36
are our management decisions. If we can develop
knowledge about how one management decision
may lead to new ecological phenomena (because
we understand the ecological patterns and processes that it affects), we may be able to follow the
thread of change in process to the necessity for a
new management policy. From this point we may
be able to better address the assessments required
for sustainable development, for instance.
2.5 Theory Applied
With such a diversity of possible theoretical perspectives, the following question arises: Which one
is the right one? There is no simple answer, because it depends crucially on the ecological phenomena of interest, as well as the scale. For instance, if we focus on net primary production (NPP)
in the pelagic zone of a lake, a simple singleequilibrium model may suffice, because annual
NPP is relatively constant year after year as determined by nutrient loading. The same lake, however, may display multiple states or attractors if we
focus on the species composition of the phytoplankton that are the primary producers. There will
be a sequence of dominant phytoplankton species
over the course of a single year, and year to year
variation in the timing of phytoplankton blooms
and the dominant species involved may be great.
The temporal scale is important. For instance,
community dynamics on time scales of a few generations for the species involved may be modeled
as an approach to an attractor (e.g., a stable community composition), but if hundreds or thousands
of generations are considered, evolutionary
processes will make former attractors unstable-a
dynamic that might be better represented by an
edge-of-chaos system with infinite transients,
rather than stable attractors.
Spatial scale is also important. A fIre in a small
forest stand will be perceived as a massive and sudden change in state. From a broader landscape perspective, there may be a shifting mosaic of burnt and
unburnt forest stands, with the overall proportions
remaining relatively constant over time. At the regionallevel, fIre is not so much an external perturbation as an intrinsic part of the system dynamics.
A simple equilibrium model may be suffIcient to
represent the relative constancy of mosaic composition over time; but if we focus on the fluctuations
away from the long-term mean, there is evidence
that forest fires display power law size-frequency
relationships suggestive of self-organized criticality (Drossel and Schwabl, 1992; Malamud et aI.,
1998).
A Theoretical Framework for Ecological Assessment
Hierarchy and landscape theories and techniques
improve our abilities to conduct ecological assessments by providing us with methods that we can
use to design research around important assessment
issues. Figure 2.1 suggests that there is a focal level
N toward which research, measurement, and monitoring should be directed. It also suggests that the
research, whether modeling of ecological systems
or field-based research, need only account for three
levels, as suggested by Johnson (1996) and Turner
(1995). Remember that functions at one level produce the structure of the next higher level (i.e.,
physiological processes grow and maintain the
trees). Often we fInd that it is change or variability at the higher level that controls or constrains the
system (i.e., climatic factors constrain forest expansion regionally and globally). The level at
which the inquiry begins is arbitrary (Allen et aI.,
1987). This decision determines which observations, scales, and measurements are appropriate. N
is defined uniquely for the question at hand; it may
be defined by purely scientifIc interest or in response to ecological crisis and societal concerns.
Figure 2.3 is one (far from unique) conception
of a phenomena and measurement criteria hierarchy. It could be any set of interesting and related
phenomena. By related, we mean that there is a
connection between the processes and functioning
of one level and the expression of phenomena at a
higher or lower level. More familiar are space-time
diagrams in which structures, functions, systems,
or other lists of ecologically interesting and related
things are arranged along a diagonal of their inherent time and space scales. In this figure the
time-space dynamics are combined in one line
across the top, and the phenomena are arranged so
that some of the possible measurement criteria can
be associated with each phenomenon. Note that we
have defined a slow-large level, which we call
landscape. This is arbitrary. Note, too, that there
are spatially arrayed processes that are appropriate
criteria at most scales. For instance, measurement
of spatial cover produced by a population of plants
is amenable to landscape analytical techniques.
The phenomena along the diagonal-cell, organism, population, community, and ecosystem!
landscape-may each be considered as a focal level
N. If the population level is chosen as the level of
investigative focus, then the level of the organism
is the N - 1 level of mechanisms, and community
is the N + 1 control level. Among the criteria that
could be chosen to describe the population level are
intraspecifIc competition effects, genetic diversity,
spatial dispersion and biomass production of the
population, and dispersal or invasion rate of
propagules or offspring. Understanding of the
are our management decisions. If we can develop
knowledge about how one management decision
may lead to new ecological phenomena (because
we understand the ecological patterns and processes that it affects), we may be able to follow the
thread of change in process to the necessity for a
new management policy. From this point we may
be able to better address the assessments required
for sustainable development, for instance.
2.5 Theory Applied
With such a diversity of possible theoretical perspectives, the following question arises: Which one
is the right one? There is no simple answer, because it depends crucially on the ecological phenomena of interest, as well as the scale. For instance, if we focus on net primary production (NPP)
in the pelagic zone of a lake, a simple singleequilibrium model may suffice, because annual
NPP is relatively constant year after year as determined by nutrient loading. The same lake, however, may display multiple states or attractors if we
focus on the species composition of the phytoplankton that are the primary producers. There will
be a sequence of dominant phytoplankton species
over the course of a single year, and year to year
variation in the timing of phytoplankton blooms
and the dominant species involved may be great.
The temporal scale is important. For instance,
community dynamics on time scales of a few generations for the species involved may be modeled
as an approach to an attractor (e.g., a stable community composition), but if hundreds or thousands
of generations are considered, evolutionary
processes will make former attractors unstable-a
dynamic that might be better represented by an
edge-of-chaos system with infinite transients,
rather than stable attractors.
Spatial scale is also important. A fIre in a small
forest stand will be perceived as a massive and sudden change in state. From a broader landscape perspective, there may be a shifting mosaic of burnt and
unburnt forest stands, with the overall proportions
remaining relatively constant over time. At the regionallevel, fIre is not so much an external perturbation as an intrinsic part of the system dynamics.
A simple equilibrium model may be suffIcient to
represent the relative constancy of mosaic composition over time; but if we focus on the fluctuations
away from the long-term mean, there is evidence
that forest fires display power law size-frequency
relationships suggestive of self-organized criticality (Drossel and Schwabl, 1992; Malamud et aI.,
1998).
A Theoretical Framework for Ecological Assessment
Hierarchy and landscape theories and techniques
improve our abilities to conduct ecological assessments by providing us with methods that we can
use to design research around important assessment
issues. Figure 2.1 suggests that there is a focal level
N toward which research, measurement, and monitoring should be directed. It also suggests that the
research, whether modeling of ecological systems
or field-based research, need only account for three
levels, as suggested by Johnson (1996) and Turner
(1995). Remember that functions at one level produce the structure of the next higher level (i.e.,
physiological processes grow and maintain the
trees). Often we fInd that it is change or variability at the higher level that controls or constrains the
system (i.e., climatic factors constrain forest expansion regionally and globally). The level at
which the inquiry begins is arbitrary (Allen et aI.,
1987). This decision determines which observations, scales, and measurements are appropriate. N
is defined uniquely for the question at hand; it may
be defined by purely scientifIc interest or in response to ecological crisis and societal concerns.
Figure 2.3 is one (far from unique) conception
of a phenomena and measurement criteria hierarchy. It could be any set of interesting and related
phenomena. By related, we mean that there is a
connection between the processes and functioning
of one level and the expression of phenomena at a
higher or lower level. More familiar are space-time
diagrams in which structures, functions, systems,
or other lists of ecologically interesting and related
things are arranged along a diagonal of their inherent time and space scales. In this figure the
time-space dynamics are combined in one line
across the top, and the phenomena are arranged so
that some of the possible measurement criteria can
be associated with each phenomenon. Note that we
have defined a slow-large level, which we call
landscape. This is arbitrary. Note, too, that there
are spatially arrayed processes that are appropriate
criteria at most scales. For instance, measurement
of spatial cover produced by a population of plants
is amenable to landscape analytical techniques.
The phenomena along the diagonal-cell, organism, population, community, and ecosystem!
landscape-may each be considered as a focal level
N. If the population level is chosen as the level of
investigative focus, then the level of the organism
is the N - 1 level of mechanisms, and community
is the N + 1 control level. Among the criteria that
could be chosen to describe the population level are
intraspecifIc competition effects, genetic diversity,
spatial dispersion and biomass production of the
population, and dispersal or invasion rate of
propagules or offspring. Understanding of the