The Agroecosystem Concept - Agroecology: The Ecology of Sustainable Food Systems - page 30

SUN

Human Inputs

Atmosphere
and Rain

Animals and
Animal Products

Crop plants

Loss
Loss

Consumption
and Markets

Soil

Nutrients
Energy

Decomposers

Loss

Loss

FIGURE 2.7 Functional components of an agroecosystem. In addition to the natural inputs provided by the atmosphere and the
sun, an agroecosystem has a whole set of human inputs that come from outside the system. An agroecosystem also has a set of
outputs, labeled here as “Consumption and Markets.”

characteristics as species diversity, nutrient cycling, and
habitat heterogeneity.

TABLE 2.2
Important Structural and Functional Differences
Between Natural Ecosystems and Agroecosystems
Natural Ecosystems

T HE A GROECOSYSTEM AS A U NIT OF A NALYSIS

Agroecosystems

We have so far described agroecosystems conceptually; it
remains to explain what they are physically. In other
words, what is the thing we are talking about when we
discuss the management of an agroecosystem? This is first
of all an issue of spatial boundaries. The spatial limits of
an agroecosystem in the abstract, like those of an ecosys-
tem, are somewhat arbitrary. In practice, however, an
“agroecosystem” is generally equivalent to an individual
farm, although it could just as easily be a single farm field
or a grouping of adjacent farms.
Another issue involves the relationship between an
abstract or concrete agroecosystem and its relationship
and connection to the surrounding social and natural
worlds. By its very nature, an agroecosystem is enmeshed
in both. A web of connections spreads out from every
agroecosystem into human society and natural ecosys-
tems. Coffee drinkers in Seattle are connected to coffee-
producing agroecosystems in Costa Rica; the Siberian
taiga may experience impacts from conventional corn
production systems in the U.S.
In practical terms, however, we must distinguish
between what is external to an agroecosystem and what
is internal. This distinction becomes necessary when
analyzing agroecosystem inputs, since something cannot
be an input unless it comes from outside the system. The

Net productivity Medium High

Trophic interactions Complex Simple, linear

Species diversity High Low

Genetic diversity High Low

Nutrient cycles Closed Open

Stability (resilience) High Low

Human control Independent Dependent

Temporal permanence Long Short

Habitat heterogeneity Complex Simple

Source: Odum, E. P. 1969. Science 164: 262-270.

The key ecological differences between natural ecosys-
tems and agroecosystems are summarized in Table 2.2.
Although sharp contrasts have been drawn between
natural ecosystems and agroecosystems, actual systems
of both types exist on a continuum. On one side of the
continuum, few 'natural' ecosystems are truly natural in
the sense of being completely independent of human
influence; on the other side, agroecosystems can vary
greatly in their need for human interference and inputs.
Indeed, through application of the concepts presented in
this text, agroecosystems can be designed that come
close to resembling natural ecosystems in terms of such