Agriculture Reference
In-Depth Information
16 Agroecosystem Diversity
and Stability
Both agroecosystems and natural ecosystems are made up
of organisms and the nonliving physical environment in
which the organisms live. The three preceding chapters
have been concerned primarily with the organismal, or
biotic, components of these systems, at the level of pop-
ulations and communities. In this chapter, we begin to add
the abiotic components of ecosystems to the picture,
thereby reaching the ecosystem level of study. At
this level, we look at systems as wholes, gaining a more
complete picture of their structure and functioning.
The complexity that characterizes whole systems is
the basis for ecological interactions that are a crucial
foundation for sustainable agroecosystem design. These
interactions are largely a function of the diversity of a
system.
Diversity is at once a product, a measure, and a foun-
dation of a system's complexity — and therefore, of its
ability to support sustainable functioning. From one
perspective, ecosystem diversity comes about as a result
of the ways that the different living and nonliving com-
ponents of the system are organized and interact. From
another perspective, diversity — as manifested by the
complex of biogeochemical cycles and the variety of
living organisms — is what makes the organization and
interactions of the system possible.
In this chapter, we first explore what it means to man-
age agroecosystems as whole systems, taking advantage
of their emergent qualities. We then examine biodiversity
in natural ecosystems, the value of diversity in an agro-
ecosystem setting, how diversity is evaluated, and the
possible role of island biogeography theory in managing
diversity. Finally, we explore the connections between
ecological diversity, stability, and sustainability in terms
of developing a framework for agroecosystem design and
management.
plants in its fields). Management that works at this level
can take advantage of a huge array of beneficial interac-
tions and processes.
M ANAGING THE W HOLE S YSTEM
Agroecology emphasizes the need to study both the parts
and the whole. Although the concept of the whole being
greater than the sum of its parts is widely recognized, it
has been ignored for a long time by modern agronomy
and technology, which emphasize the detailed study of the
individual crop plant or animal as a way of dealing with
the complex issues of farm production and viability. We
have learned a great deal from specialization and a narrow
focus on the yield of the crop components of farming
systems, but an understanding of the entire farm (and
the whole food system) must also be developed to fully
understand agricultural sustainability and implement
sustainable management practices.
When agroecosystem management considers the
opportunities presented by the emergent qualities of whole
systems, the paradigm of controlling conditions and pop-
ulations is replaced by the paradigm of managing them.
Under the management paradigm, we are always striving
to consider the effects on the whole system of any action
or practice, and we deliberately design practices that build
on whole-system functioning and emergent qualities.
Under the conventional approach, the attempt to rigidly
control and homogenize all the conditions separately too
often results in the elimination of beneficial relationships
and interferences, leaving only negative interferences and
interactions. Conventional management practices work pri-
marily at the individual or population level of the system,
rather than at the community and ecosystem levels, where
more complex interactions can take place.
The problems inherent in the population level, control-
oriented conventional approach are readily seen in the way
it has been applied to pest, weed, and pathogen control
during the past several decades. Based on the principle
that the only good bug or weed is a dead one, an incredible
array of technologies have been developed to remove or
eliminate each target pest from the cropping system. These
technologies have simplified agroecosystems in various
ways — for example, by eliminating the predators of the
target pests. In simplified agroecosystems, however, pest
invasions become more common and pernicious, and the
WHOLE-SYSTEM APPROACHES AND
OPPORTUNITIES
In Chapter 15, we saw how the interactions among the
populations of a crop community lead to emergent qual-
ities that exist only at the community level. At the eco-
system level, another set of emergent qualities exists that
makes the agroecosystem much greater than the sum of
its parts (or the farm much greater that the sum of the crop
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