Dryland Degradation: Symptoms,
Stages, and Hypothetical Cures
W. R. J. Dean
Suzanne J. Milton
Morné du Plessis
W. Roy Siegfried
Abstract—We studied shifts in the relative abundance and species richness of native plants and animals in dwarf shrubland in
the Karoo, South Africa. With heavy grazing, there is a shift in
the ratio of perennial to annual plants, both in abundance and
species, and an increase in seeds on the soil surface. Along with
this, there is a shift in the abundance of seed-harvesting ants,
because there is more available seed in the heavily grazed rangeland. There are also changes in the abundance of certain rootfeeding insects, such as cicadas. The relative abundance of members of bird foraging guilds changes, with increases in the numbers
of nomadic granivores. We conclude that, as rangelands degrade,
changes in the ratio of ephemeral to perennial plants and concomitant changes in seed abundance cause a cascade that runs
through the granivore community, increasing variability in populations and making rangelands more difficult to restore.
Although ecologists have expressed some concern about
the apparent progressive loss of secondary productivity and
diversity of arid and semi-arid rangelands over the past
century (see review in Milton and others 1994), shifts in
the abundance of plant species other than those palatable
to domestic livestock or losses of native animals have received less attention. Although no specific study has demonstrated that heavy grazing by domestic livestock has led
to the extinction of species and the loss of species richness
from an area, several studies have shown that changes in
habitat diversity caused by grazing can affect some animal
species (see review in Milton and others 1994).
Karoo rangelands lack stability but are apparently
strongly resistant (see Pimm 1986). We have suggested
elsewhere that degradation in Karoo rangelands proceeds
in measurable stages, with changes in the relative abundance of plants (Milton and others 1994). Severely degraded
rangelands in general may not return to their original state
even when rested for decades (Westoby and others 1989;
O’Connor 1991). Conservation problems in the arid parts
of South Africa have recently received some attention
(Siegfried 1989) and it is of interest to examine which
In: Roundy, Bruce A.; McArthur, E. Durant; Haley, Jennifer S.; Mann,
David K., comps. 1995. Proceedings: wildland shrub and arid land restoration symposium; 1993 October 19-21; Las Vegas, NV. Gen. Tech. Rep.
INT-GTR-315. Ogden, UT: U.S. Department of Agriculture, Forest Service,
Intermountain Research Station.
Richard Dean, Sue Milton, and Morné du Plessis are Senior Research
Officers, and Prof. Roy Siegfried is Director of the Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7700,
South Africa.
178
species, other than palatable plants, are affected by changes
in rangeland status. Our preliminary findings suggest that
initial changes involve subtle differences in plant and animal populations and guild structures as rangelands degrade.
The reduction in the relative abundance of some species
may lead to cascading effects and the disruption of seed
dispersal and other mutualistic relationships. As part of
a study aimed at quantifying the status of semi-arid and
arid rangelands, we present preliminary data on the changes
in vegetation and seed abundance, and in insect and avian
assemblages in heavily grazed Karoo rangeland.
Methods and Study Sites
We censused seeds and birds once a month for 36 months
at 40 randomly selected sites in dwarf shrubland on sheep
and Angora goat ranches in the southern Karoo. Of these
40 sites, we classified three as lightly grazed, 19 as moderately grazed, and 18 as heavily grazed. All sites were situated between Tierberg Karoo Research Centre (33°10' S,
22°17' E) and Rietbron (32°54' S, 23°10' E), South Africa.
Rainfall for the general area averages about 160 mm per
year, with a coefficient-of-variation of about 22%. There
is a slight seasonal difference in the pattern of rainfall between the two places with Tierberg receiving about 10%
more of its rain in winter.
Seeds were censused by sweeping the soil surface with a
mini-vacuum cleaner for two 30-s periods at each site. The
first sweep was over bare soil and the second sweep under
the cover of vegetation. Samples were bagged and sorted
later in the laboratory, when all seeds were counted. Birds
were censused for 5 min at each site, and all birds seen
within 100 m were counted.
In addition, we intensively sampled three sites at
Tierberg Karoo Research Centre. Tierberg KRC has been
fully described by Milton and others (1992). Our study
sites at this locality were:
• A fenced exclosure 1 km square from which all domestic livestock have been excluded for the last 7 years
(the Tierberg study site).
• Rangeland immediately adjacent to the Tierberg site
on which Merino sheep have been stocked at 1 sheep
per 6 ha for the past 40 years.
• Adjacent rangeland that has a history of overstocking
and heavy grazing by ostriches and wool and mutton
sheep until about 1960, and that subsequently has
been less heavily stocked.
The number of seeds on the soil surface is shown in Figure 3. Although there is little difference in mean numbers,
the data show a trend towards increasing numbers of seeds
on the soil surface as grazing pressure increased. Surface
seed assemblages in lightly grazed sites were dominated
by perennial shrubs, whereas seed assemblages in heavily
grazed sites were dominated by annual forbs and annual
grasses.
Insects
Numbers of ants caught in pit-traps at Tierberg showed
trends towards an increase in numbers of granivorous ants
with increased grazing pressure (Fig. 4), but there were no
clear trends in insectivorous ants. Figure 5 shows numbers of harvester ant nest-mounds at Tierberg, supporting
the trend shown by the pit-trap data. Counts of cicada
Figure 1—The effect of grazing pressure
on the abundance of ephemerals and
woody shrubs at 40 sites in the southern
Karoo. Lightly grazed, n = 3, moderately
grazed, n = 19, heavily grazed, n = 18.
On all three Tierberg sites, we pit-trapped insects once a
month for 24 months (methods given in Dean and Griffin
1993), censused cicada (Quintillia sp.) emergence holes in
random 10-m2 plots, counted harvester ant (Messor capensis) nest-mounds on 1-km x 10-m transects and counted
Tractrac Chats (Cercomela tractrac) and Karoo Chats (C.
schlegelii) on transects 1 km long once a week for 100 weeks.
All birds seen within 50 m on either side of the transect line
were counted, so we effectively sampled an area of 10 ha.
Figure 3—The number of seeds on
the soil surface in relation to grazing
pressure. Sites as Figure 1.
Results
Vegetation
Plants showed a general trend towards more ephemerals
and fewer perennials with increasing grazing pressure
(Fig. 1). Total plant cover (of all species) decreased with
increasing grazing pressure (Fig. 2).
Figure 4—The number of granivorous
and insectivorous ants pit-trapped at
Tierberg. For lightly grazed areas, n = 40
traps, and for moderately and heavily
grazed areas, n = 20 traps.
Figure 2—Changes in relative
plant cover with increasing grazing
pressure. Sites as Figure 1.
179
Figure 5—The number of harvester
ant (Messor capensis) nest-mounds
counted in 1-km x 10-m transects at
Tierberg. For each site, n = 10.
emergence holes (Fig. 6) show decreasing numbers of holes
with increasing grazing pressure.
Birds
Numbers of birds followed similar patterns to ants, with
granivorous species increasing with increasing grazing pressure (Fig. 7A), and similar inconclusive results in insectivores (Fig. 7B). Numbers of resident species of birds did not
show clear trends (Fig. 8A), but numbers of nomadic birds
increased with increasing grazing pressure (Fig. 8B). Resident Tractrac Chats and Karoo Chats censused at Tierberg
showed diametrically opposed trends, with the Tractrac
Chat decreasing with increasing grazing pressure (totally
absent from the heavily grazed site), and the Karoo Chat
increasing in numbers with increasing grazing pressure
(Fig. 9).
Figure 6—The number of cicada
(Quintillia sp.) emergence holes in
10-m2 plots at Tierberg. For each
site, n = 10.
Figure 7—The number of
granivorous (A) and insectivorous (B) birds counted in 10ha plots. Sites as Figure 1.
Figure 8—The number of resident (A) and nomadic (B) birds
counted in 10-ha plots. Sites
as Figure 1.
180
and Dean 1992). Changes in the abundance of certain rootfeeding insects, such as cicadas, can affect the amount of
water in the soil because infiltration is significantly improved in patches where there are cicada emergence holes
(Dean 1992).
The relative abundance of members of bird foraging guilds
changes with grazing pressure. Granivorous birds apparently increase in numbers with increasing grazing pressure,
a pattern also found by Baker and Guthery (1992) in south
Texas. Tidemann (1990), however, found that granivorous
birds (grass-finches) were negatively affected by cattle grazing in northern Australia. On heavily grazed rangelands
in the southern Karoo, there are also increases in the numbers of such nomadic species as sandgrouse, granivorous
larks and buntings that feed on the seeds of ephemerals
(Maclean 1993). This increase in the number of nomads
indicates a lack of stability in seed resources, supported to
some extent by the high variance of the samples obtained
in the seed sweeps (Fig. 3).
These preliminary data suggest that, as rangelands degrade, changes in the ratio of ephemeral to perennial plants
and concomitant changes in seed abundance cause a cascade that runs through the granivore community, resulting
in increased variability in populations (see Pimm 1986).
This will influence both the value of the area as a rangeland
and its potential for conservation and restoration. In natural, lightly grazed rangelands, the biomass and composition
of vegetation varies with random variations in rainfall.
The avifauna is largely resident, with fluctuations in the
proportion and abundance of nomadic species depending on
particular rainfall events. At the next stage, reductions in
the recruitment of palatable plants allows populations of
unpalatables to increase (Milton and others 1994) and produce abundant seed. This allows harvester ants to increase
in abundance, increasing the number of nutrient patches
(Dean and Yeaton 1994) and providing germination sites
for seedlings (Dean and Yeaton 1992) that further increase
the number of unpalatable plants. Plant species that fail
to recruit are lost or drastically reduced in numbers, together with mutualistic relationships with animals and
other plants (for example, nurse and nursed plants). As
the rangelands degrade further, the biomass and productivity begins to fluctuate as ephemeral plants benefit from the
loss of perennial cover, and the number of nomadic seedeating animals increases disproportionately. Rangelands
finally reach a stage where vegetation is largely absent,
seed numbers have been markedly reduced and few animals of any species are evident.
Restoration of animal communities, particularly invertebrates and birds, can be achieved only through the restoration of habitats and the recolonization of the area by animals from adjacent areas. The use of mixed species patches
of plants, particularly including species that produce juicy
fruits, and the creation of other microhabitats will encourage the colonization of the area by animals. We have no
data on the time needed for the restoration of severely degraded rangelands, but observations suggest that recovery
and restoration time is in the order of decades, and that
the more extensive a degraded patch, the more time it will
require to restore.
Figure 9—The numbers of Tractrac
Chats (Cercomela tractrac) and Karoo
Chats (Cercomela schlegelii) counted
on 1-km transects at Tierberg.
Discussion
Vegetation
The total canopy cover of perennial plants was generally
lower in sites with a history of heavy grazing. In the hot
summer months, many shrubs were leafless and the intershrub gaps were bare. After autumn rains, the inter-shrub
gaps on over-grazed range were colonized by annual grasses
(Enneapogon, Aristida and Karoochloa spp.) and forbs
(Galenia, Leysera, Trianthema, Tribulus and Gazania spp.).
The densities and compositions of these ephemeral assemblages differed greatly from year to year. A decrease in perennial biomass or cover and an increase in ephemeral biomass has been observed elsewhere in arid rangelands that
have been overgrazed (Andrew and Lange 1986). Temporal fluctuations in ephemeral cover influence both forage
flow for domestic livestock and the availability of resources
for granivores.
Certain insects benefit from the resources provided by
both ephemeral and perennial plants of heavily grazed
sites. Seed-harvesting ants apparently take the most
abundant, most available seed, and are not constrained
by the toxicity of the plant (Milton and Dean 1993). For
example, the harvester ant appears to increase in abundance in karroid shrublands with grazing regimes that
favor medium- to large-seeded plants (Milton and Dean
1993). However, in semi-arid grassland, Vorster and others (1992) found that the number of harvester ant nests
was correlated with grazing pressure, and that the lowest
nest density was found on “continually overgrazed” sites.
Cicadas appear to be dependent on a stable food resource
in the form of roots of perennial shrubs, because the nymphs
are subterranean feeders on root xylem sap. Cicada density
was positively correlated with woody shrubs, and decreased
as ephemerals and grazing pressure increased (Milton
181
Acknowledgments
Milton, S.J; Dean, W.R.J. 1992. An underground index of
rangeland degradation: cicadas in arid Karoo shrublands,
southern Africa. Oecologia. 91: 288-291.
Milton, S.J.; Dean, W.R.J. 1993. Selection of seeds by harvester ants Messor capensis in relation to condition of
arid rangeland. J. Arid Environ. 24: 63-74.
Milton, S.J.; Dean, W.R.J.; Kerley, G.I.H. 1992. Tierberg
Karoo Research Centre: history, physical environment,
flora and vertebrate fauna. Trans. Roy. Soc. S. Afr. 48:
15-46.
Milton, S.J; Dean, W.R.J.; du Plessis, M.A.; Siegfried, W.R.
1994. A conceptual model of arid rangeland degradation.
BioScience. 44: 70-76.
O’Connor, T.G. 1991. Local extinction in perennial grasslands: a life-history approach. Amer. Nat. 137: 735-773.
Pimm, S.L. 1986. Community stability and structure. In:
Soulé, M.E., ed. Conservation biology: the science of scarcity and diversity. Sunderland, MA: Sinauer Associates:
309-329.
Siegfried, W.R. 1989. Preservation of species in southern
African reserves. In: Huntley, B.J., ed. Biotic diversity in
southern Africa: concepts and conservation. Cape Town:
Oxford University Press: 186-201.
Tidemann, S.C. 1990. Relationships between finches and
pastoral practices in Northern Australia. In: Pinowski,
J.; Summers-Smith, J.D., eds. Granivorous Birds in the
Agricultural Landscape. Warsaw: Polish Scientific Publishers: 305-315.
Vorster, H.; Hewitt, P.H.; van der Westhuizen, M.C. 1992.
Nest density of the granivorous ant Messor capensis
(Mayr) (Hymenoptera: Formicidae) in semi-arid grassland of South Africa. J. Afr. Zool. 106: 445-450.
Westoby, M.; Walker, B.; Noy-Meir, I. 1989. Opportunistic
management for rangelands not at equilibrium. J. Range
Manage. 42: 266-274.
This report is a contribution to the Desertification Programme of the FitzPatrick Institute, University of Cape
Town. The Programme is funded by the Foundation for
Research Development, the Department of Environment
Affairs and the Southern African Nature Foundation. Attendance at the Wildland Shrub and Arid Land Restoration
Symposium was funded by the Foundation for Research
Development and the FitzPatrick Institute.
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