1.3 Basic Ecological Principles and Their Relation to Ecological Assessments
19
bounce back to its original state. For example,
forests that undergo pest outbreaks as part of their
natural cycle are unstable (populations oscillate
strongly), but they are resilient (they bounce back).
It is important to clarify what is meant by resilience
and stability before ecosystem response to change
can be assessed. In light of the nonlinear, hierarchical, and nonequilibrium characteristics of ecosystems, assessment protocols must be designed
and interpretations made using consistent terminology.
1.3.2 Ecosystems Can be Viewed
Spatially and Temporally within
Organizational Levels
The concepts of scale and pattern are interwoven
(Hutchinson, 1953; Levin, 1992). Complex ecosystem patterns, landscapes, and the multitude of
processes that form them exist within a hierarchical framework (Allen and Starr, 1982; Allen et aI.,
1984; O'Neill et al., 1986). Scale dependency is
very significant because the relationships between
ecological processes (and the patterns that they create) change with spatial scale (Turner, 1990; Davis
et al., 1991). In recent years, considerable attention
has been directed to describing the formal hierarchical organization of ecological systems. As applied to ecological assessments, hierarchy theory
provides a needed framework for the description of
the components of an ecosystem and their scaled
relations.
The following four tenets of hierarchy theory are
required for an understanding of landscape patterns
and their dynamics (Allen et aI., 1984; O'Neill et
al., 1986).
1. The whole-part duality of systems states that
every component of a system, ecological or otherwise, is a whole and a part at the same time.
For example, a forest (a whole) is made up of
trees (the parts). However, at larger spatial
scales, the forest is part of a regional landscape.
In this case, the regional landscape is the whole
and the forest becomes a part. The notion of
whole-part duality is very important to the characterization of ecological systems because it
must be clearly stated which role a component
plays at a given scale of examination.
2. Patterns, processes and their interactions can be
defined at multiple spatial and temporal scales.
These scales need to be clearly identified in the
assessment process and should be appropriate to
the issues being addressed.
3. No single scale of ecological organization is correct for all purposes, although scientists often
provide information and interpretations on ecological systems at a single scale or limited number of scales.
4. The definition of an ecological hierarchy (component patterns and processes) is dictated by the
critical issues or policy decisions that need to
be addressed in an ecological assessment.
Because a primary goal of ecosystem management is to maintain natural ecological patterns over
time (sustainability), an understanding of pattern
persistence is critical to most ecological assessments. One major consequence of the hierarchical
organization of ecological systems is that nonequilibrium dynamics or spatial heterogeneity at one
scale can be translated into eqUilibrium at a higher
level (O'Neill et al., 1986; Urban et al., 1987;
Levin, 1992). Accordingly, pattern persists within
a hierarchical framework because a pattern may be
stable at one level and not at another (Rabel, 1990).
Therefore, ecological pattern should be analyzed at
more than one scale (Rabel, 1990) and ecological
assessments need to consider all levels of appropriate ecological organization (Baker, 1992; Levin,
1992; Urban et al., 1987). Examples of hierarchical organization within vegetation patterns and social systems are shown in Figure 1.3.
1.3.3 Ecosystems Have Biophysical,
Economic, and Social Limits
In response to change, ecological systems may follow different pathways. Eventually, the processes
that trigger change come into balance with the
processes that lead to ecological organization. This
state is called an optimal operating point (Kay,
1991) and may be used to define the limit of ecosystem development along one pathway. It has been
suggested that ecosystems have more than one such
point (or limit) (Holling, 1986; Kay, 1991) and that
such limits are more appropriately described as "a
set of points in state space whose membership
changes over time" (Kay, 1991) or as "evolution at
the edge of chaos" (Bak and Chen, 1991; Kauffman and Johnson, 1991; Costanza et al., 1993).
The concept that ecosystems may have multiple
limits, some more stable than others, should be considered in the design of ecological assessments. For
example, the climax community is the endpoint of
ecological succession and as such exemplifies an
optimum operating point (Kay, 1991). However,
after a disturbance, succession may proceed along
multiple pathways, and a given pathway may stop
before the endpoint of succession is reached. Fur-
19
bounce back to its original state. For example,
forests that undergo pest outbreaks as part of their
natural cycle are unstable (populations oscillate
strongly), but they are resilient (they bounce back).
It is important to clarify what is meant by resilience
and stability before ecosystem response to change
can be assessed. In light of the nonlinear, hierarchical, and nonequilibrium characteristics of ecosystems, assessment protocols must be designed
and interpretations made using consistent terminology.
1.3.2 Ecosystems Can be Viewed
Spatially and Temporally within
Organizational Levels
The concepts of scale and pattern are interwoven
(Hutchinson, 1953; Levin, 1992). Complex ecosystem patterns, landscapes, and the multitude of
processes that form them exist within a hierarchical framework (Allen and Starr, 1982; Allen et aI.,
1984; O'Neill et al., 1986). Scale dependency is
very significant because the relationships between
ecological processes (and the patterns that they create) change with spatial scale (Turner, 1990; Davis
et al., 1991). In recent years, considerable attention
has been directed to describing the formal hierarchical organization of ecological systems. As applied to ecological assessments, hierarchy theory
provides a needed framework for the description of
the components of an ecosystem and their scaled
relations.
The following four tenets of hierarchy theory are
required for an understanding of landscape patterns
and their dynamics (Allen et aI., 1984; O'Neill et
al., 1986).
1. The whole-part duality of systems states that
every component of a system, ecological or otherwise, is a whole and a part at the same time.
For example, a forest (a whole) is made up of
trees (the parts). However, at larger spatial
scales, the forest is part of a regional landscape.
In this case, the regional landscape is the whole
and the forest becomes a part. The notion of
whole-part duality is very important to the characterization of ecological systems because it
must be clearly stated which role a component
plays at a given scale of examination.
2. Patterns, processes and their interactions can be
defined at multiple spatial and temporal scales.
These scales need to be clearly identified in the
assessment process and should be appropriate to
the issues being addressed.
3. No single scale of ecological organization is correct for all purposes, although scientists often
provide information and interpretations on ecological systems at a single scale or limited number of scales.
4. The definition of an ecological hierarchy (component patterns and processes) is dictated by the
critical issues or policy decisions that need to
be addressed in an ecological assessment.
Because a primary goal of ecosystem management is to maintain natural ecological patterns over
time (sustainability), an understanding of pattern
persistence is critical to most ecological assessments. One major consequence of the hierarchical
organization of ecological systems is that nonequilibrium dynamics or spatial heterogeneity at one
scale can be translated into eqUilibrium at a higher
level (O'Neill et al., 1986; Urban et al., 1987;
Levin, 1992). Accordingly, pattern persists within
a hierarchical framework because a pattern may be
stable at one level and not at another (Rabel, 1990).
Therefore, ecological pattern should be analyzed at
more than one scale (Rabel, 1990) and ecological
assessments need to consider all levels of appropriate ecological organization (Baker, 1992; Levin,
1992; Urban et al., 1987). Examples of hierarchical organization within vegetation patterns and social systems are shown in Figure 1.3.
1.3.3 Ecosystems Have Biophysical,
Economic, and Social Limits
In response to change, ecological systems may follow different pathways. Eventually, the processes
that trigger change come into balance with the
processes that lead to ecological organization. This
state is called an optimal operating point (Kay,
1991) and may be used to define the limit of ecosystem development along one pathway. It has been
suggested that ecosystems have more than one such
point (or limit) (Holling, 1986; Kay, 1991) and that
such limits are more appropriately described as "a
set of points in state space whose membership
changes over time" (Kay, 1991) or as "evolution at
the edge of chaos" (Bak and Chen, 1991; Kauffman and Johnson, 1991; Costanza et al., 1993).
The concept that ecosystems may have multiple
limits, some more stable than others, should be considered in the design of ecological assessments. For
example, the climax community is the endpoint of
ecological succession and as such exemplifies an
optimum operating point (Kay, 1991). However,
after a disturbance, succession may proceed along
multiple pathways, and a given pathway may stop
before the endpoint of succession is reached. Fur-