Environmental Engineering Reference
In-Depth Information
input and output budgets or determining the controls on system metabolism made great
progress under these assumptions (
Likens 1992
). However, as research on ecosystem struc-
ture and function has advanced, heterogeneity began to claim more attention from ecolo-
gists. Still, concerted focus on the implications of heterogeneity for ecosystem science is
relatively new (
Turner and Cardille 2007
). Therefore, a body of empirical generalizations
about heterogeneity and ecosystems does not yet exist. It is important, however, to have a
clear way to guide the search for new knowledge about heterogeneity and ecosystems.
This chapter outlines key features of a framework for addressing heterogeneity in and sur-
rounding ecosystems.
THE NATURE OF HETEROGENEITY
Heterogeneity
is a technical term for the differentiation in structure or process over
three-dimensional space or over time in any system. In more familiar terms, heterogeneity
translates into such commonly observed spatial patterns as patchiness, or the variation
across large landscapes (
Figure 10.1
). Such heterogeneity or patchiness has been used for a
long time in some biological subjects and ecological subdisciplines. For example, because
evolution uses the genetic differences among individuals as a key explanatory process,
there are countless examples of studies of variation among individuals and its implications
for selection and differential fitness (
Futuyma 2009
). Interest in variation has been impor-
tant for a long time in population ecology as well, with its emphasis on within-population
heterogeneity in age, sex, and size (
Futuyma 2009
). Heterogeneity in community ecology
is illustrated by regional differences or differences along gradients in species composition
(
Scheiner and Willig 2005
). The evolutionary and population ecologist's focus on differen-
tiation between individuals and populations is beginning to be transferred to ecosystem
science as the bridge between species biology and ecosystem science is increasingly
explored (
Jones and Lawton 1995
).
The examples just mentioned expose an important aspect of heterogeneity. It can be
identified within any kind of ecological unit, such as populations, communities,
ecosystems, and individuals. Qualitative or quantitative differences among individual ele-
ments of each kind of unit result in heterogeneity (
Figure 10.2
). Heterogeneity can exist as
(1) richness of entities of interest, (2) the frequency of those entities in a collection, or
(3) the spatially or temporally explicit configuration of the entities. Population heterogene-
ity is expressed in the genetic or demographic differences mentioned earlier, for example.
However, heterogeneity can also be identified as a matter of spatial arrangement at partic-
ular spatial or temporal scales. At a scale of hundreds of meters, an animal population in
which bachelor males form separate herds will likely have different dynamics than
unstructured herds. An additional example at the scale of many meters or kilometers is a
set of plant communities that differ in such attributes as height, layering, or composition
as a result of soil fertility or moisture differences.
Examples also exist at finer scales. Across only tens of meters, heterogeneity within
plant communities can result from the biotic interactions among neighboring individuals.
The examples of heterogeneity throughout this chapter can be differentiated based on
whether they arise from differences expressed among the kinds of elements or patches of
