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ROBUSTNESS IN RANGELAND MANAGEMENT:
AN APPLICATION OF RESILIENCE ANALYSIS IN
MANAGED ECOSYSTEMS
BRIAN WALKER
OVERVIEW
This is a short overview of three papers presented on the theme of robustness in
rangelands:
1. Carpenter, SC, Walker, BH, Anderies, M and Abel, N. 2001. From metaphor to
measurement: Resilience of what to what? ECOSYSTEMS 4: 765-781.
2. Walker BH and Janssen MA. 2002. Rangelands, pastoralists and governments:
interlinked systems of people and nature. Phil. Trans. R. Soc. London. B (2002) 357,
719-725.
3. Anderies JM, Janssen MA and Walker BH. 2002. Grazing management, resilience and
the dynamics of a fire driven rangeland. Ecosystems 5:23-44.
The last two papers are concerned only with rangelands. The first is a general paper on
resilience, that includes rangelands, and describes what resilience is and how it applies in
two detailed examples - lakes and rangelands. It emphasizes the need to consider
resilience of what to what. The Walker and Janssen (2002) paper considers the rangeland
ecosystem and the managers as a complex adaptive system. It explores the outcomes of
management and policy strategies using an agent based model and an optimisation
approach using a genetic algorithm. Finally, the paper by Anderies et al (2002) provides
a detailed account of a model of resilience dynamics in the grassland component of a
savanna rangeland. What follows are some of the insights that emerge from the three
papers plus some general observations arising from work on resilience.
Multiple states in ecosystems are common. Scheffer et al (2002) and Folke et al (2002)
describe examples from a wide range of ecosystem types. Work in progress on
thresholds in social and ecological systems, as part of a joint activity between the SFI
Robustness Project and Resilience Alliance, has so far identified over 60 examples in the
literature. They encompass natural and managed ecosystems from the tundra to the
tropics, including terrestrial, freshwater and marine systems. Regime shifts can be both
endogenous and externally induced. Here we are interested in those shifts where
management, ususally aimed at maximising profit or efficiency of resource use, leads
unwittingly to changes in the positions of the stable and unstable equilibria of the system,
reducing the distance between them and increasing the likelihood that some external
disturbance will “flip” the system into an alternate, less desirable, state.
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The paper by Anderies et al (2002) addresses the interactive dynamics of three variables
that change at different speeds under the combined effects of fire and grazing - the leafy
shoots of grasses (fast dynamics), the roots/crown of grasses (slower) and the shrubs
(slowest). The two grass variables can exist in two domains; one with high levels of both
and one in which both are virtually absent. The distance between the unstable saddle
point that separates them and the high stable equilibrium is a measure of the resilience of
the system to variations in grazing pressure. With increasing amounts of shrubs this
distance declines, making the system more and more vulnerable to variations in grazing
pressure induced, for example, through variations in rainfall.
Managing for resilience amounts to managing threshold positions. Where sufficient is
known about the system to model it adequately, such models can be used to guide
management. In the absence of full understanding we look to the use of general
principles, aiming to increase those attributes of ecosystems that enhance resilience, eg
the response diversity of species, spatial heterogeneity (eg Peterson 2002), ecological and
social ‘memory’.
Walker et al (2002) offer a diagrammatic representation of the dynamics of a managed
rangeland system in terms of two dominant variables – a biophysical one (woody
vegetation, or shrubs) and a socio-economic one (debt:income ratio) (Fig 1). The
threshold position on the woody gradient is influenced by a number of factors, including
the proportion of perennial:annual species, and biophysical resilience can be increased
through fostering perennial species. The social threshold (some level of the debt:income
ratio) can be increased through increasing the diversity of income streams, especially offfarm income. This approach to managing thresholds was used by Fernandez et al (2002)
to explain and suggest solutions to a range of examples of system degradation.
debt : income ratio
(i)
B
A
(ii)
D
C
B
A
D
C
woody vegetation
B
(iii)
D
A
C
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Fig 1. (Fig 4. from Walker et al 2002). Hypothesized trajectories of a 2-variable system
through time. The dotted lines indicate thresholds along the two gradients of woody
vegetation and debt:income. In iii) changes in attributes of the biophysical and social
systems have resulted in changes in the positions of the thresholds, increasing the
resilience of the desired configuration (A).
Resilience vs robustness. Resilience developed as a notion of the domain of attraction of
a particular state – the ball and cusp metaphor of a basin around an equilibrium. Few if
any systems are ever in equilibrium, but centripetal dynamics around a shifting
equilibrium state keep the system within a configuration of states. Examination of a
variety of systems (eg Gunderson and Holling 2002) extended the threshold concept to
the notion that the system learns from disturbances, and self-organises. Only by
periodically being subjected to the range of disturbances under which it developed does
the system maintain the attributes that enable it to respond to and recover from such
disturbances. An ecosystem in which fire is completely eliminated eventually loses the
species that are adapted to withstanding fire. A rangeland that is never subjected to
heavy grazing soon loses the grass species that are able to withstand such grazing,
through possession of (for example) attributes like large root:shoot ratios, anti-herbivore
chemical defence and fibrous, tough leaves. These same attributes make the species less
competitive (for light, water and nutrients) against fast-growing species that channel all
their photosynthetic production into growth of new leaves.
In accordance with all of the above, resilience is defined as:
- The amount of change a system can undergo and still retain the same controls on
function and structure (still be within the same basin of attraction)
- The degree to which the system is capable of self-organization
- The degree to which the system expresses capacity for learning and adaptation
Resilience and robustness (as described in Erica Jen’s excellent answer to “Stable or
robust? What’s the difference?”) share many features, especially persistence in the face
of various perturbations. The two terms seem to be much the same if judged on the basis
of the 17 definitions of robustness on the SFI Robustness website. If there’s a difference
it might be that, in the use of the term robustness, there is often a connotation of robust
policies – ie a policy that will enable the system being used to persist under a variety of
external conditions and assumptions without resulting in a change in system state or
configuration. “Resilience” is usually applied to the system itself, not policies for its
management. A resilient system is forgiving in the face of management interventions and
will have a wider range of policies that are robust than will a non-resilient system.
Perhaps the two terms are the same!
References
Fernandez RJ, Archer ERM, Ash AJH, Dowlatabadi H, Hiernaux PHY, Reynolds JF,
Vogel CH, Walker BH and Wiegand T. 2002. Degradation and recovery in socioecological systems: A view from the household/farm level. Ch 17, Pp 297 – 323 IN
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Reynolds JF, Stafford Smith M (Eds.) Global Desertification: Do Humans Cause
Deserts?, Dahlem University Press, Berlin.
Folke C, Carpenter SC, Elmqvist T, Gunderson LH, Holling CS, and Walker BH. 2002.
Resilience and Sustainable Development: Building adaptive capacity in a world of
transformations. AMBIO 31:437 – 440
Peterson, G.D. 2002. Contagious disturbance, ecological memory, and the emergence of
landscape pattern. Ecosystems 5(4):329-338.
Scheffer, M, Carpenter, SC, Foley, J, Folke, C and Walker BH. 2001. Catastrophic Shifts
in Ecosystems. NATURE, 413: 591-596.
Walker B, Carpenter S, Anderies A, Abel N, Cumming C, Janssen M, Lebel L, Norberg
J, Peterson G D, and Pritchard R. 2002. Resilience management in social-ecological
systems: A working hypothesis for a participatory approach. Conservation Ecology 6(1):
14. [online] URL: http://www.consecol.org/vol6/iss1/art14