Disturbance, Succession, and Agroecosystem Management - Agroecology: The Ecology of Sustainable Food Systems - page 237

17 Disturbance, Succession, and

Agroecosystem Management

The ecological concepts of disturbance and recovery
through succession have important application in agro-
ecology. Agroecosystems are constantly undergoing
disturbance in the form of cultivation, soil preparation,
sowing, planting, irrigation, fertilizer application, pest
management, pruning, harvesting, and burning. When dis-
turbance is frequent, widespread, and intense — as it is in
conventional agriculture — agroecosystems are limited to
the earliest stages of succession. This condition enables
high productivity but requires large inputs of fertilizer and
pesticides, and tends to degrade the soil resource over
time.
More sustainable food production can be achieved by
moving away from dependency on continual and excessive
disturbance and allowing successional processes to generate
greater agroecosystem stability. Based on our understand-
ing of disturbance and succession in natural ecosystems,
we can enhance the ability of agroecosystems to maintain
both fertility and productivity through appropriate man-
agement of disturbance and recovery.

recovery process that occurs in agriculture usually takes
place in sites that formerly had other biotic components,
we will focus our attention here on the secondary succes-
sion process.

THE NATURE OF DISTURBANCE

Although natural ecosystems give the impression of being
stable and unchanging, they are constantly being altered
on some scale by events such as fire, wind storms, floods,
extremes of temperature, epidemic outbreaks, falling
trees, mudslides, and erosion. These events disturb eco-
systems by killing organisms, destroying and modifying
habitats, and changing abiotic conditions. Any of these
impacts can change the structure of a natural ecosystem
and cause changes in the population levels of the organ-
isms present and the biomass they store.
Disturbance can vary in three dimensions:

• Intensity of disturbance can be measured by the

amount of biomass removed or the number of
individuals killed. The three types of fire
described in Chapter 10 provide good examples
of variation in disturbance intensity: surface
fires usually create low-intensity disturbance,
whereas crown fires cause high-intensity
disturbance.

DISTURBANCE AND RECOVERY IN
NATURAL ECOSYSTEMS

A long-standing tenet of ecology is that following a dis-
turbance, an ecosystem immediately begins a process of
recovery from that disturbance. Recovery takes place
through the relatively orderly process of succession, which
was introduced in Chapter 2. In the broadest sense, eco-
logical succession is the process of ecosystem develop-
ment, whereby distinct changes in community structure
and function occur over time.
Ecologists distinguish two basic types of succession.
Primary succession is ecosystem development on sites
(such as bare rock, glaciated surfaces, or recently formed
volcanic islands) that were not previously occupied by
living organisms or subject to the changes that the biotic
components can bring to bear on the abiotic components.
Secondary succession is ecosystem development on sites
that were previously occupied by living organisms, but
had some or all of those organisms removed by fire, flood-
ing, severe wind, intense grazing, or some other event.
Depending on the intensity, frequency, and duration of the
disturbance, the impact on the structure and function of
the ecosystem will vary, as will the time required for
recovery from the disturbance. Since the disturbance and

• Frequency of disturbance is the average amount

of time between each disturbance event. The
longer the time span between disturbances, the
greater the ability of the ecosystem to fully
recover after each disturbance.

• Scale of disturbance is the spatial scope of the

disturbance, which can vary from a small, local-
ized patch to the entire landscape. The small
gap in the forest canopy created by an individual
tree falling is a small-scale disturbance, whereas
the massive destruction of a powerful hurricane
is very large-scale.

All three characteristics of disturbance are often inter-
twined in complex ways. Fire, for example, may occur
with varying frequency; it may be distributed over the
landscape in a patchy manner; and where it does occur
it may burn some areas very intensely and others hardly
at all.

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