. Study of this level of organization
is called autecology or physiological ecology. It is con-
cerned with how a single individual of a species performs
in response to the factors of the environment and how the
organism's particular degree of tolerance to stresses in the

23