Environmental Engineering Reference
In-Depth Information
CHAPTER
10
Ecosystem
s in a Heterogene
ous World
Steward T.A. Pickett
1
and Mary L. Cadenasso
2
1
Cary Institute of Ecosystem Studies, Millbrook, NY;
2
Department of Plant Sciences,
University of California, Davis
INTRODUCTION
The ecosystem concept has few fundamental assumptions (
Pickett and Cadenasso
2002
). In fact, the core idea—that an ecosystem constitutes the interaction among a group
of organisms with one another and with the physical environment within a specified area
(
Tansley 1935
)—requires no additional assumptions (see Chapter 1). Yet, the history of
ecology is rich with definitions of the ecosystem that assume such things as materially
closed boundaries, equilibrium dynamics, or autotrophy (
Golley 1993
). Because the core
definition is silent about such things, specific models are free to adopt any number of
assumptions in their attempt to apply the concept to real or ideal situations. However,
assumptions need to be identified as such, and clearly specified as attributes of models of
ecosystems, not the core concept itself. Oddly, whether ecosystems are assumed to be spa-
tially heterogeneous or homogeneous, and the relevance of heterogeneity outside their
boundaries, have been among the largely unexamined features of ecosystem models.
As ecologists have been able to amass more data on how systems behave, they have
seen the need to relax the restrictive assumption of internal homogeneity of ecosystems
(
Golley 1993
). Empirical study has richly shown that ecosystems are internally heteroge-
neous, and that such patchiness could be significant to the functioning of ecosystems
(
Kolasa and Pickett 1991; Lovett et al. 2005
). Similarly, as they have been able to observe
systems over broader spatial contexts, they have abandoned the assumption that the exter-
nal environment of an ecosystem is a uniform bath or quantitative field (
Forman and
Godron 1986
).
The assumptions that ecosystems were homogeneous and embedded in uniform spatial
contexts were very helpful in the early development of ecosystem science where the prin-
cipal goal was to elucidate the fundamentals of how ecosystems worked. In fact, assessing
