Agriculture Reference
In-Depth Information
many of the homeostatic mechanisms (Rappaport, 1971; Pimental and Hall, 1984).
In addition, new avenues for nutrient and energy losses—such as export of produce,
leaching, and erosion—develop (Gliessman and Amador, 1980). Socioeconomic
considerations become the most significant determinants of the functional state of
the resulting ecosystem.
Despite the control of ecosystem processes that the farmers maintain, agroeco-
systems are in a constant state of flux as they respond to such other influences as cli-
matic changes and geographic variation (Clapham, 1983). Managing agroecosystems
effectively requires an understanding of the signals generated within an ecosystem
and how these signals—in their totality—influence the various functions and states
of the ecosystem (Clapham, 1983).
1.4.1 s
y s t e m s
t
h e o r y
A system has been defined as an abstract concept of a whole (Checkland et al., 1990),
consisting of a group of parts or components that interact according to some kind of
process (Odum, 1983) and behave as a whole in response to stimuli applied to any
of the parts (Spedding, 1988). To distinguish this from the common usage of the
word system (real-world arrangement of things or processes), Checkland (1988) and
Checkland and Scholes (1990) suggested the use of an alternate term:
holon
. This
term was coined by Koestler (1978), who spoke of reality as Janus faced, like the
two-faced Roman god. He referred to each unit (person, organism, etc.) as a holon
and the nested hierarchy of which they are a part as a
holarchy
. The interactive com-
bination of parts within a holon confers new properties to the system over and above
those of the individual components that constitute it. Properties arising from inter-
actions between parts—termed
emergent properties
(Checkland et al., 1990)—are
only apparent when taking an overview of the system.
An ecosystem can be described as a holon that exhibits the emergent property
of having a capacity to regulate and organize its own internal structure and function
and to mitigate stresses imposed from outside (Rapport et al., 1985). This property—
termed
integrity
(Kay, 1991)—imparts to the holon a capacity to perpetuate itself
over time even within a fluctuating environment. Sustainability would therefore
result when a holon has the capacity to maintain both its integrity and its productiv-
ity over all the foreseeable fluctuations.
A distinctive feature of ecosystems is that they can be described as occurring
in nested hierarchies (Waltner-Toews and Wall, 1997), where entities at different
scales are nested within each other in concentric layers. The nested hierarchies form
holarchies, with each nested entity considered a holon (Checkland, 1981). Several
holarchies can be described for agroecosystems, depending on the features (e.g.,
ecological, cultural, social, or economic) used to delineate the holons within it. For
example, a biophysical holarchy can be defined as consisting of fields nested within
farms, catchments, watersheds, drainage basins, agroecozones, and larger biore-
gions. A socioeconomic holarchy can be conceptualized as individuals nested within
households, villages, larger administrative or sociopolitical boundaries, all the way
to the global community. Each level in a holarchy has its own emergent properties. It
contributes to the nature of, and is affected by, levels above and below it. Each level
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