Environmental Engineering Reference
In-Depth Information
that exists in settled areas. Contrasts in the mixtures and details of these three major types
of cover can result in different ecological functions.
CONCLUSIONS: ECOSYSTEMS IN TIME
AND SPACE
Understanding heterogeneity is an important frontier for ecosystem science. However,
ecosystem ecology has come to focus on heterogeneity much more recently than the spe-
cialties of evolutionary, population, community, and landscape ecology. Heterogeneity can
exist in both space and time. Spatial heterogeneity can appear as discrete patches with
functionally important boundary zones, or as continuous gradients. Temporal heterogene-
ity appears as relatively short secondary successions, and as extraordinarily long primary
successions on ancient substrates (
Figure 10.10
). Where an ecosystem is located on a suc-
cessional gradient is an important aspect of its context, and can explain much about
resource limitation, and whether disturbance—or management—will lead to dominance
by organisms that demand more or fewer resources than current dominants. The ability of
the resulting ecosystem to retain or acquire resources depends on its position along a suc-
cessional gradient. Temporal variation important to ecosystems also includes short-term
climate extremes, longer-term climate cycles, and now, the anthropogenic trajectory of
global change, along with transient variation.
The two dimensions of heterogeneity can interact. Heterogeneity within ecosystems is
broadly associated with successional status, and the external heterogeneity of an ecosys-
tem will have different implications depending on its successional condition. This interac-
tion is key to sorting out how readily invaded ecosystems can be, and thus has great
practical significance. The essence of the ecosystem concept—the interaction between biota
and the physical environment in a place—is embodied in understanding successional con-
texts. Indeed, the ecosystem concept was originally formulated to solve problems in suc-
cession theory.
Heterogeneity can be caused by multiple agents. Some agents and circumstances pro-
duce pulses of heterogeneity, while others act as persistent presses of altered conditions.
The complexity of ecological events is only hinted at by the contrast between the press
and pulse classes. Furthermore, both human causes and biophysical causes of heterogene-
ity are in play, often in the same ecosystems. Anthropogenic heterogeneity is often inten-
tionally designed, and consequently has a relatively fine scale matching the spatial array
of institutional and household controls of land, and straight or regular curvature based on
efficiency of transport, property ownership, or on aesthetics.
Ecosystem heterogeneity can be important both within and outside of the ecosystem
boundaries selected for a particular research question (
Figure 10.14
). Internal heterogeneity
is currently less well understood than the effects of external heterogeneity. Both these
kinds of ecological heterogeneity may exist at various scales. Indeed, both the study of
ecosystems and the study of landscapes are less about a particular scale or location on
some ideal, nested hierarchy from Earth to atom, and more about a research perspective
or criterion. Landscape ecology asks about the reciprocal interactions among causes,
nature, and effects of spatial and temporal heterogeneity. Its guiding question is applicable
