Disturbance, Succession, and Agroecosystem Management - Agroecology: The Ecology of Sustainable Food Systems - page 240

TA B L E 1 7 . 2
Some Examples of Intermediate Disturbance
in Natural Ecosystems

TABLE 17.3
Desirable Ecological Characteristics of
Agroecosystems in Relation to Successional
Development

Frequency Scale Intensity Nature of disturbance

High Small low Natural wind-felling of

trees in forests

Successional Stage of
Greatest Development

Low Large high Hurricane damage to coral

reef or coastal tropical forest

Benefit to
Agroecosystem Characteristic Early Middle Late

High Medium low Removal of above-ground

biomass by grazing
herbivores in grasslands

High species
diversity

Reduced risk of
catastrophic
crop loss

Medium Medium medium Ice and sleet damage to trees

in temperate forests

High total
biomass

Larger source of soil
organic matter

Medium Medium low Surface fires in dry-summer

tropical forests

High net
primary
productivity

Greater potential for
production of
harvestable biomass

Complexity
of species
interactions

Greater potential for
biological control

agroecosystem at the early stages of succession, where a
greater proportion of gross productivity is available as net
productivity or harvestable biomass. But in order to
develop more stable systems that are much less dependent
on human interventions and polluting, non-renewable
inputs, we must do much more to take advantage of natural
ecosystem recovery processes. Our knowledge of the suc-
cessional process in natural ecosystems can be used both
to aid agroecosystems in their recovery from the impacts
of human-induced disturbance and to introduce distur-
bances in a planned manner.
Simply stated, the task is to design agroecosystems
that on the one hand take advantage of some of the ben-
eficial attributes of the early stages of succession, yet on
the other hand incorporate some of the advantages gained
by allowing the system to reach the later stages of succes-
sion. As shown in Table 17.3, only one desirable ecological
characteristic of agroecosystems — high net primary pro-
ductivity — occurs in the early stages of successional
development; all the others do not become manifest until
the later stages of development.
The challenge for research, then, is to develop ways of
integrating disturbance and development so as to take best
advantage of both extremes. This involves learning how to
use successional processes for installing and developing an
agroecosystem, as well as for re-introducing disturbance and
recovery at appropriate times in the life of the system.

Efficient
nutrient
cycling

Diminished need
for external
nutrient inputs

Mutualistic
interference

Greater stability;
diminished need for
external inputs

takes its toll on other developmental processes and makes
stability impossible.
Another approach to agroecosystem management is
to “mimic nature” by installing a farming system that uses
as a model the successional processes that go on naturally
in that location (Soule and Piper, 1992; Ewel, 1999; Jackson,
2002). Through such an approach — sometimes called the
“analog model” or “natural systems agriculture” — we
can establish agroecosystems that are both stable and
productive.
Under a scheme of managed succession, natural suc-
cessional stages are mimicked by intentionally introducing
plants, animals, practices, and inputs that promote the
development of interactions and connections between
component parts of the agroecosystem. Plant species (both
crop and non-crop) are planted that capture and retain nutri-
ents in the system and promote good soil development.
These plants include legumes, with their nitrogen-fixing
bacteria, and plants with phosphorus-trapping mycor-
rhizae. As the system develops, increasing diversity, food
web complexity, and level of mutualistic interactions all
lead to more effective feedback mechanisms for pest and
disease management. The emphasis during the develop-
ment process is on building a complex and integrated
agroecosystem.
Such a strategy may require more intensive human
management, but because processes and interactions are
internalized within the agroecosystem, it should lead to

A LLOWING S UCCESSIONAL D EVELOPMENT

Agriculture has long taken advantage of disturbance to
keep farming systems in the earlier stages of succession.
This is especially true for annual cropping systems, where
no part of the ecosystem is allowed to progress beyond
the early pioneer stage of development. In this stage, the
system can produce large amounts of harvestable material,
but keeping an agroecosystem at this high output level