Environmental Engineering Reference
In-Depth Information
ecosystems and heterogeneity. Ultimately, the question to be answered is, “How does het-
erogeneity affect ecosystem structure and function?”
TOWARD A FRAMEWORK FOR SPACE AND
TIME HETEROGENEITY
Constructing a way to think about heterogeneity in ecosystems can take many forms.
Here, we summarize the fundamentals of spatial heterogeneity; link spatial to temporal
heterogeneity, which can be summarized in the concept of patch dynamics; introduce
some distinctive features of human-induced heterogeneity; and end with a pulse-press tax-
onomy of kinds of temporal events generating heterogeneity.
Spatial Heterogeneity
Heterogeneity can appear as differentiation in either space or time. These two kinds of
heterogeneity are linked conceptually and analytically ( Dutilleul 2011 ), yet it is important
to recognize their distinct characteristics. For example, spatial heterogeneity appears as
three-dimensional patchiness or gradients such that different places are different from one
another—the structure, composition, or processes underway at each place differ. Such het-
erogeneity may appear on fine scales, existing within a defined ecosystem, or on very
coarse scales, encompassing what may be defined by researchers as several ecosystems.
Spatial heterogeneity can take contrasting forms. In the case of patchiness, differentiation
is abrupt at a given scale, while for gradients, differentiation at that same scale is gradual.
Gradients are often detected directly and linearly through space. However, gradients may
sometimes be abstracted from spatial mosaics, as in ordination, and do not necessarily
appear as concrete, linear transects on the ground (Austin 2005). The principle of eco-
logical differentiation along gradients is one of ecology's oldest ideas. It suggests that any
area isolated for study on some part of a gradient would exhibit biotic differentiation
within it that paralleled the environmental gradient, and would exhibit differences in com-
position, three-dimensional architecture, or process compared to adjacent areas along the
gradient. Important exchanges of matter, energy, or influence may occur between the focal
ecosystem and other ranges of the gradient. Most of the examples given in this chapter to
this point have emphasized spatial heterogeneity.
Patch Dynamics and the Linkage of Time and Space
Patch dynamics provides a way to both summarize spatial heterogeneity, and relate it
to temporal heterogeneity. The time dimension of heterogeneity is incorporated in the con-
cept of patch dynamics. Patch dynamics is a conceptual and modeling approach that
recognizes the spatial patterning at a variety of scales and for various ecological criteria,
including individuals, populations, communities, ecosystems, and landscapes ( Pickett and
Rogers 1997 ). Patches, as three-dimensional bodies, appear in aquatic systems as volumes
of water that are formed and move differentially based on temperature, density, or
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