23
Dynamic Terrestrial Ecosystem
Patterns and Processes
Stephanie P. Wilds and Peter S. White
23.1 Introduction
Ecological assessments often begin with the inventory of a site's biological resources and culminate in the generation of a static, two-dimensional
map or other fixed report of present site conditions.
Using this approach, we may fail to appreciate the
short- and long-term dynamics of the populations
and ecosystems that we are assessing. Spatial and
temporal dynamics of patterns and processes are
important to consider for at least three reasons.
First, the very resources we assess may be changing, with consequences for the conclusions of the
assessment itself. Second, the dynamics have implications for whether and under what conditions
the populations and ecosystems are sustainable on
the site of interest. Finally, key processes may occur at broader spatial scales or on longer temporal
scales than we would otherwise consider in the assessment; that is, the populations and communities
at one site may be attributed in part to the spatial
context surrounding that site or the occurrence of
rare past events. In essence, the processes responsible for the biological resources on the site may
themselves not be contained within the site being
assessed, and the resources may not be in balance
with current site processes. Overlooking the effects
of dynamic patterns and processes can therefore
lead to misinterpretations of observations and erroneous conclusions concerning a given site's condition and functional nature.
Ecosystem dynamics can encompass a broad
spectrum of phenomena, including global climate
change, succession, exotic invasions, and episodic
disturbances. These phenomena can be classified
into three general categories. Gradual changes in
the environment, such as climate change or longterm geomorphic and soil development, occur at
broad scales and over very long time periods. Dis338
turbances are relatively discrete, disruptive events
and may include fires, floods, storms, wind, ice,
droughts, freezes, and disease. Natural periodicities, such as seasonal variations and semiperiodic
environmental fluctuations like those caused by the
Southern Oscillation, include annual hydrologic
cycles and regular temperature- and solar radiationdependent fluctuations (DeAngelis and White,
1994) (Figure 23.1). These three classes of ecosystem dynamics readily interact with one another;
global warming, for example, may alter the hydrologic cycle, and seasonal wet or dry periods influence the probability of droughts, floods, or fire
events. Often, the immediate effects of both gradual environmental change and natural periodicities
(e.g., changes in precipitation caused by the Southern Oscillation) are disturbances to the existing
ecosystem. In this chapter, we focus on disturbances as the prime agents of change in dynamic
pattern; natural periodicities and gradual changes
occur on such a broad scale that they are beyond
the scope of most assessment efforts.
Disturbance regimes are part of a more general
group of agents of pattern formation that also includes the biophysical template (e.g., physical landform constraints, precipitation patterns, and substrate conditions) and biotic processes (e.g.,
dispersal, colonization, extinction and other demographic processes) (Urban et al., 1987; Bourgeron
and Jensen, 1994). By altering the availability of
space and resources and creating patchy environments, disturbances playa particularly critical role
in the determination of landscape pattern and in the
development of ecosystem composition, structure,
and function.
Disturbances are also important in maintaining
biological diversity. A site's ecological attributes
result from the way disturbances interact with environmental gradients, substrates, and topography
Dynamic Terrestrial Ecosystem
Patterns and Processes
Stephanie P. Wilds and Peter S. White
23.1 Introduction
Ecological assessments often begin with the inventory of a site's biological resources and culminate in the generation of a static, two-dimensional
map or other fixed report of present site conditions.
Using this approach, we may fail to appreciate the
short- and long-term dynamics of the populations
and ecosystems that we are assessing. Spatial and
temporal dynamics of patterns and processes are
important to consider for at least three reasons.
First, the very resources we assess may be changing, with consequences for the conclusions of the
assessment itself. Second, the dynamics have implications for whether and under what conditions
the populations and ecosystems are sustainable on
the site of interest. Finally, key processes may occur at broader spatial scales or on longer temporal
scales than we would otherwise consider in the assessment; that is, the populations and communities
at one site may be attributed in part to the spatial
context surrounding that site or the occurrence of
rare past events. In essence, the processes responsible for the biological resources on the site may
themselves not be contained within the site being
assessed, and the resources may not be in balance
with current site processes. Overlooking the effects
of dynamic patterns and processes can therefore
lead to misinterpretations of observations and erroneous conclusions concerning a given site's condition and functional nature.
Ecosystem dynamics can encompass a broad
spectrum of phenomena, including global climate
change, succession, exotic invasions, and episodic
disturbances. These phenomena can be classified
into three general categories. Gradual changes in
the environment, such as climate change or longterm geomorphic and soil development, occur at
broad scales and over very long time periods. Dis338
turbances are relatively discrete, disruptive events
and may include fires, floods, storms, wind, ice,
droughts, freezes, and disease. Natural periodicities, such as seasonal variations and semiperiodic
environmental fluctuations like those caused by the
Southern Oscillation, include annual hydrologic
cycles and regular temperature- and solar radiationdependent fluctuations (DeAngelis and White,
1994) (Figure 23.1). These three classes of ecosystem dynamics readily interact with one another;
global warming, for example, may alter the hydrologic cycle, and seasonal wet or dry periods influence the probability of droughts, floods, or fire
events. Often, the immediate effects of both gradual environmental change and natural periodicities
(e.g., changes in precipitation caused by the Southern Oscillation) are disturbances to the existing
ecosystem. In this chapter, we focus on disturbances as the prime agents of change in dynamic
pattern; natural periodicities and gradual changes
occur on such a broad scale that they are beyond
the scope of most assessment efforts.
Disturbance regimes are part of a more general
group of agents of pattern formation that also includes the biophysical template (e.g., physical landform constraints, precipitation patterns, and substrate conditions) and biotic processes (e.g.,
dispersal, colonization, extinction and other demographic processes) (Urban et al., 1987; Bourgeron
and Jensen, 1994). By altering the availability of
space and resources and creating patchy environments, disturbances playa particularly critical role
in the determination of landscape pattern and in the
development of ecosystem composition, structure,
and function.
Disturbances are also important in maintaining
biological diversity. A site's ecological attributes
result from the way disturbances interact with environmental gradients, substrates, and topography