2
A Theoretical Framework
for Ecological Assessment
S. J. Turner and Alan R. Johnson
2.1 Introduction
Ecological assessment of natural and human-managed systems need not be a shot in the dark or a
hopeful guess about processes and impacts. There
is a surprisingly large body of theory that can inform ecological assessment and the prediction of
change for ecosystems. This body of theory can be
used to structure the way that we investigate systems, the way we interpret the results of our investigations, and the management decisions that we
make based on the assessment. It is critically important that we narrow any assessment to those
things that specifically and coherently address the
focal question of the assessment (e.g., forest health,
biodiversity, sustainable economic development).
We have theories that can help us to do so. Paramount among them are theories of hierarchical organization' landscape ecology, and system dynamics. These theories provide us with a conceptual
framework within which we can address the most
important and difficult questions facing ecological
science and management. How do we understand the
ecosystems that we manage so that we can make decisions that will sustain these systems for our future?
The first step in understanding ecosystems, as
Ricklefs and Schluter (1993, p. 350) suggest, is to
"recognize that ecology, evolution, geography, and
history are different facets of a single set of
processes and the patterns they generate. One cannot isolate anyone system of a particular dimenThis work was supported in part by the U.S. Environmental Protection Agency through the Office of Research
and Development and by the U.S. Department of Agriculture, Iomada Agricultural Research Station. The authors thank P. S. Bourgeron and the anonymous reviewers for helpful comments on the manuscript.
sion from processes and structures at a smaller scale
embedded within it or from those at a larger scale
containing it." This recognition must be incorporated explicitly into the design and implementation
of research for ecological assessment. This is the
point of departure for this chapter. We stress the
importance of identifying the appropriate observation and measurement criteria and the appropriate
temporal and spatial scales for measurements. We
also address the dynamics of change-how we can
find out how ecological systems may be altered
with time and how we can predict what may happen next. We begin with hierarchy theory and landscape ecology and move on to systems dynamics;
we then use these concepts as a backdrop for making critical choices about ecological assessments.
2.2 Ecosystems as Hierarchies
2.2.1 Nestedness of Systems
Ecosystems are groups of interacting, interdependent parts (e.g., species, resources) linked to each
other by the exchange of energy, matter, and information (Costanza et aI., 1993). Ecosystem is a scaleindependent concept, so an ecosystem can be of any
size that the investigator chooses to study (Allen
and Hoekstra, 1992). This scale independence is
very important, because our management units and
our questions do not always neatly fit into convenient boundaries. Ecosystems can also be profitably
viewed as hierarchical in organization. For instance,
integral to hierarchy theory is the idea of the
whole-part duality (Koestler, 1967; Allen and Starr,
1982; Allen et al., 1987). Any part of the system
being investigated is simultaneously embedded in
the larger context of the whole ecosystem and itself
29
A Theoretical Framework
for Ecological Assessment
S. J. Turner and Alan R. Johnson
2.1 Introduction
Ecological assessment of natural and human-managed systems need not be a shot in the dark or a
hopeful guess about processes and impacts. There
is a surprisingly large body of theory that can inform ecological assessment and the prediction of
change for ecosystems. This body of theory can be
used to structure the way that we investigate systems, the way we interpret the results of our investigations, and the management decisions that we
make based on the assessment. It is critically important that we narrow any assessment to those
things that specifically and coherently address the
focal question of the assessment (e.g., forest health,
biodiversity, sustainable economic development).
We have theories that can help us to do so. Paramount among them are theories of hierarchical organization' landscape ecology, and system dynamics. These theories provide us with a conceptual
framework within which we can address the most
important and difficult questions facing ecological
science and management. How do we understand the
ecosystems that we manage so that we can make decisions that will sustain these systems for our future?
The first step in understanding ecosystems, as
Ricklefs and Schluter (1993, p. 350) suggest, is to
"recognize that ecology, evolution, geography, and
history are different facets of a single set of
processes and the patterns they generate. One cannot isolate anyone system of a particular dimenThis work was supported in part by the U.S. Environmental Protection Agency through the Office of Research
and Development and by the U.S. Department of Agriculture, Iomada Agricultural Research Station. The authors thank P. S. Bourgeron and the anonymous reviewers for helpful comments on the manuscript.
sion from processes and structures at a smaller scale
embedded within it or from those at a larger scale
containing it." This recognition must be incorporated explicitly into the design and implementation
of research for ecological assessment. This is the
point of departure for this chapter. We stress the
importance of identifying the appropriate observation and measurement criteria and the appropriate
temporal and spatial scales for measurements. We
also address the dynamics of change-how we can
find out how ecological systems may be altered
with time and how we can predict what may happen next. We begin with hierarchy theory and landscape ecology and move on to systems dynamics;
we then use these concepts as a backdrop for making critical choices about ecological assessments.
2.2 Ecosystems as Hierarchies
2.2.1 Nestedness of Systems
Ecosystems are groups of interacting, interdependent parts (e.g., species, resources) linked to each
other by the exchange of energy, matter, and information (Costanza et aI., 1993). Ecosystem is a scaleindependent concept, so an ecosystem can be of any
size that the investigator chooses to study (Allen
and Hoekstra, 1992). This scale independence is
very important, because our management units and
our questions do not always neatly fit into convenient boundaries. Ecosystems can also be profitably
viewed as hierarchical in organization. For instance,
integral to hierarchy theory is the idea of the
whole-part duality (Koestler, 1967; Allen and Starr,
1982; Allen et al., 1987). Any part of the system
being investigated is simultaneously embedded in
the larger context of the whole ecosystem and itself
29