17
Paradigm Integration between Equilibrium and
Non-equilibrium Concepts for Evaluating Vegetation Dynamics in
Rangeland Ecosystems
Takehiro SASAKI
Graduate School of Life Sciences, Tohoku University,
6-3 Aoba, Aramaki, Aoba-ku, Sendai 980-8578, Japan
e-mail: [email protected]
Abstract
There are two major paradigms involved in applied ecological research on rangeland ecosystems: the
equilibrium paradigm and non-equilibrium paradigm. The former idea posits that communities will respond
in a sequential and predictable manner to the environment and disturbances such as grazing. The latter idea
has minimized ecosystem regulation and stability and placed greater emphasis on external factors such as
climatic variability and episodic events, implying that ecosystems are less predictable than indicated by the
equilibrium concept. Previous literature has suggested that both equilibrium and non-equilibrium paradigms
must be incorporated into rangeland management perspectives if the solid science of rangeland ecology is to
continue to underpin them. However, the current state-of-the-art in rangeland management appears to be
progressing at the extremes of the equilibrium–non-equilibrium continuum. Consequently, there is uncertainty as to whether the current knowledge is the key to successful environmental management of rangeland
ecosystems. Here, I have synthesized the extensive debates on rangeland ecology from a rangeland management perspective, and merged the two major paradigms by placing the idea of ecological thresholds at the
core of the study framework. The clear implication is that both equilibrium and non-equilibrium paradigms
must be incorporated into vegetation management in rangeland ecosystems. Effective vegetation management based on both paradigms can prevent adverse changes in states before ecological thresholds are
reached, while maintaining or enhancing the ecological resilience of rangeland ecosystems.
Key words: arid and semi-arid rangelands, ecological theory, ecological threshold,
non-equilibrium dynamics, rangeland management
1. Introduction
Predicting the impacts of livestock grazing on natural communities has become a major concern in recent
rangeland ecology research, especially where grazing is
widespread and its impacts may be in conflict with
sustainable use of natural resources and biodiversity
conservation (e.g., Milchunas and Lauenroth, 1993;
Diaz et al., 2001; Vesk & Westoby, 2001; Pakeman.
2004; Vesk et al.. 2004; Diaz et al., 2007). Major
advances have been made in revealing the patterns and
processes of vegetation changes associated with grazing,
generalizing these patterns within a given landscape or
region, and reassessing the appropriate paradigm for
describing vegetation dynamics.
There are two major paradigms involved in applied
ecological research on rangeland ecosystems: the
equilibrium and non-equilibrium paradigms. The former
idea posits that communities will respond in a sequential
Global Environmental Research
14/2010: 17-22
printed in Japan
and predictable manner to the environment and disturbances such as grazing (Clements, 1936; Dyksterhuis,
1949). The latter idea has minimized ecosystem regulation and stability and placed greater emphasis on external factors such as climatic variability and episodic
events, implying that ecosystems are less predictable
than indicated by the equilibrium concept (Wiens, 1984;
De Angelis & Waterhouse, 1987; Ellis & Swift, 1988;
Westoby et al., 1989). Briske et al. (2003) proposed that
both equilibrium and non-equilibrium ideas be incorporated into rangeland management perspectives in order
that the solid science of rangeland ecology continue to
underpin them. However, the current state-of-the-art in
rangeland management appears to be progressing at the
extremes of the equilibrium-non-equilibrium continuum
(Wiens, 1984; Briske et al., 2003). Consequently, there
is uncertainty as to whether the current knowledge is the
key to successful environmental management of rangeland ecosystems (Briske et al., 2003).
©2010 AIRIES
18
T. SASAKI
Here, I synthesize the extensive debates on this field
from a rangeland management perspective, and try to
merge these two major paradigms. This paper is divided
into three parts. Firstly, I briefly review the paradigms,
and examine strengths and weaknesses of three conceptual models among them. Secondly, I propose a study
framework for paradigm integration, placing the ideas of
ecological threshold (Bestelmeyer, 2006; Groffman
et al., 2006) at its core. Thirdly, I provide an example of
an empirical demonstration in the Mongolian rangelands
which supports the paradigm integration and I discuss
how both equilibrium and non-equilibrium paradigms
can be incorporated into vegetation management in
rangeland ecosystems.
2. Major Paradigms in Rangeland Ecology
2.1 Equilibrium paradigm
The equilibrium paradigm (represented by the “range
model” in rangeland ecology: sensu Dyksterhuis, 1949)
represents vegetation change along a single axis defined
by the successional theory of Clements (1936). It
assumes that ecosystems possess the capacity for internal regulation through negative feedback mechanisms,
including intense intra- and interspecific competition
and plant–animal interactions (De Angelis & Waterhouse, 1987). In the range model, vegetation dynamics
are considered best characterized as continuous and
reversible change (Clements, 1936; Dyksterhuis, 1949).
The interpretation of equilibrium vegetation dynamics
may therefore be strongly influenced by the degree of
managerial involvement imposed within a given system.
The main management applications based on the range
model are to provide information about “increasers” and
“decreasers” (sensu Dyksterhuis, 1949) as a diagnostic
tool for rangeland ecosystems (e.g., Diaz et al., 2001;
McIntyre & Lavorel, 2001; Vesk & Westoby, 2001;
McIntyre et al., 2003; Vesk et al., 2004) and to describe
detailed successional pathways within a given site (e.g.,
Lauenroth & Laycock, 1989; Pakeman et al., 1997;
Coffin et al., 1998) (Fig. 1a).
2.2 Non-equilibrium paradigm
Recently, however, some rangeland ecologists have
proposed that the understanding of rangeland ecosystems fully based on the equilibrium paradigm may
misdirect land management efforts, sometimes leading
to degradation of ecosystems (Wiens, 1984; Ellis &
Swift, 1988; Walker, 1993). This is due to the equilibrium paradigm’s undervaluing the potential existence of
discontinuous and irreversible vegetation dynamics, and
the importance of climatic variability and episodic
events on ecosystem behavior (Wiens, 1984; Ellis &
Swift, 1988). Alternatively, a non-equilibrium paradigm
has emerged that minimizes ecosystem regulation and
stability and places greater emphasis on discontinuous
and irreversible vegetation dynamics. This paradigm
implies that ecosystems are less predictable than previously indicated by the equilibrium paradigm and that
alternative models are required in order to account for
stochastic dynamics (Wiens, 1984; Ellis & Swift, 1988).
Of the models within the non-equilibrium paradigm,
the non-equilibrium persistent model has postulated that
vegetation dynamics in rangeland ecosystems is driven
primarily by periodic and stochastic climatic factors,
and that grazing impact plays a relatively small role in
determining the biomass or productivity and composition of vegetation (Wiens, 1984; De Angelis & Waterhouse, 1987; Ellis & Swift, 1988). Some rangeland
ecosystems are reported to be driven by such nonequilibrium dynamics (Fernandez-Gimenez & AllenDiaz, 1999; Jackson & Bartolome, 2002; Walker &
Wilson, 2002; Richardson et al., 2005) and others are
not (Fernandez-Gimenez & Allen-Diaz, 1999; Fynn &
O’Connor, 2000; Diaz et al., 2001; Walker & Wilson,
2002). However, much of the prevailing rhetoric in
rangeland science and management today emphasizes
the non-equilibrium nature of most rangelands and the
inappropriateness of equilibrium-based models such as
the range model as the basis for rangeland management
(Briske et al., 2003). The perception that vegetation
dynamics is driven entirely by infrequent and unpredictable events reduces the opportunity for observation and
experience to be incorporated into management models
and decreases incentives for adaptive management
(Watson et al., 1996; Fernandez-Gimenez & Allen-Diaz,
1999; Illius & O’Connor, 1999; Walker & Wilson, 2002;
Buttolph & Coppock, 2004) (Fig. 1b). Hence, the debate
has forced a more comprehensive interpretation of
vegetation dynamics along the entirety of the equilibrium–non-equilibrium continuum (Fernandez-Gimenez
& Allen-Diaz, 1999; Illius & O’Connor, 1999; Walker &
Wilson, 2002; Buttolph & Coppock, 2004). Recent
rangeland studies have suggested that a continuum
of systems exists rather than a stark dichotomy
between equilibrium and non-equilibrium rangelands
(Fernandez-Gimenez & Allen-Diaz, 1999; Illius &
O’Connor, 1999; Walker & Wilson, 2002; Buttolph &
Coppock, 2004).
Another model within the non-equilibrium paradigm
is the state-and-transition model that focuses on describing quasi-stable vegetation states, predicting the circumstances that trigger transitions to species-different states,
and modelling these changes (Westoby et al., 1989;
Laycock, 1991; Bestelmeyer et al., 2003, 2004;
Stringham et al., 2003; Briske et al., 2005, 2006, 2008).
This model emphasizes the nonlinearity of vegetation
responses to grazing and other environmental perturbations (Westoby et al., 1989; Laycock, 1991; Bestelmeyer et al., 2003; Briske et al., 2005). In particular,
ecological thresholds (see the next section) have become
a focal point in rangeland management through their
relationship to state-and-transition models because the
identification of thresholds is necessary in order to
recognize the various quasi-stable states that can potentially exist at a given ecological site (Briske et al., 2005,
2006, 2008). Previous studies (McIntyre & Lavorel
2001; Pakeman, 2004; Diaz et al., 2007) suggested that
Paradigm Integration between Equilibrium and Non-equilibrium Concepts
increased grazing favored a suite of attributes associated
with fast regeneration and growth, including annual life
cycles and ruderal strategies, whereas decreased grazing
favored perennial species, regeneration from a buried
seed bank and species that were relatively palatable
(Fig. 1). The state-and-transition model assumes that
there is an ecological threshold in the shifts between
these two quasi-stable states (Westoby et al., 1989;
Bestelmeyer et al., 2003; Stringham et al., 2003; Briske
et al., 2005, 2006, 2008) and that adverse changes in
states beyond the ecological threshold might essentially
be irreversible (Scheffer & Carpenter, 2003) (Fig. 1c).
However, fewer studies (e.g., Friedel, 1991, 1997;
Bestelmeyer et al., 2004) have actually examined
whether there is an ecological threshold in such shifts
(Fig. 1c). Recognition of nonlinear vegetation dynamics
in real ecosystems (Briske et al., 2005; Peters &
Havstad, 2006; Peters et al., 2006) will provide a strong
incentive for an alternative or substantially modified
(a)
Vegetation
attributes
Increaser
19
evaluation procedure that would accommodate a broader
spectrum of vegetation dynamics than the classical
equilibrium model (Bestelmeyer et al., 2003; Stringham
et al., 2003; Briske et al., 2005). This is because continuous and reversible vegetation dynamics prevails
within stable vegetation states, whereas discontinuous
and irreversible dynamics can occur when ecological
thresholds are surpassed and one quasi-stable state replaces another.
3. What is An Ecological Threshold?
An ecological threshold is defined as a point or zone
at which relatively rapid change occurs from one
ecological condition to another along a gradient in a
prevailing disturbance regime (Radford & Bennett,
2004; Radford et al., 2005; Bestelmeyer, 2006;
Groffman et al., 2006). Many rangeland studies have
reported that the impacts of concentrated grazing on
vegetation dynamics generally lead to marked reductions in forage resources, mainly due to shifts in the
community composition from dominance by perennial
grasses and forbs toward dominance by unpalatable
forbs and weedy annuals (e.g., Fernandez-Gimenez &
Allen-Diaz, 2001; McIntyre & Lavorel, 2001; Todd,
2006) (Fig. 2). In Mongolian rangeland ecosystems,
Sasaki et al. (2008) found strong evidence for the exis-
Decreaser
Grazing intensity
Vegetation
attributes
(b)
No practical
prediction
Grazing intensity
Rainfall
(c)
Perennial grass
dominated
Potential
threshold
Potentially
irreversible
Annual forb
dominated
Fig. 1 The illustration of management applications
based on three conceptual models in rangeland ecology; (a) range model, (b) nonequilibrium persistent model, (c) state-andtransition model.
Fig. 2 Severe livestock grazing generally shifts the community composition from dominance by perennial
grasses and forbs (dominated by Allium polyrrhizum;
upper panel) toward dominance by unpalatable forbs
and weedy annuals (dominated by Chenopodium
album; lower panel).
20
T. SASAKI
2.0
1.5
0
1.0
0.5
1
0.0
2
-0.5
(b)
2.5
-1
Similarity in species composition
(represented by DCA axis 1)
(a)
0
200
400
600
800
1000
0
500
1000
1500
2000
Distance (m) from the source of grazing gradient
Fig. 3 Typical examples of threshold changes in plant species composition along a grazing gradient in Mongolian
rangelands (Sasaki et al., 2008). Best-fit models of species composition (represented by scores of DCA axis
1) as a function of distance from the gradient source are shown. Dashed lines indicate the 95% bootstrap
confidence interval and solid vertical lines indicate an ecological threshold (see text for definition). Sasaki et
al. (2008) generally found a piecewise shape in responses of species composition to grazing; exceptionally,
they found a sigmoid logistic response along a grazing gradient. Both models strongly indicate a threshold
response of vegetation to grazing in Mongolian rangelands.
tence of an ecological threshold in such shifts along a
grazing gradient across all ecological sites, even though
the vegetation types, edaphic conditions, landscape positions and climatic conditions differed among the sites
(Fig. 3; Sasaki et al., 2008). This suggests that vegetation responses to grazing in the study areas were essentially nonlinear. Because Sasaki et al. (2008) obtained
this evidence from snapshot data, the repeatability of
this ecological threshold across several years needs to be
examined to account for high rainfall variability in arid
and semi-arid regions such as Mongolia.
4. A Study Framework That Allows Integration
of Equilibrium and Non-equilibrium
Paradigms
Here, I propose a study framework that incorporates
both equilibrium and non-equilibrium vegetation dynamics (Westoby et al,. 1989; Watson et al., 1996;
Bestelmeyer et al., 2003; Briske et al,. 2003, 2005) with
the understanding that ecosystems need not be classified
exclusively as either equilibrium or non-equilibrium
from a rangeland management perspective (FernandezGimenez & Allen-Diaz, 1999; Jackson & Bartolome,
2002; Walker & Wilson, 2002; Briske et al., 2003). In
this framework, I argue that greater knowledge of the
nature and behavior of thresholds in response to the
impacts of grazing across observation years (i.e., the
existence of an ecological threshold and its repeatability) is essential for sustainable management of rangeland ecosystems. Continuous and reversible vegetation
dynamics would prevail within the stable vegetation
state before ecological thresholds are reached. In addition, I focus on the responses of functional groups (e.g.,
perennial grasses, annual forbs) to grazing in relation to
the ecological threshold with repeated measurements
from the same sites to account for rainfall variability
across Mongolian rangeland ecosystems. Consequently,
this procedure allows important signs to be extracted
that could forewarn of the occurrence of threshold
changes in the vegetation state. The study framework
should thus allow for generalization of ecological
knowledge based on both equilibrium and nonequilibrium paradigms.
5. Applying the Study Framework to
Rangeland Management: An Empirical
Example in Mongolian Rangeland
Ecosystems
Figure 4 shows typical examples of significant and
similar responses in functional groups to grazing at each
site across observation years (from 2006 to 2008).
Repeatability was observed in the ecological threshold
along a grazing gradient (Sasaki et al., unpublished
manuscript). This provides strong evidence that real
accumulated damage by livestock grazing on rangeland
ecosystems still remains and can be observed even
under fluctuations in rainfall. In general, dominant functional groups at each site drastically decreased prior to
the crossing of an ecological threshold. The data from
recovery experiments established along the grazing
gradients suggested that beyond the ecological threshold
the prior the vegetation state cannot recover after
short-term livestock exclusion (Sasaki et al., unpublished manuscript). Therefore, the first step in annual
management under system uncertainty must be to identify sharp decreases or increases in the cover of these
species—definitive signs that the grazing intensity will
soon cross the threshold level.
Paradigm Integration between Equilibrium and Non-equilibrium Concepts
200
400
600
800 1000
1
0
-1
-3
-2
Cover of shrubs
2
10
5
0
-5
-10
Cover of perennial grasses
5
0
2006
2007
2008
0
(c) Site C
(b) Site B
-5
-10
Cover of perennial grasses
(a) Site A
21
0
100
200
300
400
500
0
500
1000
1500
2000
Distance (m) from the source of grazing gradient
Fig. 4 Trends in covers of functional groups, plotted against distance from the source of the grazing gradient. Of the significant
(approximate P < 0.05 in generalized additive models; GAM) and similar response patterns of functional groups,
representative responses to the grazing gradient across years of observation (from 2006 to 2008) at several sites are shown.
Values on the y-axis are residuals, the smooth-term coefficients (s) fitted from a GAM, and scaled to have a mean of 0 across
all permanent transects within each year, where the distance is the smoothed explanatory variable. I have also contrasted
these functional groups’ responses with the repeatable ecological threshold (solid vertical line; see also text) to extract robust
and repeatable predictors of occurrence of the ecological threshold.
6. Conclusion
I have thus synthesized the extensive debates on
rangeland ecology from a rangeland management
perspective and merged two major paradigms by placing
the idea of ecological thresholds at the core of the
study framework. The clear implication is that both
equilibrium and non-equilibrium paradigms must be
incorporated into vegetation management in rangeland
ecosystems. Effective vegetation management based
on both paradigms can prevent adverse changes in states
before ecological thresholds are reached, while maintaining or enhancing the ecological resilience of rangeland ecosystems.
Acknowledgments
This work was financially supported by the Global
Environmental Research Fund of Japan’s Ministry of the
Environment (G-071), with additional support from the
Tohoku University Global COE program “Ecosystem
Adaptability Science for the Future.”
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Takehiro SASAKI
Takehiro SASAKI is a Postdoctoral Fellow in the
Ecosystem Adaptability Global COE program at
Tohoku University. His research mainly focuses
on non-linear vegetation dynamics in arid and
semi-arid ecosystems, including the patterns and
processes of nonlinear vegetation responses to
grazing, and the mechanisms of how local extinctions in communities
impact ecosystem functioning. For more information, please visit his
website: http://homepage3. nifty.com/landscape_ecology/.
(Received 25 March 2010, Accepted 13 May 2010)