Spatial Dynamics of Disturbance and Succession: Tracing the Impact of Watt’s
Unit Pattern
by
Jen Costanza
11/18/05
Introduction
Alex S. Watt’s paper entitled “Pattern and Process in the Plant Community” (Watt 1947)
has become a classic that continues to influence ecology today. In fact, publications indexed by
the Institute for Scientific Information have cited his paper a total of 671 times between January
1980 and November 18, 2005, including 32 citations since January 2005. His work has
influenced much of ecology and has led to many lines of research, including the description of
landscape pattern, the general link between pattern and ecological process, the relationship
between disturbance and succession, and the use of neutral models in ecology. In addition, the
pattern-process relationship is one of the earliest precursors of the current field of landscape
ecology (Turner 1989).
In his paper, Watt was the first to explore the ubiquitous nature of heterogeneity and
pattern on the landscape, and was able to show that for a variety of systems, the phases of
succession manifest themselves in space. Watt described how process causes pattern in
ecological communities using seven case studies, followed by a synthesis and placement of his
work in a broader ecological context. In his examples, he illustrated ways in which processes
such as gap dynamics give rise to a cyclic pattern of change that manifests itself as the "unit
pattern", in which all phases of succession are represented in patches on the landscape. In this
way, Watt described a pattern in space that represents succession in time. In one example, Watt
detailed a beechwood community in which he detailed the cyclical change and regeneration due
to gap dynamics, as well as more extreme disturbance events. Watt stated that in a constant
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environment with gap dynamics present, the plant community may be in equilibrium and show a
constant proportion of the various phases of succession, but mortality due to extreme disturbance
events such as fire or drought may cause a change in this equilibrium that may have
consequences well into the future. In addition, with high disturbance frequency relative to
recovery time, this equilibrium may be reached only rarely.
The most important contribution Watt’s paper has made has been its influence on the way
in which ecologists view succession and disturbance. Watt’s paper has become a classic because
the spatial manifestation of disturbance and succession is a ubiquitous phenomenon in ecological
systems, and because of the clarity with which he was able to portray his idea. I will show that
Watt’s description of the unit pattern has shaped our current ideas regarding the spatial patterns
of succession and disturbance, and will continue to be relevant to future research.
Disturbance, Succession and Patch Dynamics– a Shifting Paradigm: pre-1947 to 1995
During the five decades following the publication of Watt’s paper, ecologists’ ideas about
succession and disturbance underwent several major transformations as a result of Watt’s unit
pattern concept. Watt’s ideas about succession were quite different from the prevailing ideas of
the time. Earlier researchers had viewed succession as a progression in time toward a
homogenous, steady-state climax condition. Terms such as monoclimax, polyclimax, and
postclimax were used by some to describe the endpoints of succession (Clements 1936). In
addition, Gleason (1936) assumed that within a climax community, species associations were
completely random and homogenous. A community could therefore be divided into
progressively smaller pieces that still represent the whole. Watt showed that even systems that
are presumably at the climax state are heterogeneous and experience small-scale internal
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disturbances that reset the successional clock in portions of the landscape. To Watt, the unit
pattern was the smallest division of the landscape into a representation of the whole.
Shortly following Watt’s paper, Whittaker (1953) described the “climax pattern” as a
situation in which community productivity, structure, and pattern are at steady-state and are
determined by environmental factors. According to Whittaker, all climaxes were determined by
biotic, edaphic, and climatic factors, and the many terms used to describe climax states were
irrelevant since succession is ultimately a progression toward a pattern that is adapted to be
highly productive given the local environment. In this way, Whittaker expressed Watt’s unit
pattern concept in terms of the physical environment. Whittaker’s concept has been used
previously to determine the historic vegetation, or “potential natural vegetation” of a given place
using environmental characteristics as proxies (Küchler 1964).
Watt’s and Whittaker’s ideas were departures from the widely-held views of the climax
because they considered small-scale disturbance to be an autogenic process that leads to a
distinct spatial pattern on the landscape. However, both continued to accept the concept of a
climax state as the optimal condition for a site. Watt described a central tendency toward
equilibrium, and Whittaker’s ideas implied that there is a climax state for any given set of
environmental characteristics.
The progression of these ideas began with an increase in studies of gap dynamics, as well
as larger disturbances during the 1960s and 1970s. For example, Watt’s work inspired Bray
(1956) to describe the gap dynamics of a maple-basswood forest. He showed that sugar maple
(Acer saccharum) was able to take advantage of gap openings, while oak species (Quercus sp.)
established more often under dense canopies, leading to a characteristic pattern in the landscape.
Williamson (1975) studied gap dynamics in an old-growth forest and found that the forest was a
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mosaic of species of different seral stages. Gap dynamics were also incorporated into forest
growth models of succession (“gap models”) (Botkin et al. 1972).
In addition to gap dynamics, larger disturbances such as fire and intertidal fluctuations
were studied to a greater degree, especially beginning in the 1970s. Heinselman (1973) studied
fire in northern conifer forests and found that historically, fire determined the pattern of
vegetation on the landscape. The historic fire regime resulted in a mosaic effect and prevented
older stands from attaining climax and Heinselman suggested that fire “must be studied as an
integral part of the system”. In addition, studies of rocky intertidal dynamics (Levin and Paine
1974) showed that such communities are spatial and temporal mosaics that are constantly
undergoing disturbance events that disrupt succession to a climax state. These intertidal
communities were viewed as a heterogeneous system made up of essentially homogenous
patches, and equilibrium composition depended on disturbance frequency.
As a result of this heightened interest in the relationship between disturbance and
succession, an increasing number of researchers began to view disturbance as an essential and
ubiquitous process in natural systems. Therefore, the concept of climax as a stable, homogenous
state was being challenged according to that view. For example, White (1979) asked, “how much
change is allowable within the notion of a stable climax?”. As a result of these new viewpoints,
the paradigm of patch dynamics emerged, emphasizing the non-equilibrium nature of natural
communities (Pickett 1980, Pickett and White 1985). Patch dynamics describes the pattern of
patch creation in time and space, as well as the changes in individual patches throughout time.
The patch dynamic concept rejects the traditional view of succession toward a steady state and
instead incorporates disturbance as a fundamental, internal process. Patch dynamics recognizes
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that disturbance plays a ubiquitous role in shaping most communities, and it is the nonequilibrium nature of communities that promotes coexistence of species.
Patch dynamics is a progression of Watt’s view of succession as the unit pattern since it
focuses on the spatial dynamics of succession. The patch dynamic concept not only incorporates
small, gap-scale disturbance events, but also includes large disturbances such as fire as part of
the natural system. This concept was soon applied to the design of nature reserves in the
“minimum dynamic area” idea (Pickett and Thompson 1978). This idea states that reserve design
should be based on the smallest area that preserves the disturbance regime and internal
colonization processes.
At about the same time as the patch dynamic concept was emerging, the concept of the
“shifting mosaic steady state” (Bormann and Likens 1979, Sprugel and Bormann 1981) was also
introduced. This concept essentially describes patch dynamics at a larger scale. Bormann and
Likens found that patterns may shift in space due to disturbances, but the overall proportion of
the landscape in a given successional stage will achieve a steady state over time. One example of
a shifting mosaic steady state is wave-regeneration of fir in boreal forests (Sprugel and Bormann
1981). These are systems in which mature trees at the leading edge of a patch are killed by wind,
and subsequently regenerate. At any given point in time, all phases of regeneration are present in
a “wave” through the landscape. The shifting mosaic steady state focuses the tendency of a
system toward equilibrium, while patch dynamics describes non-equilibrium heterogeneity.
The perceived distinction between these two theories caused confusion among ecologists
and led to research through the 1980s and 1990s that expanded on the patch dynamic and shifting
mosaic steady state ideas. In particular, the initial concept of the shifting mosaic steady state was
shown to be invalid for systems experiencing frequent or large-scale disturbance events. Turner
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et al. (1993) showed that landscape-scale equilibrium occurs only for relatively infrequent,
small-extent disturbances.
In addition, hierarchy theory (O'Neill et al. 1986) began to be applied to ecological
systems to show that landscapes are organized into patterns that differ with spatial and temporal
scales. Thus, patterns at one scale are nested within dynamics at higher levels. Wu and Loucks
(1995) suggested that hierarchy theory be combined with patch dynamics to create a new
paradigm for describing landscape successional pattern and process called “hierarchical patch
dynamics”. In their paper, Wu and Loucks stated that much of the controversy over equilibrium
vs. non-equilibrium ideas, and homogeneity vs. heterogeneity comes from the scale-dependence
of these phenomena: a non-equilibrium system at a fine scale may be at equilibrium at coarse
scale. The shifting mosaic steady state idea describes spatial pattern over multiple patches in
large area, while patch dynamics focuses on dynamics between individual patches. Hierarchical
patch dynamics examines multiple scales. The theory creates a framework in which ecological
systems are nested hierarchies of patch mosaics. The dynamics at one scale are the result of a
composite of patch dynamics at different scales. According to the framework, disturbance and
succession occur at an intermediate scale, but the patchiness that results affects both finer- and
coarser-scale processes and patterns. Therefore, Watt’s unit pattern idea had progressed and was
being applied at various scales. Putting patch dynamic hierarchy theory to the test is the subject
of much current research, as well as an area ripe for future advance.
Expanding Research: 1996 to 2005
Since the mid-1990s, the concept of disturbance as a natural process that leads to spatial
variability in succession has led to an expansion in research on the patch dynamics of
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disturbance. In particular, several themes have emerged, including the study of succession
following large, infrequent disturbances (LIDs), the application of variability in disturbance to
management, and modeling patch dynamics across multiple scales.
The occurrence of large-scale disturbances in the 1980s such as the eruption of Mount St.
Helens in 1980 and the 1988 fires in Yellowstone National Park has led to an expansion of
research on large, infrequent disturbances (LIDs). The large extent of LIDs leaves little residuals
such as survivors and seed sources. Therefore, they have ecological consequences that differ
from smaller, more frequent disturbances. Turner et al. (1998) showed that succession following
LIDs differs from smaller, infrequent disturbances. LIDs tend to produce novel successional
pathways, while smaller disturbances result in succession that is predictable. In addition, Foster
et al. (1998) showed that LIDs interact with the landscape and vegetation to produce legacies that
can last for many years into the future.
Modeling patch dynamics across multiple scales is another important area of current
research. Wu and Levin (1997) expanded on earlier gap models by providing a model that treats
ecological systems as hierarchical dynamic mosaics of patches that interact at a higher level to
determine the overall structure and function of the system. They linked within-patch dynamics
and between-patch dynamics to simulate spatial heterogeneity and its effects on ecological
processes. In this way, they put into practice the hierarchical patch dynamics paradigm proposed
by Wu and Loucks. In addition, Urban (2005) discussed the combination of gap models with
other models, a cellular automaton and a stage-based transition model, to create two different
meta-models. The meta-models incorporated gap processes, along with landscape-level
processes such as disturbance. However, the use of multiple-scale models is just beginning. They
show promise in assisting the study of gap and disturbance processes across spatial scales.
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Another field of current research has been inspired by the variability of disturbance
regimes and succession. The concept of historic or natural range of variability (Landres et al.
1999) states that while disturbance events may keep natural systems from achieving a static or
quantitative equilibrium, systems should experience a qualitative equilibrium, or variation within
bounds, over time. Accepting this variability and approximating the natural, dynamic variation of
disturbance and succession systems has become a goal for many land managers and
conservationists. Tinker et al. (2003) showed that the patchiness that resulted from clearcutting
the Targhee National Forest was outside the natural range of variability of nearby Yellowstone
National Park. Therefore, management in the Targhee was not in accordance with natural
conditions. Assessing natural variation in systems continues to be a complicated task in ecology,
since variability depends on the spatial and temporal scale with which it is assessed.
Summary and Future Directions
Watt’s idea that successional phases manifest themselves in space and give rise to
heterogeneity in almost all systems led to a shifting paradigm from traditional Clementsian
succession to the shifting mosaic steady state and patch dynamics, to hierarchical patch
dynamics. The current view of ecological systems as dynamic mosaics of patches formed by
disturbance and subsequent succession has resulted from Watt’s unit pattern concept. However,
there are several key areas of research that need to be addressed. First, it is clear that these
dynamics vary by spatial and temporal scale, and hierarchy theory has provided a starting point
for scaling processes. However, there is a need for a solid framework that describes the ways in
which community-level patch dynamics scale up to ecosystem-level processes and is grounded in
empirical evidence. Additionally, the feedback between disturbance events and vegetation
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pattern on the landscape should be investigated more explicitly. Specifically, more research is
needed on the compounded impacts of multiple disturbances on the landscape. Similarly, while
alternate stable states and resilience have been investigated thus far, we still lack a good
understanding of whether and under what circumstances the concept is worthwhile. The pursuit
of these research areas and others will ensure that Watt’s legacy continues well into the future.
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Excellent job putting your paper in context, tracing ideas prior to your classic through to recent
research.
26/26
Excellent work. Very much forward looking.
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