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Rangelands at equilibrium and non-equilibrium: Recent developments in the debate

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Abstract

This paper reviews the predictions and management implications of two current paradigms in the ecology and management of arid and semi-arid rangelands. The equilibrium model stresses the importance of biotic feedbacks such as density-dependent regulation of livestock populations and the feedback of livestock density on vegetation composition, cover and productivity. Range management under this model centres on carrying capacity, stocking rates and range condition assessment. In contrast, non-equilibrium rangeland systems are thought to be driven primarily by stochastic abiotic factors, notably variable rainfall, which result in highly variable and unpredictable primary production. Livestock populations are thought to have negligible feedback on the vegetation as their numbers rarely reach equilibrium with their fluctuating resource base. Recent studies suggest that most arid and semi-arid rangeland systems encompass elements of both equilibrium and non-equilibrium at different scales, and that management needs to take into account temporal variability and spatial heterogeneity.
Journal of
Arid
Environments
Journal of Arid Environments 62 (2005) 321–341
Rangelands at equilibrium and non-equilibrium:
recent developments in the debate
S. Vetter
Department of Botany, Rhodes University, Grahamstown 6140, South Africa
Received 11 March 2004; received in revised form 17 November 2004; accepted 23 November 2004
Available online 8 February 2005
Abstract
This paper reviews the predictions and management implications of two current paradigms
in the ecology and management of arid and semi-arid rangelands. The equilibrium model
stresses the importance of biotic feedbacks such as density-dependent regulation of livestock
populations and the feedback of livestock density on vegetation composition, cover and
productivity. Range management under this model centres on carrying capacity, stocking rates
and range condition assessment. In contrast, non-equilibrium rangeland systems are thought
to be driven primarily by stochastic abiotic factors, notably variable rainfall, which result in
highly variable and unpredictable primary production. Livestock populations are thought to
have negligible feedback on the vegetation as their numbers rarely reach equilibrium with their
fluctuating resource base. Recent studies suggest that most arid and semi-arid rangeland
systems encompass elements of both equilibrium and non-equilibrium at different scales, and
that management needs to take into account temporal variability and spatial heterogeneity.
r2005 Elsevier Ltd. All rights reserved.
Keywords: Range condition; Overgrazing; Key resources; Rainfall variability; Heterogeneity; Scale
dependence; Mobility
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0140-1963/$ - see front matter r2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jaridenv.2004.11.015
Tel.: +27 46 6038595; fax: +27 46 6225524.
E-mail address: s.vetter@ru.ac.za (S. Vetter).
1. Introduction
For most of this century, there has been concern about the sustainability of
communally grazed rangelands in Africa and other parts of the world. Pastoral
systems are commonly viewed as overstocked, overgrazed, degraded and unproduc-
tive (e.g. Lamprey, 1983;Sinclair and Fryxell, 1985), and this has resulted in
widespread interventions to reduce stock numbers in an attempt to halt degradation.
Overgrazing is commonly thought to be inevitable in communal pastoral systems
because people keep more livestock than they need for a variety of reasons (e.g.
Herskovits, 1926;Lamprey, 1983), and because of the problems inherent in
communal ownership of the resource, where individual benefit is maximized at the
expense of the communal resource (Hardin, 1968). Increasing human population
pressure, encroachment of rangelands by other land use, control of livestock diseases
and the breakdown of traditional resource management structures are thought to
contribute to the degradation problem.
This way of viewing and managing communal grazing systems has come under
considerable criticism regarding its underlying ecological and economic assumptions,
and the idea that communal rangelands are necessarily mismanaged is now widely
challenged (e.g. Sandford, 1983;Homewood and Rodgers, 1987;Ellis and Swift,
1988;Abel and Blaikie, 1989;Behnke and Scoones, 1993;Behnke and Abel, 1996;
Sullivan, 1996;Sullivan and Rohde, 2002). From a broader debate about
interpretations of desertification, and the identification of pastoralists as its major
causative agent (e.g. Homewood and Rodgers, 1987;Leach and Mearns, 1996;
Dodd, 1994), a debate around the ecological dynamics and appropriate management
of semi-arid rangelands has developed. This debate arose in the 1980s in response to
a growing concern that interventions aimed at stabilizing spatially and temporally
variable rangelands were inappropriate and damaging to pastoral livelihoods
(Sandford, 1983;Ellis and Swift, 1988).
This coincided with an increasing recognition that equilibrium dynamics were
difficult or impossible to demonstrate conclusively in many ecological systems
(Wiens, 1977, 1984, 1989a;Connell and Sousa, 1983;DeAngelis and Waterhouse,
1987), and it was suggested that a different paradigm was necessary to describe
non-equilibrium systems. Ellis and Swift (1988) and Westoby et al. (1989) ap-
plied non-equilibrium concepts to rangelands and pointed out that a fundamental
misunderstanding of their ecological dynamics was leading to inappropriate
and failed interventions. The debate gained momentum in the early 1990s after
two international workshops around emergent new paradigms in rangeland
ecology and socio-economics, which resulted in the publication of two influential
books on range ecology (Behnke et al., 1993) and pastoral strategies to
cope with uncertainty (Scoones, 1994). The ‘‘new rangeland ecology’’ posits
that traditional, equilibrium-based rangeland models have not taken into account
the considerable spatial heterogeneity and climatic variability of semi-arid
rangelands, and that mobility, variable stocking rates and adaptive manage-
ment are essential for effectively and sustainably utilizing semi-arid and arid
rangelands.
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Central to the debate is the relative importance of biotic and abiotic factors in
driving primary and secondary production in rangelands, and the consequences of
this regarding the potential for grazing-induced rangeland degradation. The
equilibrium model stresses the importance of biotic feedbacks between herbivores
and their resource, while the non-equilibrium model sees stochastic abiotic factors as
the primary drivers of vegetation and livestock dynamics. The debate has stimulated
much new research, and many researchers now agree that both equilibrium and non-
equilibrium dynamics are found in rangelands, often at different times or governing
different parts of the resource (e.g. Fernandez-Gimenez and Allen-Diaz, 1999;Illius
and O’Connor, 1999, 2000;Desta and Coppock, 2002;Briske et al., 2003). However,
the equilibrium ideas of stability and predictability have remained pervasive in
ecology and range management and also in the social sciences (Scoones, 1999).
The challenge is to understand under what circumstances different dynamics
apply, since the two models have fundamentally different consequences for policy
and management. Interventions based on the equilibrium model focus on reducing
stocking rates and increasing stability, while the non-equilibrium paradigm
advocates opportunistic stocking strategies and promotes mobility. The different
predictions of these two paradigms also determine whether livestock numbers can be
allowed to increase without threatening degradation. Possibly the most widely cited
and debated argument of the non-equilibrium rangeland ecology is that plant
composition and biomass in semi-arid rangelands are primarily driven by rainfall
and not by grazing pressure, that animal numbers are kept below equilibrium
densities by frequent droughts, and that degradation of the vegetation as a result of
overgrazing is thus unlikely (Ellis and Swift, 1988;Behnke and Scoones, 1993;
Sullivan and Rohde, 2002). Since overgrazing is a problem commonly attributed to
communal tenure, whether or not intensive grazing is damaging to the environment
has profound and far-reaching consequences for the persistence of pastoral systems.
For example, many pastoralist groups have been removed from their traditional
grazing areas because they are seen as a threat to wildlife conservation in East Africa
(e.g. Homewood and Rodgers, 1987;Brockington, 2002). In southern Africa, the
debate about degradation and low productivity in communal rangelands has
influenced policy on land reform (Cousins, 1996).
This paper examines the predictions and management implications of the
equilibrium and non-equilibrium paradigms, reviews some recent attempts to test
their predictions and discusses the current status of the debate.
2. The ecological debate
2.1. What determines the size and productivity of livestock populations?
One of the ongoing debates in range ecology is that regarding the relative
importance of density-dependent interactions and abiotic factors in determining herd
productivity, reproduction and mortality from year to year (Ellis and Swift, 1988;
Illius and O’Connor, 1999;Sullivan and Rohde, 2002). In a grazing system with
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relatively predictable rainfall and hence forage production, livestock populations are
regulated in a density-dependent manner via competition for food resources. As
population size nears carrying capacity, increased competition for resources leads to
reduced herd productivity. A sign of density dependence is that population growth
rates decrease with increasing population size because of the effects of competition
on reproductive and mortality rates. This is exemplified by the Jones and Sandland
(1974) model of the effect of stocking rate on cattle weight gain.
Rainfall in different years affects grass production and composition (Dye and
Spear, 1982;O’Connor, 1985, 1994, 1995) and hence the effective carrying capacity
at different times. If livestock populations are near carrying capacity, and hence
already competing for resources, they are likely to experience population crashes in
drought years when resources become scarce (Caughley, 1979). If livestock
populations are well below the ecological carrying capacity, drought mortality is
reduced because livestock are buffered against such stress events by greater forage
and body fat reserves. Thus the recommended management practice is the
maintenance of conservative stocking rates which can be maintained in drier years.
In grazing systems with very high climatic variability, forage availability varies to
such a great degree with rainfall that herbivore population dynamics are driven by
rainfall via its direct effect on forage availability in any given year. In such non-
equilibrium systems, density-dependent interactions such as competition for
resources play a minor role in regulating populations (Wiens, 1977;Ellis and Swift,
1988). Mortality is high and density-independent during severe droughts,
particularly droughts lasting longer than 1 year (Homewood and Lewis, 1987;Ellis
and Swift, 1988;Scoones, 1990;Oba, 2001). Livestock numbers build up during
series of wet years. Population size thus fluctuates dramatically, and cannot track
rainfall closely because of the time it takes populations to recover from crashes.
The dichotomy between density-dependent and abiotically driven population
dynamics is an oversimplification of the range of situations found in reality. The
strength of density-dependent interactions varies over time and in space. For
example, density-dependent dynamics in non-drought years can alternate with
density-independent mortality during droughts and subsequent recovery (e.g.
Scoones (1990) in southern Zimbabwe; Desta and Coppock (2002) in southern
Ethiopia). In a grazing experiment examining the relative importance of rainfall and
stocking rate on plant composition and primary and secondary production, Fynn
and O’Connor (2000) found that density-dependent consumer–resource coupling
was largely limited to drought periods and was greater at high stocking rates. This
example illustrates that a measure of grazing pressure (the number of livestock per
unit of available forage) is more informative—if harder to quantify—than stocking
rate (the number of animals per area) in systems where livestock numbers and
rainfall vary over time.
It is exceedingly difficult to infer density-dependent mechanisms—or their
absence—from livestock population census data. Part of this problem lies in the
extreme difficulty and cost involved in obtaining detailed enough data on population
size, mortality, fecundity and migration over a long enough time series. Simulation
models are a useful tool for exploring the mechanisms which regulate livestock
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dynamics under different conditions in such variable systems. More fundamentally, a
focus on the phenomenon of density-dependence does not in itself provide an
explanation of the underlying mechanisms of the consumer–resource dynamics, for
example how they are affected by seasonal variability and spatial heterogeneity in
forage quality and quantity (Owen-Smith, 2002).
Illius and O’Connor (1999, 2000) question the relevance of non-equilibrium
concepts to arid grazing systems and argue that variability in arid and semi-arid
rangelands is not the outcome of qualitatively different ecological dynamics. They
propose that livestock populations in arid and semi-arid grazing areas are regulated
in a density-dependent manner by key resources. Key resources are defined in terms
of the key factor determining livestock populations, usually survival through the
season of plant dormancy, and thus the forage available during the dry season.
Herbivore populations are in long-term equilibrium with the key resources, while
being largely uncoupled from forage resources that are only available in the wet
season, which can be classed as non-equilibrium resources (Illius and O’Connor,
2000). So far, these models have not been tested in the field, nor has the existence and
nature of key resources and non-equilibrium resources been explored in many
rangeland systems outside southern Zimbabwe, where their role was highlighted by
Scoones (1993, 1995).
2.2. What determines the composition and productivity of vegetation communities?
The equilibrium paradigm is based on the assumption that every environment has
a carrying capacity determined by biophysical characteristics, such as the mean
annual rainfall, soil type and other biophysical characteristics, which determine its
potential primary production (East, 1984;Bell, 1982;Fritz and Duncan, 1994). The
actual carrying capacity of an area at any given time is determined by range
condition, which is assessed as a function of grass composition, biomass and cover
and is interpreted as a stage in plant succession (Dyksterhuis, 1949;Foran et al.,
1978;Trollope, 1990). The response of the vegetation to grazing pressure is linear
and reversible, and can be manipulated predictably with stocking rates. No, or very
light, grazing allows the vegetation to reach its climax stage, whereas heavy grazing
pushes it back to a pioneer stage dominated by weedy or unpalatable grass and forb
species typical of disturbed environments. Continuous intense grazing leads to
vegetation changes such as the replacement of palatable grasses by less palatable
plant species, replacement of perennial grasses by annuals, bush encroachment,
lower standing biomass and reduced basal cover (e.g. Kelly and Walker, 1976;
Coppock, 1993;Ash et al., 1995;Todd and Hoffman, 1999;Fynn and O’Connor,
2000). These in turn are predicted to result in a decrease in forage quality and
quantity, increased variability of primary production, accelerated soil erosion and
ultimately an irreversible decline in animal production. Rainfall is thought to affect
the vegetation via a similar mechanism where drought reduces range condition by
pushing the vegetation community towards a pioneer stage, while high rainfall
improves range condition. Rainfall and stocking rate interact, with low rainfall
exacerbating the effects of high stocking rate, and high rainfall mitigating them.
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Observations that vegetation responses to grazing, drought and fire are often not
linear and reversible have led to the suggestion that thresholds exist between
different rangeland states (Friedel, 1991), and to the development of state-and-
transition models as an alternative to the rangeland succession model (Westoby et
al., 1989). These incorporate multiple successional pathways, multiple steady states,
thresholds of change, and discontinuous and irreversible transitions (Stringham et
al., 2003). Changes from some states, e.g. bush encroachment, can be irreversible
over management time-scales and require major management inputs. Management
of such systems should be opportunistic and take advantage of, or create, conditions
which allow switches to a more desirable state. Although state-and-transition models
are considered to characterize rangelands not at equilibrium (Westoby et al., 1989;
Briske et al., 2003), they have been applied to rangelands in conjunction with
succession-based models (Phelps and Bosch, 2002).
In some systems, no such distinct states can be distinguished and forage
availability and composition every year are primarily determined by stochastic
abiotic factors such as rainfall. Due to the short duration of the growing season, the
high frequency of droughts and the great inter- and intra-annual variation of rainfall
in semi-arid rangelands, the available amount of forage fluctuates considerably
between years. Livestock numbers are unable to track these sharp fluctuations, and
the dynamics and productivity of the vegetation and livestock are thus uncoupled
most of the time. Two-year droughts, which are accompanied by severe mortalities,
also occur regularly. Herd size builds up gradually in wetter years following a
drought, during which time the vegetation is relatively lightly grazed. In a system
such as this, degradation due to overgrazing is unlikely, since animals seldom if ever
reach densities at which they provide a negative feedback on the vegetation (Ellis and
Swift, 1988). Whereas the equilibrium model views drought as concentrating the
effects of herbivory on scarce resources (Illius and O’Connor, 1999), the non-
equilibrium model sees drought as relieving the pressure of high stocking densities,
by making grazeable vegetation unavailable (Sullivan and Rohde, 2002) and by
inducing density-independent livestock mortality which reduces grazing pressure
(Ellis and Swift, 1988). It has been suggested that forage limitation in cold winters
and density-independent livestock mortality during extreme cold events result in
similar non-equilibrium dynamics in the cold rangelands of northern Asia (Kerven,
2004).
Research supports the suggestion that equilibrium and non-equilibrium are
extremes along a continuum and that many systems encompass elements of both
(Wiens, 1984, 1989a;Ellis et al., 1993;Ellis, 1994;Stafford Smith, 1996;Oba et al.,
2000;Desta and Coppock, 2002;Sullivan and Rohde, 2002). Evidence from arid
environments with high rainfall coefficients of variability (C.V.) suggests that these
systems are well described by the non-equilibrium model (Ellis and Swift, 1988;
Ward et al., 1998, 2000a; Sullivan, 1998, cited in Sullivan and Rohde, 2002;
Fernandez-Gimenez and Allen-Diaz, 1999). In the arid areas studied, vegetation
cover, composition and productivity were strongly determined by rainfall, while
grazing intensity had a negligible influence. In more mesic areas with rainfall C.V. of
less than 30%, grazing-induced changes such as bush encroachment (Desta and
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Coppock, 2002) and changes in grass composition (Fernandez-Gimenez and Allen-
Diaz, 1999) were found. This is consistent with the prediction (Ellis et al., 1993;Ellis,
1994) that non-equilibrium dynamics predominate in areas where rainfall C.V.
exceeds 33%.
One of the reasons some arid rangelands appear to be resilient to
long-term intensive grazing is that the grass sward is dominated by annual
grasses, which do not germinate or establish in the absence of rainfall. Grasses
grow from a seed bank in subsequent wet years, with biomass production
more or less proportional to the amount of rainfall. As Sullivan and Rohde (2002)
argue, there may be literally no grass to overgraze in a drought year. Studies on
annual grasslands of the Sahel have, however, revealed changes in plant
composition, productivity and soil characteristics in response to grazing. Turner
(1999) found that long-term grazing history affected the composition and peak
biomass production of annual grasslands in the Sahel, even though no vegetation
responses to short-term grazing impacts could be detected. Grazing can influence
annual vegetation by leading to reduced seed production of preferred or more
grazing-exposed species, or by favouring species with short life cycles, heterogeneous
germination patterns or competitive advantages under low litter cover (Turner, 1999
and references therein). Hiernaux (1998) similarly found changes in species
composition of annual Sahelian grasslands which resulted from differences in
grazing tolerance. However, there was no correlation between grazing response and
palatability and productivity as predicted by the rangeland succession model.
Grazing can also alter soil conditions, leading to shifts in plant composition (Turner,
1998;Hiernaux et al., 1999), although this was not found to be the case in Namibia
(Ward et al., 1998). The susceptibility of soils to grazer-induced changes such as
crusting, compaction and accelerated erosion is related to texture, with sandy soils
being more resilient than clay soils in arid areas, and the reverse being the case at
high rainfall (Walker et al., 2002). Grazing can also affect nutrient and water cycling.
The loss of perennial shrubs, which accumulate nutrients in hot spot ‘‘islands’’ under
their canopies, can lead to overall nutrient losses at the landscape scale (Schlesinger
et al., 1990;Allsopp, 1999).
In systems dominated by perennial grasses, high grazing pressure can exacerbate
drought mortality of grass tussocks and hinder post-drought establishment of
seedlings (O’Connor, 1991, 1994;O’Connor and Pickett, 1992). Compositional
changes and local extinction of grass species such as Themeda triandra following
drought are greater under heavy grazing than under light or no grazing (O’Connor
1995;Fynn and O’Connor, 2000). Perennial grasses invest less in reproduction from
seed than annual grasses, and their dispersal, recruitment and establishment is
therefore often seed-limited. As grass tufts die and grasses fail to re-establish, more
soil becomes exposed and hence vulnerable to erosion. O’Connor and Roux (1995)
found that the long-term response to grazing was most pronounced in longer-lived
plants, whereas the growth of annual grasses directly responded to rainfall from year
to year.
In certain areas, long-term high grazing pressure has resulted in persistent and
resilient vegetation assemblages dominated by grazing-tolerant or grazing-resistant
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plant species. Examples are the grazing lawns of the Serengeti (McNaughton, 1979),
the shortgrass steppe of the USA (Milchunas et al., 1988), perennial Aristida
junciformis grasslands of Transkei in South Africa (McKenzie, 1982) and annual
grasslands with Indigofera cliffordiana dwarf shrubs in northern Kenya (Oba et al.,
2000). Oba et al. (2000) found that high Indigofera mortality accompanied complete
exclusion of ungulate herbivory for longer than 5 years, and this was followed by an
increase in the percentage of bare ground after 8 years. The authors concluded that
grazing was essential in maintaining the productivity and diversity of the vegetation
and that a lack of grazing, rather than overgrazing, leads to rangeland degradation.
However, care must be taken with the definition and assessment of degradation, and
when trying to extrapolate such findings to other regions.
It is increasingly recognized that the quality, quantity and seasonal availability
of forage differs between parts of the landscape, and that people and livestock
do not utilize all areas at the same frequency and intensity. Herded animals
use the landscape differently to animals kept in paddocks, resulting in
different impacts on different parts of the landscape. Some areas are more
resilient to transformation than others, either because livestock cannot access
them for prolonged periods (e.g. annual grasslands, grazing areas far from
permanent water) or because the dominant plant species are tolerant of heavy
defoliation (e.g. stoloniferous grasses). Patterns and processes in such heterogeneous
landscapes are scale dependent, such that inferences about large-scale behaviour
cannot reliably be made on the basis of smaller-scale observations. All patterns and
processes are best described at a particular scale, but no single scale can collectively
describe population, community and ecosystem level processes (Wiens, 1989b;Briske
et al., 2003;Hobbs, 2003). Many range ecologists are struggling to overcome the
mismatch between the scales of ecological investigation and those at which ecological
processes in rangelands take place. While there is now a plethora of experimental
results at the plot scale, larger-scale data from heterogeneous landscapes are still
scarce.
2.3. Are non-equilibrium rangelands prone to degradation?
Much of the heat of the debate has been generated by the assertion that non-
equilibrium rangelands are not vulnerable to degradation. In some cases this has
been embraced so readily that concerns about degradation and the relevance of
stocking rates were completely dismissed (e.g. Dikeni et al., 1996 in South Africa).
This has led to concern about the ecological consequences of uncritically adopting
the non-equilibrium paradigm for management, for example in areas which are not
predominantly experiencing non-equilibrium dynamics (Illius and O’Connor, 1999;
Fernandez-Gimenez and Allen-Diaz, 1999;Cowling, 2000;Desta and Coppock,
2002). Areas where non-equilibrium concepts would be inappropriate include less
drought-prone rangelands at the more mesic end of the spectrum, but also arid,
climatically variable areas where mobility of pastoralists has been severely restricted,
or where the provision of seasonally scarce resources such as feed and water is
reducing the temporal variation in animal growth even though rainfall and plant
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growth are low in drought years. Many studies suggest that the sustainability of non-
equilibrium rangelands is dependent on drought (or other factors) periodically
reducing livestock numbers and thus keeping grazing pressure below levels that are
likely to cause degradation over the long term. A similar effect is achieved by moving
livestock to less drought-affected parts of the landscape. An important question is
whether this post-drought reduction in grazing pressure is a general requirement for
rangeland sustainability.
Illius and O’Connor (1999) predict that areas with a greater ratio of key to non-
equilibrium resources are more prone to grazing-induced degradation, as a bigger
key resource allows the non-equilibrium resource to be more heavily utilized during
the dry season. Introducing supplementary feed would have the same effect as
increasing the key resource and thus increase the risk of degradation as this allows
high grazing pressure to be maintained in an area during and after the dry season.
Buying in of livestock, especially breeding stock, can speed up the recovery of the
herd to its pre-drought size. For example, most livestock owners in South African
communal areas have cash incomes from migrant labour, local employment,
remittances and/or pensions (Cousins, 1998), making purchases of feed and livestock
possible. Data from a communal area in South Africa show that high livestock
numbers are increasingly being maintained through the provision of feed and buying
animals after droughts (Vetter and Bond, 1999;Vetter, 2003), and that livestock
numbers thus remain high during and after droughts. The provision of large
amounts of subsidized supplementary feed, as is common in North Africa, the
Middle East and China and was widespread in northern Asia during the Soviet era,
has been observed to result in rangeland degradation (Seligman and Perevolosky,
1994;Kerven et al., 2003, 2004). An important research challenge is thus to
understand the ecological consequences of restricting mobility in spatially
heterogeneous areas, and of providing seasonally scarce resources such as water
and feed in temporally variable environments.
It is now widely acknowledged that while many assessments of degradation were
exaggerated and their attributed causes have been oversimplified, degradation has
occurred in many semi-arid rangelands. The causes of this are complex, and the
underlying causes can occur at larger scales than can be influenced by the land users
(Ward et al., 2000b;Reynolds and Stafford Smith, 2002;Kerven et al., 2003).
Common proximate causes include sedentarization of pastoralists or supplementary
feeding, both of which lead to continuous, heavy utilization of parts of the range.
The effects are usually spatially heterogeneous and often difficult to quantify,
especially effects on secondary production which tend to be masked by spatial
heterogeneity (Ash et al., 2002). Usually degradation takes place over time-scales
much greater than those at which management decisions are made, and this disparity
in scales has led land users not to perceive degradation as a concern (Abel, 1993;
Biot, 1993;Reynolds and Stafford Smith, 2002). A recently developed synthetic
framework, which recognizes the joint roles of biophysical and socio-economic
factors at different scales in causing desertification, marks substantial progress in
understanding the drivers and effects of degradation in arid rangelands (Stafford
Smith and Reynolds, 2002).
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3. Management of equilibrium and non-equilibrium rangelands
Planning and management of African communal rangelands has generally
followed the equilibrium model and the assumption that these systems are
overstocked and degraded. This has led to government interventions such as
destocking schemes, conversion of communal areas into individually managed
‘economic units’ and settling of nomadic pastoralists into group ranches (Sandford,
1983;Ellis and Swift, 1988;Archer et al., 1989;Boonzaier et al., 1990;Rohde et al.,
1999). The main focus of these interventions has been on preserving natural
resources, with the additional intention of increasing livestock production and
offtake, often for export or city markets. These schemes have met with widespread
resistance, not least because they ignored the objectives of the pastoralists who derive
a multitude of benefits from multi-species herds (Coughenour et al., 1985), many of
which are non-consumptive (Shackleton et al., 2000). It is argued that these benefits
are maximized at higher stocking rates than commercial farming objectives such as
beef production (Sandford, 1983;Abel and Blaikie, 1989;Wilson and MacLeod,
1991;Behnke and Abel, 1996). Interventions often seemed to create or exacerbate,
rather than solve, degradation problems and left many people economically worse
off than before (Ellis and Swift, 1988;Hoffmann and Ashwell, 2001).
3.1. Dealing with temporal variability and drought
The recommended management practice for commercial farmers in semi-arid and
drought-prone environments is to maintain low enough stocking rates to ensure
sufficient forage in years of low rainfall. There is thus acknowledgement of climatic
variability in equilibrium-based range management, but the proposed solution is to
achieve stability by maintaining livestock at densities that are unlikely to exceed the
reduced carrying capacity of dry years. The short-term economic benefits of keeping
higher livestock numbers nevertheless encourage many commercial farmers to
overstock (Ash et al., 2002), and some governments discourage this by making
drought or other subsidies conditional on following recommended stocking rates.
It is argued that management based on constant and conservative stocking rates is
inappropriate and costly to pastoralists in such variable systems, as they would be
unable to make use of all the available forage in wet years, and would still overstock
in very dry years (Sandford, 1982, 1983;Behnke and Scoones, 1993). The
opportunity cost of conservative stocking rates increases with increasing rainfall
variability and more conservative stocking rates (Sandford, 1982, 1983;Stafford
Smith, 1996). Pastoralists employ a variety of strategies to cope with the variability
of their environment (Sandford, 1983;Ellis and Swift, 1988;Scoones, 1994). Instead
of aiming to keep animal numbers constant, pastoralists allow herd size to change
with rainfall (Sandford, 1983, 1994;Toulmin, 1994). Drought risks are minimized
not by maintaining conservative stocking rates, but rather by allowing livestock
numbers to increase in wet years. While livestock owners risk substantial losses
during a severe drought, having a large herd at the beginning of the drought ensures
that at least some part of the herd survives. The bigger the herd belonging to an
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individual in a communal system, the greater is the number likely to survive, and
larger herds thus provide greater security during droughts.
The effectiveness of this strategy depends on how pre-drought livestock density
affects livestock survival, condition and post-drought recovery, particularly of
breeding females. If mortality is completely density-independent, the number of
livestock before the drought does not affect the number that die, and keeping low
livestock numbers will thus not reduce drought mortality. If livestock mortality is
density-dependent during drought, the effect of high pre-drought livestock numbers
needs to be taken into consideration. Reducing livestock numbers before they reach
densities where they exacerbate drought mortality, as suggested by Desta and
Coppock (2002), would be an appropriate strategy under these conditions.
Management of livestock numbers in response to drought must take into account
the variables of interest to pastoralists—i.e. the benefits derived from the livestock
herd in the years between droughts, and the survival and recovery of the herd during
and after the drought. To the livestock owner, the percentage mortality of the
regional herd is not so much of interest as the number of animals per household that
survives the drought. If the number of livestock surviving a drought is the same
regardless of initial density, and the benefits derived from livestock are proportional
to livestock number, a strategy of maximizing livestock numbers between droughts
would be sensible. This results in a higher percentage mortality, as well as a greater
number of livestock lost, but greater benefits derived between droughts and the same
number of livestock after the drought. This scenario assumes that there are no short-
or long-term effects on the vegetation if livestock numbers are high at the onset of
drought. Short-term effects on vegetation productivity affect post-drought recovery
of livestock, while long-term effects are of concern for the long-term sustainability of
maintaining high stocking rates. Very often, there is a mismatch of the time-scale on
which the benefits (short-term) and costs (long-term) of heavy grazing occur.
In the short term, the benefits commonly exceed the costs, favouring the
maintenance of high stocking rates even when there is a long-term risk of
degradation (Ash et al., 2002).
It is argued that appropriate management in climatically variable grazing systems
should aim at supporting flexible responses to droughts, such as pre-empting
drought mortality by marketing surplus animals, and offering opportunities to re-
stock by buying in animals (Sandford, 1983;Toulmin, 1994;Behnke and Abel,
1996). Opportunistic strategies are being recognized as better alternatives to
constant, conservative stocking rates, even in commercial systems (Mentis et al.,
1989;Danckwerts et al., 1993). However, the economic efficiency and environmental
sustainability of tight tracking strategies, particularly those which rely on buying
stock after droughts, are still debated (e.g. Sandford, 1994;Illius et al., 1998;
Campbell et al., 2000). Rainfall is such an important driving variable in rangeland
systems, and its variability so high, that simulation modelling is necessary for
exploring the economic outcomes of different stocking strategies. These outcomes
are influenced by the goods and commodities considered, the economic criteria used
to measure outputs and the sequence of wet and dry years over which the system is
modelled (Sandford, 2004).
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3.2. Dealing with spatial heterogeneity
Pastoral strategies also make use of spatial heterogeneity, as resource availability
and rainfall are not evenly distributed across the landscape (Sandford, 1983;Ellis
and Swift, 1988;Scoones, 1995) and spatial heterogeneity acts as a buffer to climatic
variability (Ash et al., 2002). Pastoralists in arid areas are usually mobile and will
move their animals to the best available grazing, covering different areas over the
course of the year and between years. Such movements may be in the form of
transhumance, which follows a more or less predictable pattern between wet and dry
season resources, or more opportunistic movement tracking less predictable patterns
of productivity, often caused by patchy rainfall patterns (Coughenour, 1991;Bayer
and Waters-Bayer, 1994;Niamir-Fuller and Turner, 1999;Fernandez-Gimenez and
Swift, 2003). These movement patterns are thought to enable farmers to maintain
high stocking rates even in dry years without putting continuous pressure on the
grazing resource throughout the year (Coughenour, 1991;Ellis et al., 1993;Stafford
Smith, 1996).
Many rangelands contain small, highly productive areas which make a
disproportionately large contribution to the area’s total forage production.
Examples are riverine areas and drainage lines, which support green grass
growth throughout most of the year, or croplands where animals can
graze on crop residues in the dry season. Such key resource areas may be
responsible for maintaining total animal numbers considerably higher than
the predicted carrying capacities, which are based on a homogeneous
landscape (Scoones, 1993, 1995). In areas where semi-arid rangelands border on
areas where crop production is possible, nomadic pastoralists and settled
agriculturalists may have mutually beneficial arrangements where livestock use crop
residues in the dry season, allowing the crop farmer to make use of manure. Where
such relationships break down, the total number of livestock that can be kept is
considerably reduced as exploitation of the entire rangeland by pastoralists relies on
mobility and access to crop residues in the dry season (Bayer and Waters-Bayer,
1994). The same happens when the most productive grazing areas are converted to
cropland, forcing pastoralists into increasingly marginal land without access to key
resources (Homewood and Rodgers, 1987;Scoones, 1990;Dodd, 1994;Desta and
Coppock, 2002).
Ellis and Swift (1988),Scoones (1990, p. 392),Ellis et al. (1993) and Bayer
and Waters-Bayer (1994) among others also stress that to persist through droughts,
pastoralists need to be able to expand their operations into areas not normally used
for grazing, and to gain access to outside resources. Neighbouring pastoral
groups commonly have arrangements for reciprocal grazing rights that allow
movement to better pastures in drought years (Fernandez-Gimenez and
Swift, 2003). In household-level studies of livestock dynamics during
drought, Homewood and Lewis (1987),Scoones (1990) and Oba (2001)
found that mobility during droughts was a key factor contributing to herd survival.
Even in the comparatively sedentary communal livestock farming systems in South
Africa, there are reports of livestock owners gaining access to wetter areas far
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beyond their usual pastures in a devastating drought in 2003 (Alcock, 2003
unpublished).
It seems to be widely accepted that the reduction of mobility in semi-arid and arid
pastoral systems has increased the risk of degradation because of the way it
concentrates grazing pressure on the resource and reduces the opportunities for
resting parts of the vegetation (e.g. Coughenour, 1991;Perkins and Thomas, 1993;
Oba et al., 2000;Fernandez-Gimenez and Swift, 2003;Kerven et al., 2003). At the
same time, remote areas become less frequently utilized and may lose productivity in
the absence of periodic grazing (Niamir-Fuller, 1999b). Constriction of mobility is
associated with development interventions to settle nomadic pastoralists into
ranches, encroachment of rangelands by other forms of land- use such as cultivation
and conservation, increasing population densities in rangeland areas, and the
proliferation of water points, often accompanied by settlements. In sparsely
populated arid areas, grazing impact is often concentrated in piospheres or ‘sacrifice
zones’ around water points or settlements (Perkins and Thomas, 1993;Sullivan,
1999;Leggett et al., 2003), while the rest of the area is largely unaffected. In more
densely populated rangelands, such as the former ‘‘homelands’’ of South Africa
where villages are only a few kilometres apart, high grazing pressure is much more
continuous over the landscape.
There appears to be a need for developing management models which re-introduce
mobility, to buffer pastoralists against temporal variability in forage availability,
and to reduce localized degradation. When the traditional transhumant
movements of cattle ranchers in the USA and South Africa became constricted by
settled farmers early in the 20th century (Coughenour, 1991), the solution to
perceived degradation caused by the increasingly concentrated and
continuous grazing pressure was the introduction of grazing systems such as
rotational grazing and resting. These were intended to mimic the evolutionary
grazing patterns by native ungulates, which consisted of intense, localized defoliation
followed by periods of no grazing. However, in arid areas, movement in response to
variable resource availability and drought is necessary on large scales and needs to
flexible. An alternative to rotational grazing and other forms of grazing management
would be to restore mobility in rangelands. This would in many cases involve
expanding the areas under communal tenure and re-establishing access to key
resources, a strategy likely to clash with conservation agendas and other land users.
The legitimacy of mobility has been questioned and undermined in many countries,
and reinstating mobility thus requires a fundamental change in mindset (Niamir-
Fuller, 1999b).
The causes and extent of fragmentation, its costs to pastoralists and the
environment, and possible ways of reversing or mitigating it are presently the
subject of policy debate (Niamir-Fuller, 1999a,b) and large-scale research such as the
SCALE Project (Boone and Hobbs, 2003;Galvin et al., 2003;Reid et al., 2003).
Options for buffering the effects of temporal variability are moving livestock into
other areas, providing supplementary feed, selling and restocking or a combination
of the above. The viability of these options in different pastoral systems, and their
ecological and economic consequences need to be better explored.
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4. Progress and future directions
Much of the initial heat of the debate appears to have dissipated, and recent years
have seen a move from the equilibrium—non-equilibrium dichotomy towards a
greater awareness of the spatial heterogeneity and temporal variability of semi-arid
and arid rangelands. There is increased acknowledgement that rangeland manage-
ment in drylands is complex and is influenced by spatial, bio-physical, social, cultural
and economic factors at a multitude of temporal and spatial scales. The scope of
enquiry has expanded from mainly communal rangelands in sub-Saharan Africa to
include other continents and tenure systems, such as the pastoral regions of Asia
(Fernandez-Gimenez and Allen-Diaz, 1999;Kerven et al., 2003) and commercial
rangelands in Australia (e.g. Ash et al., 2002). Answers to some of the central
questions in the debate seem to be emerging. Stocking rates do matter, but the
grazing pressure and its timing and duration at any given time and part of the
landscape are of more consequence than long-term average stocking rates (e.g. Ward
et al., 1998). Degradation does not occur everywhere, but can occur in arid
landscapes, particularly if grazing pressure is concentrated on certain parts of the
landscapes for prolonged periods of time. Different areas and parts of the landscape
differ in their susceptibility to transformation, and research needs to take into
account this heterogeneity and the scale dependence which is its emergent
characteristic. Management of climatically variable rangelands should be flexible
and adaptive, but this flexibility is often undermined by fragmentation, increased
population pressure, sedentarization and lack of access to information, markets and
economic opportunities outside the rangeland system.
Progress in the debate has been hindered by a lack of clarity on the types of
systems under discussion. Nomadic pastoral systems, more settled agropastoral
systems and commercial ranching are all subject to temporal variability and spatial
heterogeneity, but management and policy options are different in different types of
rangeland systems. The research, management and policy dimensions of the debate
have narrowly concentrated on two components of the system, forage and livestock,
and how they interact and affect each other. This ignores other important
components of livelihoods in rangelands, such as harvesting and trade in other
natural resources, crop cultivation and migration in and out of the pastoral system.
Apart from differences that have evolved in traditional systems under the influence
of different climatic and ecological constraints (e.g. Ellis and Galvin, 1994),
rangelands across the globe have been affected by a variety of other factors such as
population growth, encroachment of other land use on rangelands, restriction of
mobility, government policies and interventions, access to healthcare and education,
urbanization and the different aspirations of the younger generation. Various
combinations of these factors have led to far-reaching and often profound changes in
the livelihood strategies of pastoralists, and in many areas, livestock make a
decreasing contribution to livelihoods (Shackleton et al., 2000).
Despite improved consensus or at least communication among researchers, the
translation of research findings into management recommendations and policy has
been very slow. Some of this has to do with the difficulty in extrapolating results
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from controlled experiments to larger scales. It is difficult to make confident
recommendations in unpredictable systems, and many researchers appear reluctant
to do so. There is also considerable resistance at the policy level to communal tenure,
mobility and other flexible land use practices because those too are harder to control
and predict. And while the concept of adaptive management is widely considered to
be sensible and appealing, changing the laws and policies to allow and facilitate it is
not easy. The time-scales at which adaptive management is implemented and
monitored exceed conventional research and development planning horizons, and
this further explains why it is so rarely put into practice.
There has been a growing recognition of the need to integrate the
ecological, economic, social and institutional dimensions of rangeland
research. Nevertheless, problems communicating across disciplines still persist,
and there remains a tension between those who see the debate mainly in
terms of ecological theory and those who see it in a larger socio-political context.
Some of the latter feel frustrated at the detached approach of many ecologists and
feel that policy questions should inform the natural scientist’s research agenda. A
persistent problem in these discussions is that pastoralists are still under-represented
at defining the research agenda with their needs, priorities and knowledge. They
remain in most cases subjects of research, development and policy rather than
playing an active role.
Acknowledgements
This paper arose from a workshop held at the VIIth International Rangeland
Congress in Durban, South Africa, in July 2003. The organization of the workshop,
invitation of key participants and preparation of this paper were made possible by a
grant from the International Development Research Center, Canada. Nicky Allsopp
and Timm Hoffman are thanked for their comments on this paper.
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... In systems where managers subscribe to the equilibrium approach, the underlying premise is that plant biomass is controlled by herbivores, where grazing pressure, in combination with fire, maintains a relative constant amount of forage (Ellis and Swift 1988;Westoby and others 1989). In these equilibrium systems, managers place an emphasis on controlling herbivore numbers to maintain safe stocking rates below the ecological carrying capacity and retain a functional ecosystem during dry years, thereby reducing drought mortality (Ellis and Swift 1988;Vetter 2005). In contrast, a non-equilibrium management approach allows plant biomass to be controlled by fluctuating biotic and abiotic factors, such as rainfall, fire, herbivory, and predation, and assumes a system has discrete states of alternative persistent vegetation communities with transitions occurring between states (Ellis and Swift 1988;Westoby and others 1989;Stringham and others 2003). ...
... In contrast, a non-equilibrium management approach allows plant biomass to be controlled by fluctuating biotic and abiotic factors, such as rainfall, fire, herbivory, and predation, and assumes a system has discrete states of alternative persistent vegetation communities with transitions occurring between states (Ellis and Swift 1988;Westoby and others 1989;Stringham and others 2003). Managers subscribing to the non-equilibrium approach will allow herbivore populations to fluctuate with rainfall, with animal numbers increasing during wet years due to increased fecundity and decreasing due to high mortality during dry years (Ellis E. Linden and others and Swift 1988;Vetter 2005). Mobility is key for non-equilibrium and large-scale systems, as it allows herbivores to move across the landscape in response to resource availability (Vetter 2005; Staver and others 2019; Smit and others 2020). ...
... Managers subscribing to the non-equilibrium approach will allow herbivore populations to fluctuate with rainfall, with animal numbers increasing during wet years due to increased fecundity and decreasing due to high mortality during dry years (Ellis E. Linden and others and Swift 1988;Vetter 2005). Mobility is key for non-equilibrium and large-scale systems, as it allows herbivores to move across the landscape in response to resource availability (Vetter 2005; Staver and others 2019; Smit and others 2020). When possible, rangeland management, especially in larger protected areas, has shifted toward the non-equilibrium approach to allow greater spatiotemporal flexibility, which in turn fosters landscape heterogeneity and ecological resilience (du Toit and others 2003;Cumming 2004;Vetter 2005). ...
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... Ecosystems were considered complex systems with non-linear dynamics in space and time for more than three decades [1][2][3][4]. However, only recent research focuses on tackling the complexity of ecosystem temporal dynamics with various methodologies [5][6][7][8][9][10][11][12]. ...
... An integrated approach to land management is required to address these issues. Understanding the dynamics and characteristics of rangelands is a vital part of their conservation [4,[24][25][26]. The Normalized Differentiated Vegetation Index (NDVI) was widely used to monitor, assess, and classify vegetation [27][28][29][30][31]. ...
... Arid rangelands are spatially heterogeneous [4,26], and land degradation and overgrazing can affect the landscape creating a grassland/woodland continuum [79,80]. This effect is reflected in the overlapping clusters, showing that discrete areas can have similar vegetation. ...
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... The study of the sustainability of rangelands (or desertification, which would be its opposite) requires the use of comprehensive tools and a multidisciplinary approach, since various disciplines such as ecology, economics, or agronomy are involved in its understanding and management. The need for a holistic approach in complex socio-ecosystems is recurrent [79][80][81][82][83][84][85][86], and SD is a suitable tool for this challenge. ...
... The study of the sustainability of rangelands (or desertification, which would be its opposite) requires the use of comprehensive tools and a multidisciplinary approach, since various disciplines such as ecology, economics, or agronomy are involved in its understanding and management. The need for a holistic approach in complex socioecosystems is recurrent [79][80][81][82][83][84][85][86], and SD is a suitable tool for this challenge. ...
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... The study of the sustainability of rangelands (or desertification, which would be its opposite) requires the use of comprehensive tools and a multidisciplinary approach, since various 99 disciplines such as ecology, economics or agronomy are involved in its understanding and management. The need for a holistic approach in complex socio-ecosystems is recurrent [41][42][43][44][45][46][47][48], and SD is a suitable tools for this challenge. 102 SD is a modelling methodology grounded on the theories of nonlinear dynamical systems and feedback control developed in mathematics, physics, and engineering. ...
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Rangelands are a key resource present all over the world as it covers half of the emerged lands. They are even more important in drylands, where they cover 48% of the total area. Intensification and the additional pressure added by climate change push these systems towards desertification. Over the last two decades, we have developed and applied System Dynamics (SD) models for the study of Mediterranean grasslands. In addition, we have developed procedures and analysis tools, such as global sensitivity analysis, stability analysis condition, or risk analysis, in order to detect the main drivers of these socio-ecological systems and provide indicators about their long-term sustainability. This paper reviews these works, their scientific background, and the most relevant conclusions, including purely technical and rangeland-related ones as well as our experience as systemic modelers in a world driven by field specialists.
... Enfin, la variabilité inter-annuelle de la précipitation est inhérente aux climats aride et semi-aride et peut également affecter le fonctionnement et la résilience des écosystèmes des FS. Les effets de cette variabilité inter-annuelle de la précipitation sur la régénération naturelle et la production en biomasse (fourragère) peuvent être plus importantes que celles des ruminants/folivores dans certaines zones arides (Briske et al., 2003 ;Vetter, 2005). ...
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... Por lo tanto, es necesario discutir y consensuar criterios y umbrales claros y sencillos para definir el sobrepastoreo y el riesgo de degradación de los pastizales en función de las características de los ambientes. Por último, tener en cuenta la variabilidad intra e interanual en la oferta de forraje y agua es esencial para estimar la capacidad de carga de los ecosistemas áridos y semiáridos y la correspondiente incertidumbre, así como para desarrollar prácticas y políticas de manejo (Ellis y Swift 1988;Vetter 2005), incluido el manejo sostenible de las poblaciones de herbívoros. ...
... While the 'fortress' conservation model was closely associated with anti-pastoralist rhetoric that presupposed destructive land use practices (Brockington & Homewood, 1999;Hughes, 2006), conservation biologists who have spearheaded CBC in the area surrounding Amboseli National Park (ANP) have instead considered mobile pastoralism to be an integral ecological process that enhances biodiversity within semi-arid ecosystems (Western, 1982(Western, , 1994(Western, , 1997Western & Gichohi, 1993). Their perspective generally resonates with wider understandings of pastoralism as a crucial livelihood practice in drylands that enables responses to stochastic, spatio-temporally variable rainfall and patchy key resources through flexible animal husbandry and mobility (see Behnke et al., 1993;Ellis & Swift, 1988;Scoones, 1994;Turner, 2011;Vetter, 2005). Thus, the views of conservation biologists working in the areas surrounding ANP also generally resonate with a growing understanding of the vital importance of pastoralist practices for supporting both livelihoods and global rangeland biodiversity (see Homewood & Rodgers, 1991;Homewood & Rodgers, 1984;Notenbaert et al., 2012;Sala et al., 2017;Sayre et al., 2013). ...
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... The cross-cutting nature of desertification calls for a multidisciplinary framework to tackle it, integrating co-evolving biophysical and socioeconomic models for an effective design and implementation of groundwater policies [10,34,35]. System dynamics (SD) [36] is postulated as a suitable tool for this purpose [37], because of its capacity to represent dynamic and complex interactions between diverse factors and processes, specific to different fields of knowledge [38][39][40][41]. ...
... The cross-cutting nature of desertification calls for a multidisciplinary framework to tackle it, integrating co-evolving biophysical and socioeconomic models for an effective design and implementation of groundwater policies [10,34,35]. System dynamics (SD) [36] is postulated as a suitable tool for this purpose [37], because of its capacity to represent dynamic and complex interactions between diverse factors and processes, specific to different fields of knowledge [38][39][40][41]. ...
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Groundwater degradation is a major issue on an increasingly hot and thirsty planet. The problem is critical in drylands, where recharge rates are low and groundwater is the only reliable resource in a context of water scarcity and stress. Aquifer depletion and contamination is a process of desertification. Land Degradation Neutrality is regarded as the main initiative to tackle land degradation and desertification. It is embedded in target 15.3 of the Sustainable Development Goals and focused on preventing these dynamics. Within this framework, we present an approach to assess risks of degradation and desertification in coastal basins with aquifers threatened by seawater intrusion. The approach utilizes an integrated system dynamics model representing the main relationships between the aquifer and an intensively irrigated area (greenhouses) driven by short- and medium-term profitability. The study area is located in a semi-arid region in Southern Spain, the Gualchos stream basin, which contains the Castell de Ferro aquifer. We found that the risk of salinization of the aquifer is 73%, while there is a 70% risk that the system would increases its demand for surface water in the future, and the chance of doubling the current demand is almost 50%. If the current system of reservoirs in the area were not able to satisfy such an increase in demand because of climate change, the basin would be at a serious risk of desertification.
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Forest Landscape Restoration (FLR) is an important way to address the problems of climate change and land desertification. However, there has been significant controversy about the high cost of restoration and whether it is economically feasible. Most cost-benefit analyses of ecological restoration plans have focused on a single ecosystem, without considering the complexity and relevance of the ecosystem. These studies have also not considered the large number of potential important benefits and real opportunity costs, creating the possibility of bias in the cost-benefit analysis. This study applies theoretical analysis tools from non-equilibrium ecology, combining a land system change model and economic analysis to conduct a comprehensive cost-benefit analysis of China's FLR program. The research results show that: (1) The benefits of China's implementation of the FLR policy exceed the costs, with positive net benefits. (2) After fully considering the cost of FLR, including the true opportunity cost, the net benefit of forest landscape restoration in China is between 60 trillion yuan and 110 trillion yuan. (3) Different types of commitment goals impact the success of the recovery plan. Specifically, quantifiable targets better support successful FLR implementation. The article concludes that it is worthwhile to implement forest landscape restoration in China, although the profit margin of the ecological plan is smaller than generally thought. The results provide a scientific basis for the government to formulate FLR policies and other ecological restoration plans.
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The adaptation of herbivore behaviour to seasonal and locational variations in vegetation quantity and quality is inadequately modelled by conventional methods. Norman Owen-Smith innovatively links the principles of adaptive behaviour to their consequences for population dynamics and community ecology, through the application of a metaphysiological modelling approach. The main focus is on large mammalian herbivores occupying seasonally variable environments such as those characterised by African savannahs, but applications to temperate zone ungulates are also included. Issues of habitat suitability, species coexistence, and population stability or instability are similarly investigated. The modelling approach accommodates various sources of environmental variability, in space and time, in a simple conceptual way and has the potential to be applied to other consumer-resource systems. This text highlights the crucial importance of adaptive consumer responses to environmental variability and is aimed particularly at academic researchers and graduate students in the field of ecology.
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Outlines the way in which the apparently precise technical ecological concept of overgrazing now covers a whole range of different meanings, from the trivial to the serious. It looks first at conventional wisdom on the subject and illustrates this with examples of ecological studies of vegetation and livestock parameters that are taken as incontrovertible support for environmental deterioration through overgrazing. It then investigates ideas of carrying capacity and stocking levels, the problem of divergent management aims and appropriate measures of productivity. Theoretical models of pasture dynamics illustrate the diversity of situations labelled as overgrazed, the differences in response to management intervention. Finally, two examples illustrate the use of the overgrazing concept in development planning both past and present. -from Authors
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Knowledge of livestock population dynamics is important to better understand functional attributes and development potential of pastoral production systems. With a focus on the Borana system of semi-arid Ethiopia for 1980-97, the main objectives of this research were to: (1) Characterize cattle population trends; (2) determine associations of rainfall and stocking rate with change in cattle numbers; and (3) estimate economic losses from cattle mortality. We predicted that the regional cattle population trend would consist of a "boom and bust" cycle with long periods of gradual herd growth punctuated by drought-induced losses. We expected that cattle losses would occur when high stocking rates were combined with large rainfall deficits. Such observations would refute the idea that cattle numbers were erratic and purely controlled by rainfall variation, as predicted by non-equilibrium theory. Cattle dynamics were quantified using herd histories from interviews of 56 households living in 4 sites. Data were aggregated to portray regional cattle population trends and quantify economic losses. Regression analysis was employed to assess associations of rainfall variation and stocking rate with cattle dynamics using 2 approaches: (1) Regional using aggregate herd data, empirical rainfall records, and calculated estimates for stocking rates; and (2) local using site-specific herd data along with recall of rainfall and stocking rate dynamics. Overall, results confirmed that cattle numbers followed a boom and bust cycle. Average cattle holdings dropped front 92 to 58 head/household between 1980 and 1997, respectively. Droughts in 1983-5 and 1991-3 resulted in the deaths of 37 to 42% of all cattle, respectively, up to 15-times higher than net sales. Over 17 years our target population of 7,000 households lost 700,000 cattle with a capital asset loss valued at USD 45 million. Statistical results were more difficult to interpret. Our regional approach indicated neither rainfall nor stocking rate were significantly associated with cattle mortality. We felt this interpretation was erroneous, however, due to a probable-but unmeasured-decline in key grazing resources that lowered carrying capacity, increased herd instability between successive droughts, and undermined relationships among model parameters. Our local approach was somewhat clearer in that results indicated cattle losses were significantly and consistently associated with rainfall deficits, and occasionally associated with high stocking rates that varied by site. We were concerned, however, about respondent bias and possible error in these results. We concluded that the strongest information we had was simply the aggregate pattern of herd dynamics. When aligned with empirical rainfall records and augmented with data from another dramatic cattle crash in 1998-9, we make the case that stocking rate indeed appears to influence the likelihood that a dry year will reduce cattle numbers. We concluded that the Boran live in a dynamic and productive equilibrial system where land-use change has interacted with rainfall variation to create a vicious cycle of massive cattle losses every 5 to 6 years. Improving human welfare under such circumstances should focus on creating a virtuous cycle based on more timely livestock sales, alternative investment of revenues, and sustainable economic diversification.
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The dynamics of South African rangelands have classically been interpreted according to equilibrium models of which succession theory is the archetype. The state of a rangeland has been measured by its plant species composition, on the grounds that this reflects the seral stage, the degree of recent retrogressive impact by pastoral use, and the quality of the rangeland for livestock production. The state of arid and semi-arid rangelands that cover most of South Africa, however, can be viewed as having unique histories determined by individual combinations and sequences of events involving floods, droughts, fire, grazing and other factors. -from Authors
Chapter
The response to grazing of the dominant vegetation types on rangelands in the Mediterranean Basin is reviewed. These vegetation communities are not only well adapted to heavy grazing, but low grazing pressure can have undesirable ecological and management consequences. It is suggested that a new approach to management of Mediterranean Basin rangelands should take into account the special characteristics of the vegetation and the rangeland habitats, the specific human and biological history of the region, and the changing socio-economic environment.