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El Niño as a Window of Opportunity for the Restoration of Degraded Arid Ecosystems

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Most arid ecosystems have suffered from severe overexploitation by excessive wood harvesting, overgrazing, and agriculture, resulting in depletion of vegetation biomass and soil erosion. These changes are often difficult to reverse due to positive feedbacks that tend to stabilize the new situation. In this paper, we briefly review evidence for the idea that different states in these ecosystems might represent alternative equilibria and present a graphic model that summarizes the implications for their response to changing environmental conditions. We show how, in the light of this theoretical framework, climatic oscillations such as El Niño Southern Oscillation (ENSO) could be used in combination with grazer control to restore degraded arid ecosystems. We also present evidence that, depending on grazing pressure, ENSO episodes can trigger structural and long-lasting changes in these ecosystems.
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El Nin˜ o as a Window of
Opportunity for the Restoration of
Degraded Arid Ecosystems
Milena Holmgren
1
* and Marten Scheffer
2
1
Silviculture and Forest Ecology Group, Department of Environmental Sciences, Wageningen University, P.O. Box 342, 6700 AH
Wageningen, The Netherlands; and
2
Aquatic Ecology and Water Quality Management Group, Department of Environmental
Sciences, Wageningen University, P.O. Box 8080, 6700 DD Wageningen, The Netherlands.
ABSTRACT
Most arid ecosystems have suffered from severe
overexploitation by excessive wood harvesting,
overgrazing, and agriculture, resulting in deple-
tion of vegetation biomass and soil erosion. These
changes are often difficult to reverse due to pos-
itive feedbacks that tend to stabilize the new sit-
uation. In this paper, we briefly review evidence
for the idea that different states in these ecosys-
tems might represent alternative equilibria and
present a graphic model that summarizes the im-
plications for their response to changing environ-
mental conditions. We show how, in the light of
this theoretical framework, climatic oscillations
such as El Nin˜ o Southern Oscillation (ENSO)
could be used in combination with grazer control
to restore degraded arid ecosystems. We also
present evidence that, depending on grazing pres-
sure, ENSO episodes can trigger structural and
long-lasting changes in these ecosystems.
Key words: arid ecosystems; semiarid ecosys-
tems; rangelands; alternative stable states; vege-
tation shifts; land degradation; desertification; El
Nin˜ o; ENSO; climatic oscillation; ecosystem res-
toration.
INTRODUCTION
There is increasing evidence that the El Nin˜ o South-
ern Oscillation (ENSO) has strong effects on the
dynamics of plant and animal populations in a wide
range of terrestrial ecosystems ranging from arid
and semiarid ecosystems to tropical and boreal for-
ests (Holmgren and others 2001). During an El
Nin˜ o episode, rainfall increases dramatically in cer-
tain areas of the world, while severe droughts occur
in other regions. These increases in rainfall can be
four to ten times that of a “normal” year. The
phenomenon lasts approximately 1 year before the
climate conditions reverse. The next phase, known
as La Nin˜ a, produces climate patterns roughly op-
posite to those found during an El Nin˜ o episode.
The oscillation between El Nin˜ o and La Nin˜a is
irregular but typically occurs once every 3–6 years
(Allan and others 1996). Although the effect of
global warming on ENSO oscillations are difficult to
predict, recent high-resolution climatic models sug-
gest that the frequency of El Nin˜ o–like conditions
can be expected to increase over the coming de-
cades (Timmermann and others 1999).
Interannual variability in precipitation is strongly
associated with ENSO events in many arid and
semiarid ecosystems. Increased rainfall during an
ENSO event is crucial for plant recruitment and
productivity in these ecosystems. Short-term re-
sponses are often spectacular due to an extraordi-
nary increase in the cover of annual species as well
as significant increases in growth, fruit, and seed
production among perennial herbs and woody spe-
cies (for example, see Gutie´ rrez and others 1997;
Polis and others 1997). Higher trophic levels have
also been observed to respond to the plant produc-
Received 21 March 2000; accepted 29 November 2000.
*Corresponding author; e-mail: Milena.Holmgren@btbo.bosb.wau.nl
Ecosystems (2001) 4: 151–159
DOI: 10.1007/s100210000065 E
COSYSTEMS
© 2001 Springer-Verlag
151
tivity pulse (Jaksic and others 1997; Meserve and
others 1999; Grant and others 2000), leading to
changes in ecosystem structure and functioning
that can be quite intricate (Polis and others 1998).
One may imagine that an ecosystem essentially just
tracks fluctuations in environmental conditions. In-
deed, various long-term studies of El Nin˜ o effects on
terrestrial ecosystems show such a tracing of environ-
mental fluctuations with delays on different trophic
levels (Holmgren and others 2001). However, this is
not necessarily the case in ecosystems with alternative
stable states or more appropriately, alternative “do-
mains of attraction” or “dynamical regimes.” Under
the same environmental conditions (for example,
rainfall), an ecosystem may be in either of two (or
more) distinct states (for instance, bare soil or peren-
nial grassland). Positive feedbacks may cause such
multiple stable states, a concept now beginning to be
recognized as an inherent property of a wide range of
ecosystems (Carpenter forthcoming). In this case, a
rare extreme event, such as high rainfall, may trigger
structural shifts to a different ecosystem state, which
remains extant even after the environmental condi-
tions revert.
Here, we briefly review evidence for the hypoth-
esis that arid ecosystems have alternative vegeta-
tion states depending on grazing and water avail-
ability and present a graphic model to summarize
the implications for the response to El Nin˜ o South-
ern Oscillation. We then present several field ob-
servations that support the idea that, depending on
grazing pressure, structural changes in arid ecosys-
tems may be triggered by ENSO events.
ALTERNATIVE STABLE STATES IN ARID
AND SEMIARID ECOSYSTEMS
Problems with the management of rangelands have
led to a growing consensus that these ecosystems
have distinct alternative stable states (for example
see, Westoby and others 1989; Filet 1994). Numer-
ous observations indicate that, rather than gradual
responses to changing conditions, these systems ex-
perience sudden transitions from one state to the
other when certain climatic conditions or manage-
ment actions occur (Westoby and others 1989). It is
also recognized that the ecosystem states are sepa-
rated by critical thresholds and that transitions
across such boundaries are extremely difficult to
reverse (Friedel 1991; Laycock 1991).
On a small spatial scale, patch dynamics in dry-
lands suggest the existence of alternative stable
states. Arid and semiarid ecosystems often show a
mosaic pattern, with patches that have a relatively
high biomass dispersed in a matrix of poorly vege-
tated land (for example, see Aguiar and Sala 1999).
Such patchy patterns can occur even on seemingly
homogeneous landscapes and often remain re-
markably constant over time, suggesting that the
different patch types represent alternative stable
states. For example, a comparison of the extension
of woody patches in an otherwise herbaceous ma-
trix in semiarid Chile revealed virtually no change
over a 30-year time span (Fuentes and others
1984), indicating that both types of patches are
highly resilient. Despite considerable variations in
rainfall over the 30-year period, all woody spots
remained woody and none of the open herbaceous
areas was invaded by shrubs or trees.
Although numerous distinct vegetation states
may be discerned in practice (Westoby and others
1989; Archer 1996), a rough simplification that
captures the essence of much of the discussion is
the one between woody, herbaceous, and bare veg-
etation states. In more disturbed and drier condi-
tions, the herbaceous state becomes dominated by
annual instead of perennial species (Fuentes and
others 1984; Fuls 1992), which ultimately leads to a
mostly bare condition. This is generally referred to
as “land degradation” or “desertification” and is
considered to be one of the main global ecological
threats (Kassas 1995). We briefly review the mech-
anisms that are thought to be responsible for the
stability of these distinct states, discuss the role of
spatial heterogeneity, and highlight the interactive
effects of water and grazing as major driving forces.
Mechanisms Stabilizing Alternative States
Overgrazing has become well known as a potential
mechanism for explaining alternative stable states.
The root of this theory is a classical graphic analysis
by Noy-Meir (1975) showing that the amount of
grazers needed to cause a collapse of vegetation
biomass is much larger than the amount needed to
subsequently suppress vegetation regrowth once it
is in a low-biomass state. If grazing alone were
responsible for maintaining vegetation in an over-
exploited state, we would expect that removal of all
grazers should eventually lead to recovery of the
vegetation. However, although in some cases re-
duction of herbivore density has allowed for the
recovery of degraded patches (for example, see Mc-
Naughton 1983; Belsky 1986), in other situations
vegetation did not recover (for example, see Sin-
clair and Fryxell 1985; Friedel 1991; Laycock 1991).
This implies that factors other than grazing could be
responsible for the irreversability of vegetation loss.
Soil-plant interactions are thought to play a major
role in determining the stability of perennial plant
cover (for example, see Rietkerk and van de Koppel
152 M. Holmgren and M. Scheffer
1997; van de Koppel and others 1997; Shachak and
others 1998). Perennial plants and their deposited
litter allow precipitation to be absorbed by the top-
soil and become available for uptake by plants.
When vegetation cover is lost, infiltration decreases
and water runoff increases. This triggers positive
feedbacks (for example, see Graetz 1991). Bare
ground enhances the formation of a soil crust by
direct impact of raindrops, and this physical crust
reduces further water infiltration. This reduction in
water availability decreases the possibilities of plant
establishment and growth and therefore of plant
cover. In addition, bare soils are also very suscepti-
ble to erosion by water and wind, causing a net
removal of nutrients from degraded patches. Due to
the nonlinear response of erosion to plant cover,
very small changes in plant cover close to a thresh-
old can cause very large changes in erosion (Mar-
shall 1973), which can precipitate a switch to a
barren state.
Facilitative interactions between plants are also be-
lieved to play a major role in stabilizing different
vegetation states. In arid ecosystems, even when
plant recruitment does not occur in open spaces,
seedling establishment is often possible under the
shade of existing “nurse” shrubs or trees, allowing
rejuvenation and long-term persistence of existing
vegetation. One of the main reasons behind this
nursing effect is an improvement in the seedling
water relations (Holmgren and others 1997). In the
shade of a nurse plant, air and soil temperatures are
lower, and water content of the superficial soil lay-
ers tends to remain higher (for example, see Geiger
1965; Joffre and Rambal 1988). Therefore, seed-
lings experience less thermal and water stress (for
example, see Valiente-Banuet and Ezcurra 1991;
Aguiar and Sala 1994). Other factors often contrib-
ute to this nursing effect—for example, soil nutrient
levels can be higher and herbivory levels lower
under the nurse plant (for example, Callaway
1995)—but water availability is clearly a critical
factor in arid ecosystems. The nursing effect, to-
gether with the fact that adult shrubs are relatively
less sensitive to drought and herbivory explains
why mature woody vegetation may persist and re-
juvenate in climatic regions where the establish-
ment of seedlings in the absence of nursing shade
from woody vegetation is impossible.
Fire plays a special role because it may destroy
woody plants that are relatively insensitive to graz-
ing. If certain vegetation components facilitate fire
ignition and are also relatively insensitive to fire
effects, this implies another positive feedback (for
example, see reviews in Westoby and others 1989;
Carpenter forthcoming). For instance, grasses may
produce a great deal of combustible litter, thus pro-
moting fires. However, because grasses recover
much faster from fires than shrubs, a positive feed-
back maintaining grass dominance is suggested.
In practice, a combination of factors is often re-
sponsible for observed shifts from one state to an
alternative one and the subsequent maintenance of
the latter state. For instance, human-induced fires
are thought to have caused a major decline of
Serengeti-Mara woodlands in Tanzania. However,
grazing and trampling by elephants and other her-
bivores is likely to be the factor preventing recovery
of the woodlands (Dublin and others 1990). Simi-
larly, grazing by livestock in the Sahel region has
caused vegetation loss, but the recovery of peren-
nial plant cover is strongly prevented by soil–plant
feedbacks.
Islands of Fertility and the Role of Shrubs
and Trees
Perhaps one of the most confusing aspects in the
study of land degradation is the fact that woody
plants are often seen as indicators of desertification,
whereas they also tend to provide a benign micro-
climate that facilitates the regeneration and growth
of their own offspring as well as other perennial or
annual plants. The key to understanding this para-
dox is spatial heterogeneity.
Runoff in degraded landscapes tends to lead to an
accumulation of soil and water on lower sites,
which become the most favorable places for plant
growth and therefore may develop relatively lush
vegetation. As perennial vegetation (herbaceous or
woody) traps eroded soil and water, these patches
tend to become “islands of fertility.” This patchiness
of resources and plant biomass is self-reinforcing.
As the islands of fertility become more fertile, the
desertic matrix becomes less fertile than in the sit-
uation in which resources are distributed evenly
over the landscape. Because in several regions ho-
mogeneous perennial grass cover has broken up
into a patchy mosaic of desert and fertile islands
with woody cover, shrubs are often regarded as
indicators of desertification (Schlesinger and others
1990). The new shrub state can be extraordinarily
resilient. Cattle ranchers in such areas are con-
stantly fighting against shrub encroachment to in-
crease the carrying capacity for livestock (Page
1970).
It is important to note that despite the fact that in
such disturbed landscapes, shrubs are indicators of
desertification, they are not a cause. By contrast, in
many regions, shrubs and trees may offer an im-
portant safeguard against land degradation, since
they provide a crucially important protection
El Nin˜ o and Arid Ecosystems 153
against soil erosion. Shachak and others (1998)
have demonstrated that reduction of shrub cover
increases nutrient leakage and desertification of
shrublands. Indeed, loss of forests and shrublands
has paved the way to historical land degradation in
various semiarid areas of the world. For example,
the destruction of evergreen forests in central Chile
(Fuentes 1994) and the Mediterranean basin (Pig-
natti 1995) has led to a dramatic impoverishment of
the soil and left the landscape dominated by bare
soil and an annual flora, with dispersed shrub
patches. Importantly, the invading shrubs in de-
graded shrublands usually belong to a different set
of species than the mostly larger woody species
dominating the original woodlands (see, for exam-
ple, Bisigato and Bertiller 1997).
The switches between different vegetation states
in the Mediterranean basin illustrate the effects of
loss of semiarid woodlands. This region was once
widely dominated by open woods and shrubs, but
due to excessive firewood harvesting, the landscape
has been progressively converted into one domi-
nated by grasses. Later on, it becomes dominated by
shrubs due to overgrazing. These transitions are
probably irreversible. Because of the loss of organic
soil and the failure of grasses to regenerate in open
spaces without the protection of scattered trees, a
shrub state cannot return, within an ecological time
scale, to a grass state even when grazing is relieved.
And since the transition from a grass to an arboreal
state is also very slow—if it is possible at all—the
regeneration of the original woodlands seems ex-
tremely difficult (Puigdefabregas and Mendizabal
1998). Irreversible losses of Californian woodlands
have been analyzed in this context as well (Hunts-
inger and Bartolome 1992).
AGRAPHIC MODEL
Despite confusing aspects such as the dual image of
woody plants in the literature on desertification,
the emerging grand picture is rather unambiguous.
Arid ecosystems tend to have multiple stable states
that are separated by critical thresholds of factors
such as water availability and biomass removal (for
example, through grazing). In this section, we con-
dense this observation into a simple graphic hy-
pothesis.
Woody Plants and Herbs as Alternative
Stable States
First, we focus on a simple abstract model of alter-
native stable states in vegetation biomass that we
think of as describing a single site (rather than a
landscape average). As a starting point, we take the
response of vegetation to grazing (Figure 1a) de-
scribed by Noy-Meir (1975) and others (for exam-
Figure 1. (a) Equilibrium biomass of vegetation in semiarid regions as a function of grazing pressure. The inflection points
(dots) of the curve are fold bifurcations that mark critical biomass removal rates. At grazing pressures higher than F
c
, the
vegetation can only be in a state with low biomass. At grazing pressure lower than F
r
, a high-biomass condition is the only
stable state. At intermediate grazing pressure (between F
c
and F
r
), a high-biomass state and a low-biomass state are
alternative equilibria (solid lines) of the system. Here, the dashed middle section of the sigmoidal curve represents an
unstable equilibrium that marks the border of the basins of attraction of these two stable branches. (b) Critical thresholds
of biomass removal (see 1a) as a function of water availability. Under wetter conditions, equilibrium biomass and the
critical grazing pressure for collapse (F
c
) or recovery (F
r
) are higher. A certain reduction of biomass removal rate (for
example, herbivory control) indicated by the vertical arrows may be sufficient to induce woodland recovery in a wet (El
Nin˜ o) year but not in a dry year.
154 M. Holmgren and M. Scheffer
ple, Rietkerk and van de Koppel 1997). In the ab-
sence of grazing, vegetation biomass is high,
matching the carrying capacity of the environment.
With increasing grazing pressure, vegetation bio-
mass decreases gradually until a critical threshold
(F
c
) is reached, at which point the vegetation col-
lapses to a very low level. Recovery from this over-
exploited state is only possible when grazing pres-
sure falls below another, much lower critical level
(F
r
). Note that in addition to grazing, various
mechanisms discussed earlier (facilitation, soil–
plant interactions, and fire) may in practice contrib-
ute to such a hysteresis in the response of vegeta-
tion. Note also that this simple abrupt switching
model is a starting point that can be modified for
heterogeneous environments and more than two
alternate states, as discussed later.
To see how the effects of grazing and rainfall may
interact, we extend the previous model (Figure 1a)
using the assumption that the critical grazing pres-
sure for collapse and recovery will increase with the
productivity of the environment. If conditions are
moister, the maximum grazing pressure that can be
sustained will be higher, implying that the critical
point for collapse (F
c
) will occur at higher grazing
pressure. Likewise, recovery (F
r
) may occur at
higher grazing pressure than in a drier situation.
The combined effects of water and grazing can best
be summarized by a so-called bifurcation graph
(Figure 1b). In terms of dynamic systems theory,
the critical points F
c
and F
r
are “fold bifurcations.”
Plotting these bifurcations against water availability
and grazing pressure gives a map of the system’s
behavior. In this case, the two bifurcation lines
mark three different areas. Above the collapse bi-
furcation (F
c
), only a degraded low-biomass state
exists; whereas below the recovery bifurcation (F
r
),
the high-biomass situation is the only stable state.
Between the two lines, the two states are alterna-
tive equilibria.
Note that, because we have not used an explicit
quantitative model, the picture is not computed but
rather inferred in a qualitative way from the infor-
mation we have. There are only two essential fea-
tures corresponding to two assumptions:
1. F
c
is higher than F
r
; this represents the alter-
native stable states assumption. The vegetation
collapse occurs at higher levels of biomass re-
moval and lower water availability than the
recovery.
2. F
c
and F
r
have a positive slope; this represents
the assumption that critical levels of biomass
removal for both collapse and recovery are
higher when water availability is higher.
El Nin˜o Effects
From the resulting graph, we can infer the expected
combined effects of altered levels of biomass re-
moval and changes in rainfall conditions during El
Nin˜ o. Where woodlands have been lost, they can
recover if water increases and/or biomass removal
decreases sufficiently to cross the critical line F
r
(Figure 1b). In regions where El Nin˜ o years are
wetter than intermittent periods, a rainy El Nin˜o
event will shift the system to the moister right-hand
side of the graph. In some cases, this could be
sufficient to allow the regeneration of some key
species, triggering a recovery of the woodland.
However, in other cases, the increased precipitation
during El Nin˜ o alone will not be sufficient; a reduc-
tion of the grazer density (lower biomass removal,
in general) would be needed to meet the critical
requirements (F
r
) for regeneration. Note that there
can be situations in which the complete exclusion
of grazers would be sufficient to allow regeneration
during rainy El Nin˜ o years, yet it would have no
effect during normal conditions (Figure 1b). Thus,
in cases where neither herbivore control nor rainy
El Nin˜ o events alone are sufficient to trigger wood-
land recovery, adjusting herbivore control to rainy
El Nin˜ o events may result in vegetation regenera-
tion. Note that although we use the term “El Nin˜ o,”
the idea applies to rainy events in general. It is
important to remember that in various regions of
the world, La Nin˜ a events are wet and El Nin˜o
episodes are dry.
Significantly, although the pulse of rainfall asso-
ciated with an ENSO event is a temporal condition,
a resulting recovery may be permanent due to the
resilience of woodland (an alternative stable state).
As long as the drought is not severe and biomass
removal is moderate enough to remain below the
critical line F
c
even in dry years, a woodland will
remain extant once it is well established. As men-
tioned earlier, this is due to the nursing effect of
adult trees and shrubs on seedlings and to the rel-
ative tolerance of grown individuals to grazing. The
potentially long-lasting effect of a brief regeneration
episode makes it worthwhile to invest in making
the most out of rainy years, such as the ones cor-
related to the ENSO events.
Most likely, grazer control is crucial to allow
woodland regeneration during rainy years, even if
the grazer density at the start of the year allows for
initial seedling establishment. This is because not
only plants but also herbivores respond to the in-
creases in rainfall associated with El Nin˜ o. Farmers
tend to increase the cattle stock in view of the lush
growth of herbs and grasses; in addition, popula-
El Nin˜ o and Arid Ecosystems 155
tions of natural herbivores increase following the
productivity peak in plants (see, for example,
Meserve and others 1999). Obviously, this may lead
to elimination of established seedlings and poten-
tially even leave the final condition of the vegeta-
tion deteriorated rather than improved after a rainy
year.
Beyond Two Stable States
Under dry conditions, the perennial herbaceous
vegetation can become prone to overgrazing and
erosion, leading to patches of bare soil where only
annual herbs will grow during short episodes fol-
lowing precipitation events. This bare state can be
considered to be another stable state because re-
colonization by perennial plants is extremely diffi-
cult. The graphic model can be expanded to include
this desertification process (Figure 2). Obviously,
the absolute, but also the relative, position of the
thresholds for catastrophic transitions between the
three states may differ from case to case. The usual
sequence with decreasing moisture and/or increas-
ing biomass removal is probably woodlands pe-
rennial herbs annual herbs/bare soil. However,
direct transitions from woodland to patches of an-
nual herbs and/or bare soil may also occur, depend-
ing on the relative position of the three fold bifur-
cations in the system.
Distinguishing three alternative stable states also
facilitates an understanding of the phenomenon in
which homogeneous pastures have been replaced
in some regions by woody patches in a matrix of
bare soil (Schlesinger and others 1990). Indeed, if
we plot equilibrium biomass as a function of avail-
ability of resources such as water or nutrients (Fig-
ure 2), it is easy to see that if the average resource
level sustains herbaceous vegetation, concentration
of the resource in fertile patches at the cost of
resource depletion in the rest of the matrix could
lead to bare soil with patches of woody vegetation.
Wet years may induce shifts from bare to either
herbaceous or woody vegetation depending on
grazing and other properties of the ecosystem as
reflected by the position of the bifurcation points in
models such as the one presented. Obviously, the
model is not intended to describe these complicated
problems in any detail. The main point we wish to
illustrate is that the basic idea of studying the effect
of El Nin˜ o on systems with multiple equilibria can
be easily extended beyond the simple case that we
outlined previously.
An important general implication of spatial het-
erogeneity is that switches such as those depicted in
Figure 1a may occur asynchronously at different
sites in the landscape. Depending on variation in
factors such as topographical exposure, critical wa-
ter and grazing levels leading to collapse or recovery
(F
c
and F
r
) will, in practice, differ from site to site.
Although a thorough treatment of the implications
of such heterogeneity is beyond the scope of this
paper, it is easy to see that the overall change in a
heterogeneous landscape will be more gradual than
in the simple model (Figure 1).
An example may help to clarify this. Suppose we
have a hypothetical landscape that is homogeneous
except that one half of it has a different, more fertile
soil type than the other half. With gradually in-
creasing grazing pressure, vegetation may then col-
lapse first in the unfertile part and later in the fertile
part due to the difference in plant growth rates
(much as in the reasoning for the effects of water
illustrated in Figure 1b). Thus, if we consider the
average vegetation biomass in the entire landscape,
vegetation decline occurs in two consecutive steps
Figure 2. An extension of the model presented in
Figure 1 to a situation with three alternative states—
woodlands, herbs, and bare soil. Transitions between
the states can now occur at four fold bifurcations.
Critical removal rates are indicated for collapse of
woodland (F
cw
) or herbaceous vegetation (F
ch
) and for
recovery of woodland (F
rw
) or herbaceous vegetation
(F
rh
). The three-alternative stable state model is con-
sistent with the idea that if resources such as water and
nutrients become patchily distributed, a homogeneous
herbaceous vegetation that could be sustained by the
average resource level (middle vertical dotted line) can
be transformed in a matrix of bare soil (right-hand
side) with woody patches (left-hand side). Wet years
can allow the transformation of bare soil to either
herbaceous vegetation or woodland depending on the
position of the various fold bifurcations, which are
affected (among other things) by grazer density.
156 M. Holmgren and M. Scheffer
rather than as one single collapse (as in Figure 1a).
Extending this reasoning, if many soil types rather
than just two distinct types exist, the response to
increasing grazing pressure on a landscape scale will
resemble a declining staircase with many small
steps. By continuity, it follows that a landscape with
a continuous range of local conditions will respond
in a smooth way (Figure 3). Importantly, even if
environmental heterogeneity is large, we should
expect the essential aspect of hysteresis to be pre-
served in the sense that the paths of recovery and
“collapse” are different. Obviously, real patch dy-
namics are more complicated due, for instance, to
spatial interactions between adjacent areas. How-
ever, this line of reasoning suffices to explain that
spatial heterogeneity will tend to make landscape-
scale responses more gradual than local dynamics.
DISCUSSION
Field Evidence
Several observations support the idea that ENSO
events can trigger long-lasting shifts in arid vegeta-
tion depending on grazing pressure. Rainy condi-
tions during ENSO events have been shown to trig-
ger the regeneration of woody vegetation in many
arid ecosystems (Holmgren and others 2001). For
example, in semiarid Australia, successful seedling
establishment of mulga (Nicholls 1992), as well as
Eucalyptus and conifer woodlands (Austin and Wil-
liams 1988), have been linked with wetter periods
during La Nin˜ a episodes. Similarly, in the shrub-
land–grassland transition zone of the Chihuahuan
desert, large increases in shrub cover over the past
decades have been related to episodes of increased
winter precipitation that seem to correspond largely
to El Nin˜ o events (Brown and others 1997). The
interaction of El Nin˜ o effects with grazing is nicely
illustrated by the dynamics in the Sonoran desert,
where cactus seedlings establish during rainy El
Nin˜ o years on ungrazed sites but not on sites grazed
by burros, where the nurse shrubs under which
cactus seedlings usually survive the drier periods
have been extirpated (Bowers 1997). Moreover,
long-term field experiments in northern Chile show
that shrub cover can increase during a rainy El Nin˜o
event when the main herbivores are excluded or
when predators are allowed to feed on the herbi-
vores (Gutie´rrez and others 1997).
Some of the most striking examples of the inter-
active effects of interannual variations in precipita-
tion and grazing in plant recruitment and the sub-
sequent switches in ecosystem state have been
described for semiarid Australia (Austin and Wil-
liams 1988). In the 1830s, the Pilliga scrub was an
open woodland of large Eucalyptus and Callitris trees
and grasses. Over the next 40 years, the system was
heavily invaded by shrubs due to management
practices consisting of less frequent fires and in-
creased grazing by cattle and sheep. The severe El
Nin˜ o droughts of 1876 –77 put an end to grazing.
During the subsequent rainy year of 1878, regen-
eration of the original woodland trees was so dense
that grazing became impossible. The system had
switched into a very dense forest. A few years later,
European rabbits invaded and made further regen-
eration of the original trees impossible. But the
forest remained dense until the early 1950s due to
slow sapling growth. The forest remained dense
until the early 1950s. Then a second natural exper-
iment occurred during the rainy aftermath of the
1951 ENSO when, coincidentally, rabbits were
practically eliminated by the myxoma epizootic.
Once again, the combined effects of rainy condi-
tions and absence of grazing triggered the natural
regeneration of the woodland trees, which since
then has occurred in every suitable wet year.
Using El Nin˜o for Ecosystem Restoration
These observations support the idea that the resto-
ration of degraded arid ecosystems might be
achieved in an efficient way by adjusting grazer
control to the occurrence of temporal windows of
opportunity opened by the ENSO phenomenon.
However, to apply this idea in practice, the critical
water and herbivory conditions (F
r
) for the estab-
Figure 3. Vegetation biomass averaged over a heteroge-
neous landscape is predicted to show hysteresis, but it
responds in a more gradual way than the simple base
model to increases and decreases in grazing pressure (see
Figure 1a).
El Nin˜ o and Arid Ecosystems 157
lishment of plant species in a given area must be
known. Only this information can indicate the level
of herbivory control that will be needed and
whether it is essential to wait for El Nin˜ o or not.
Field experiments with controlled herbivore density
and water additions seem the most promising ap-
proach to establishing critical thresholds. But the
ultimate test of the theory would be to perform
grazer control experiments over a range of field
sites during natural wet episodes. One such large-
scale field experiment in northern Chile has already
been following the effects of wet El Nin˜ o events on
plants and animals for several years (Jaksic and
others 1997). Past ENSO events may also be inter-
preted as natural experiments; consequently, den-
drochronological databases could be used for testing
the hypothesis that tree establishment in semiarid
regions occurs in pulses associated with wet peri-
ods. Unfortunately, due to the lack of data on past
grazing pressure, the link with herbivory is not
easily established through this line of research (for
example, see Milton and others 1997; Villalba and
Veblen 1997).
Although the use of the windows of opportunity
offered by El Nin˜ o still faces many challenges, the
possibility merits further exploration. Grazer con-
trol appears to be an essential aspect, but measures
other than reforestation and grazer control may
also be used to stimulate recovery. For instance, the
addition of large woody branches to degraded Aus-
tralian areas has proved to be very effective in im-
proving water infiltration rates as well as soil carbon
and nitrogen, and in providing a modified microcli-
mate in which the establishment and growth of
perennial grasses is greatly enhanced (Ludwig and
Tongway 1996; Tongway and Ludwig 1996). Re-
gardless of the precise measures, the evidence sum-
marized in our graphic model suggests that the
multiplicity of stable states in arid vegetation makes
it possible to design restoration strategies that can
make use of wet ENSO years to induce vegetation
and ecosystem restoration.
ACKNOWLEDGMENTS
We thank R. Urba´ for her inspiring observations on
Chilean drylands and J. A. Ludwig for his construc-
tive and thorough review of the manuscript.
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El Nin˜ o and Arid Ecosystems 159
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(1) Multiple stable states in ecosystems have been proposed on theoretical grounds, and examples have been offered, but direct tests of the predictions are lacking. A boundary between states exists if: (i) a system when disturbed from one state to another does not return to its original state once the cause of the disturbance returns to its original value; and (ii) a second factor takes over and holds the system in the new state. We examine these predictions for two stable states in the woodlands of the Serengeti-Mara ecosystem in East Africa. (2) Woodlands in natural areas of savannah Africa have declined over the past 30 years. Three general hypotheses have been proposed: (i) expanding human populations have concentrated elephants into protected areas, elephants then caused the decline of woodlands but man-induced fires prevented regeneration (two stable states); (ii) fires caused the decline and also prevented recovery (one stable state); (iii) fires caused the decline while elephants inhibited recovery through density-dependent mortality of seedlings (two stable states). (3) Two time periods, the 1960s when woodlands changed fastest and the 1980s when grasslands prevailed, produced four specific hypotheses. (i) `The 1960s elephant hypothesis' and (ii) `the 1960s fire hypothesis' hold that elephants and fire, respectively, caused woodland change. (iii) The `1980s elephant hypothesis' and `the 1980s fire hypothesis' hold that these factors, respectively, prevented woodland recovery. (4) From experiment and observation of seedling recruitment, mortality due to combinations of burning rates, elephant browsing, wildebeest trampling, and antelope browsing was estimated and used to model tree population dynamics; predictions for rates of decline and increase were compared with independent estimates from aerial photographs. (5) Maximum rates of elephant and antelope browsing could not have caused the observed decline of woodlands in the 1960s. The most conservative burning rates in the 1960s, without elephants, could have caused a decline consistent with the 1960s fire hypothesis. (6) The combined impact of fire and browsing most closely matched the observed rate of woodland loss. (7) Wildebeest grazing in the 1980s reduced dry grass and minimized fire incidence. The model predicted that fire mortality and wildebeest grazing could not maintain the present grassland state. (8) The present high elephant density was sufficient to prevent an increase in the woodlands consistent with the 1980s elephant hypothesis. Wildebeest trampling and other browsers ensures that the vegetation is currently stable in a grassland state. (9) Thus, an external perturbation, such as fire, was necessary to change the vegetation from woodland to grassland. Elephants were unable to cause such a change. Once the grassland was formed, however, elephants were able to hold it in that state. These results are consistent with the third general hypothesis that there are two stable states of woodland and grassland, the latter maintained by herbivores. (10) Simulation of conditions in the 1890s suggests that the rinderpest epidemic combined with elephant hunting could have caused the woodland regeneration observed before the 1950s. Therefore, (i) savannah woodlands may regenerate in pulses as evenaged stands, and (ii) there may have been more grassland in Africa before 1890. This longer time-scale view of the dynamics of vegetation has implications for the conservation of elephants and their habitats.
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Desertification appears to have been an irreversible perturbation. It has been possible to construct a feasible explanation of the ecological processes involved in desertification using a very simple model of grazed arid ecosystems. The model has just four components (soils, herbaceous vegetation, herbivores, people), and two external influences (external society and climate). They are connected by ten interactions. The nomadic pastoral system has persisted for a considerable time experiencing droughts known to have been as severe as those of 1969-75. Therefore we may conclude that this system had evolved as a stable, resilient strategy. A nomadic pastoral system was stabilized by the negative feedbacks between the grazed, the grazer, and the grazier. The ecological processes that comprised these feed-backs, eg energy and nutrient flow, were determined by the characteristics of arid ecosystems wherein the influence of space and time have exaggerated importance. The nomadic system of the Sahel was destabilized by the effective removal of two negative controlling feedbacks. The resultant pulse of disturbance and the drought precipitated a negative feedback oof soil erosion which desertified the productive base of the system. This pulse of response, a new and severe negative feedback, passed back up the system only to solicit a further positive feedback of destabilizing aid to renew the cycle again. -from Author
Chapter
There are many similarities between Spanish and Californian Quercus woodlands and savanna. Both are located in Mediterranean climate zones, and are used predominantly for livestock grazing. The Californian overstory is dominated by one or a combination of five Quercus species and their hybrids: Quercus douglasii H.andA., Q. agrifolia Nee., Q. wislizenii A.DC., Q. lobata Nee., and Q. englemannii Greene (blue, coast live, interior live, valley, and Englemann oaks). In southern Spain and Portugal, Quercus woodland overstory is predominantly one or a combination of two Quercus species, Quercus ilex L. (holm oak) and Quercus suber L. (cork oak). The underlying natural and semi-natural ecological dynamics of the Quercus woodlands of Spain and California are different, and it follows that the management practices employed also differ. The greatest point of contrast between California and Spain is in the intensity and diversity of management goals and practices. A state-transition model for comparing the ecological dynamics of Quercus woodlands and savanna in California and southern Spain is developed and examined. The highly simplified model is an analytic tool of use in organizing research and developing management practices. States are reached and maintained in different ways in Spain and California, but their appearance and their function in each landscape are quite similar.
Article
Fire, modified fencing and a 'grassbank' are rejuvenating rangeland. Where woody weeds once grew, the grass is stirrup high.
Book
The book begins with an overview set within an historical context followed by a synthesis of developments in theories and dynamics that have generated the present modelling approaches to forecasting the phenomenon. The second half of the book provides composites illustrating the near-global physical impacts of ENSO phases, and brief descriptions of findings from the growing number of studies examining biological, chemical and ecological impacts. The book is completed by a time sequence showing simultaneous global MSLP and SST anomalies from 1871-1994. The CD-ROM provides monthly MSLP and SST anomalies from 1871-1994.
Article
We discuss what concepts or models should be used to organize research and management on rangelands. The traditional range succession model is associated with the management objective of achieving an equilibrium condition under an equilibrium grazing policy. In contrast, the state-and-transition model would describe rangelands by means of catalogues of alternative states and catalogues of possible transitions between states. Transitions often require a combination of climatic circumstances and management action (e.g., fire, grazing, or removal of grazing) to bring them about. The catalogue of transitions would describe these combinations as fully as possible. Circumstances which allow favorable transitions represent opportunities. Circumstances which threaten unfavorable transitions represent hazards. Under the state-and-transition model, range management would not see itself as establishing a permanent equilibrium. Rather, it would see itself as engaged in a continuing game, the object of which is to seize opportunities and to evade hazards, so far as possible. The emphasis would be on timing and flexibility rather than on establishing a fixed policy. Research under the state-and-transition model would aim to improve the catalogues. Frequencies of relevant climatic circumstances would be estimated. Hypotheses about transitions would be tested experimentally. Often such experiments would need to be planned so that they could be implemented at short notice, at an unknown future time when the relevant circumstances arise.
Article
The establishment phase of Neobuxbaumia tetetzo, a giant columnar cactus, occurs mostly beneath the canopies of trees and shrubs which act as nurse plants. This pattern cannot be attributed to preferential seed dispersion, as Neobuxbaumia fruits open while still on the plant, dropping c1000 seeds fruit-1 randomly around the parent plant. Mimosa luisana is the most abundant shrub in the community. Seed germination was lowest in open spaces. In all treatments, exclusion from predators significantly increased seedling survival. Only shaded treatments had live individuals at the end of the experiment, 2 yr later. Results suggest that the nurse-plant effect between N. tetetzo and M. luisana is chiefly the result of differential survival in shaded microsites with less direct solar radiation, and consequently with lower daytime temperatures and lower evaporative demand. Field samplings were conducted in two Mexican deserts located outside the tropical belt: the Vizcaino Desert in Baja California and the Gran Desierto de Altar in Sonora. In these deserts direct solar radiation has a southern azimuth all year round. Five of six succulent species analysed showed a significant pattern of greater establishment on the shaded north sides of nurse plants. -from Authors