The Agroecosystem Concept - Agroecology: The Ecology of Sustainable Food Systems - page 29

disturbance, they remain dynamic and flexible, resilient
in the face of perturbing forces. Overall stability combined
with dynamic change is often captured in the concept of
dynamic equilibrium . The dynamic equilibrium of eco-
systems is of considerable importance in an agricultural
setting. It permits the establishment of an ecological
“balance,” functioning on the basis of sustained
resource use, which can be maintained indefinitely
despite ongoing and regular change in the form of
harvest, soil cultivation, and replanting.

open systems where considerable energy is
directed out of the system at the time of each
harvest, rather than stored in biomass, which
could otherwise accumulate within the system.
Nutrient Cycling: Recycling of nutrients is minimal in
most agroecosystems and considerable quanti-
ties are lost from the system with the harvest or
as a result of leaching or erosion due to a great
reduction in permanent biomass levels held
within the system. The frequent exposure of bare
soil between crop plants and, temporally,
between cropping seasons, also creates “leaks”
of nutrients from the system. Farmers have
recently come to rely heavily upon petroleum-
based nutrient inputs to replace these losses.
Population Regulating Mechanisms: Due to the simpli-
fication of the environment and a reduction in
trophic interactions, populations of crop plants
or animals in agroecosystems are rarely self-
reproducing or self-regulating. Human inputs
in the form of seed or control agents, often
dependent on large energy subsidies, determine
population sizes. Biological diversity is
reduced, trophic structures tend to become
simplified, and many niches are left unoccu-
pied. The danger of catastrophic pest or disease
outbreak is high, despite the intensive human
interference.
Stability: Due to their reduced structural and functional
diversity in relation to natural ecosystems, agro-
ecosystems have much less resilience than
natural ecosystems. A focus on harvest outputs
upsets any equilibrium that is established, and
the system can only be sustained if outside inter-
ference — in the form of human labor and
external human inputs — is maintained.

AGROECOSYSTEMS

Human manipulation and alteration of ecosystems for the
purpose of establishing agricultural production makes
agroecosystems very different from natural ecosystems.
At the same time, however, the processes, structures, and
characteristics of natural ecosystems can be observed in
agroecosystems.

N ATURAL E COSYSTEMS AND A GROECOSYSTEMS
C OMPARED

A natural ecosystem and an agroecosystem are dia-
grammed, respectively, in Figure 2.6 and Figure 2.7.
In both figures, flows of energy are shown as solid
lines and movement of nutrients is shown with
dashed lines.
A comparison of Figure 2.6 and Figure 2.7 reveals
that agroecosystems differ from natural ecosystems in
several key respects.

Energy Flow: Energy flow in agroecosystems is altered
greatly by human interference. Inputs are derived
from primarily human sources and are often not
self-sustaining. Thus agroecosystems become

Atmosphere
and Rain

Nutrients
Energy

SUN

Herbivorous
Consumers

Carnivorous
Consumers

Primary Producers

Decomposers

Soil

Loss

Loss

FIGURE 2.6 Functional components of a natural ecosystem. The components labeled “Atmosphere and Rain” and “Sun” are
outside any specific system and provide essential natural inputs.