Agriculture Reference
In-Depth Information
Irrigation enhances productivity but leads to the erosion of topsoil and the runoff of solu-
ble nutrients and pesticides.
Proponents of sustainable agriculture have advocated that management practices be
adopted that emulate the properties of mature ecosystems that Odum (1969) had envi-
sioned. Elliott and Coleman (1988) adopted this philosophy in their article, “Let the Soil
Work for Us.” Management practices that promote less reliance on inorganic nutrient
additions and a greater reliance on organic amendments and biotic interactions promote
internal cycling of nutrients and, if adopted, would reduce the environmental degrada-
tion that conventional practices induce. What is missing in Odum's assessment and the
philosophies of sustainable agricultural practices that followed is a critical mathemati-
cal link between the desired structural and process phenomena and the mathematical
notions of stability and persistence. This link is important because it allows us to make the
connections between ecosystem structure and function and the consequences of altering
conditions on these ecosystem properties on its persistence. Food web models offer a way
to integrate activities of organisms and the cycling of matter within ecological systems to
make the link between observation and stability.
Food web descriptions in conjunction with population and biogeochemical cycling
models have played an important role in the study of agricultural ecosystems. The descrip-
tions and models help to frame questions, interpret empirical outcomes, and convey in
numerical and visual form the complexity of interactions among species, soil organic mat-
ter, and nutrients (Parton et al., 1983; Hendrix et al., 1986; Andrén et al., 1990; Moore and
de Ruiter, 1991; de Ruiter, Moore, et al., 1993; de Ruiter et al., 1994; Swift and Anderson,
1994). Three important observations that have emerged from these studies serve as the
basis for this chapter. First, conventional agricultural management practices have resulted
in significant losses of soil organic matter (Beare et al., 1994; Vitousek et al., 1997). Second,
the patterning of nutrient flow and trophic interactions within the soil food web under
conventional management practices differ from their counterparts in native or conserva-
tion management practices in ways that favor bacteria and their consumers over fungi and
their consumers (Hendrix et al., 1986). Third, coincident with the losses in organic matter
and changes in food web interactions and nutrient flow is a decrease in stability as mea-
sured by mathematical representations derived from these interactions (de Ruiter et al.,
1995; Neutel et al., 2002).
This chapter focuses on these observations and results from models and offers propo-
sitions on how the study of trophic interactions, nutrient dynamics, and stability using
food webs can assist in management decisions to optimize production, nutrient retention,
and soil integrity in ways that mimic the mature ecosystems that are recommended for
sustainable agriculture. We start with an overview of food web descriptions and mod-
els that have been developed to study the interactions among plants and soil organisms
in natural and agricultural ecosystems. The models capture many of the structural and
functional components that Odum (1969) used to contrast early development and mature
tural and functional components to dynamic properties and stability. Here, we present
the results of theoretical exercises that change (read disturb) the observed patterns in food
web structure and assess the effects of these changes on stability. We end the chapter by
connecting the theoretical results to empirical observations from studies of agricultural
ecosystems demonstrating that agricultural management practices affect soil food webs
in ways that parallel those used in the theoretical exercises on disturbance and stability.
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