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The Return of the Giants: Ecological Effects of an Increasing Elephant Population


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Northern Botswana and adjacent areas, have the world's largest population of African elephant (Loxodonta africana). However, a 100 years ago elephants were rare following excessive hunting. Simultaneously, ungulate populations were severely reduced by decease. The ecological effects of the reduction in large herbivores must have been substantial, but are little known. Today, however, ecosystem changes following the increase in elephant numbers cause considerable concern in Botswana. This was the background for the "BONIC" project, investigating the interactions between the increasing elephant population and other ecosystem components and processes. Results confirm that the ecosystem is changing following the increase in elephant and ungulate populations, and, presumably, developing towards a situation resembling that before the reduction of large herbivores. We see no ecological reasons to artificially change elephant numbers. There are, however, economic and social reasons to control elephants, and their range in northern Botswana may have to be artificially restricted.
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© Royal Swedish Academy of Sciences 2004
Ambio Vol. 33 No. 6, August 2004
The Return of the Giants: Ecological Effects
of an Increasing Elephant Population
Christina Skarpe, Per Arild Aarrestad, Harry P. Andreassen, Shivcharn S. Dhillion, Thatayaone Dimakatso,
Johan T. du Toit, Duncan, J. Halley, Håkan Hytteborn, Shimane Makhabu, Moses Mari, Wilson Marokane,
Gaseitsiwe Masunga, Ditshoswane Modise (†), Stein R. Moe, Rapelang Mojaphoko, David Mosugelo,
Sekgowa Motsumi, Gosiame Neo-Mahupeleng, Mpho Ramotadima, Lucas Rutina, Lettie Sechele, Thato B.
Sejoe, Sigbjørn Stokke, Jon E. Swenson, Cyril Taolo, Mark Vandewalle and Per Wegge
Northern Botswana and adjacent areas, have the worldʼs
largest population of African elephant (
Loxodonta afri-
). However, a 100 years ago elephants were rare
following excessive hunting. Simultaneously, ungulate
populations were severely reduced by decease. The eco-
logical effects of the reduction in large herbivores must
have been substantial, but are little known. Today, how-
ever, ecosystem changes following the increase in ele-
phant numbers cause considerable concern in Botswana.
This was the background for the “BONIC” project, inves-
tigating the interactions between the increasing elephant
population and other ecosystem components and pro-
cesses. Results confirm that the ecosystem is changing
following the increase in elephant and ungulate popula-
tions, and, presumably, developing towards a situation
resembling that before the reduction of large herbivores.
We see no ecological reasons to artificially change el-
ephant numbers. There are, however, economic and
social reasons to control elephants, and their range in
northern Botswana may have to be artificially restricted.
Many of the large African rivers flow through areas, that, for at
least part of the year, are devoid of other surface water. In such
areas, animals that are water dependent may aggregate at the
permanent water in the dry season and disperse in the wet sea
son to areas with temporary water supplies and higher quality
and/or quantity of forage. Almost certainly, this is how the eco
system around the Chobe River functioned for millennia. About
150 years ago, however, the Chobe system changed. Hunting of
elephants for ivory had been practiced for at least 1000 years
(1), but on a small scale and, presumably, within a sustainable
level. With increasing markets in the 19
century the hunting of
elephants increased (1, 2), and towards the end of the century
elephants were rare in Botswana. The Chiefs and the Protector
ate Authorities reacted by imposing regulations on hunting of
elephants, and in the 1930s elephant numbers began to increase
(2, 3). Today, the elephant population in northern Botswana and
adjacent parts of Zimbabwe and Namibia is estimated to more
than 100 000 animals (4), constituting the largest population on
the continent (5). Mean density of elephants along the Chobe
riverfront in the dry season is about 4 animals per km
, drop-
ping to ca. 0.5 per km
during the wet season, when elephants
disperse southwards drinking from temporary pans (6).
About the same time as the elephant population reached its
nadir, other changes took place. Rinderpest, a disease spread by
cattle, came to Africa in 1887 and reached Botswana in 1896–
1897, causing heavy mortality in wild and domestic ungulates
(1). In addition, both species of rhinoceros (Ceratotherium si
mum and Diceros bicornis) were wiped out, or virtually so, by
hunters. During the 1930s and again in the early 1950s, timber
was extracted from the woodlands along the Chobe River, and a
sawmill and a village were built on the riverfront (7).
There is almost no documented evidence of vegetation com
position and structure along the Chobe River before the ele
phants and the rhinos disappeared and rinderpest caused major
decline in ungulate populations. However, the changes caused
by the decline of megaherbivores and other large herbivores
must have cascaded through the ecosystem, affecting soils, veg
etation, herbivores, and predators. Changes in fuel load, caused
both by alterations in herbivory regime and as a result of timber
extraction, probably increased fire frequency (3).
We know slightly more about the ecological processes asso
ciated with the recovery of elephant and ungulate populations.
The resulting habitat modifications have caused concern in Bo
tswana about potential adverse effects on the area’s biodiver
sity and for the loss of scenic woodlands along the river (8, 9).
Both factors have implications for the region’s lucrative tourist
industry. In particular, the loss of the
Acacia woodlands along
the riverfront (10) and the decline since the 1960s of the Chobe
bushbuck (Tragelaphus scriptus ornatus) (8) are sources of con
cern for conservationists and park managers. The increasing el
ephant population obviously plays a role in these changes, and
the ”Chobe elephant problem” has been given highest priority in
nature conservation and management by Botswana authorities;
the ultimate question being: to cull or not to cull (11, 12). How
ever, there is some uncertainty about the ecosystem processes
associated with the increasing elephant population.
In an effort by Botswana Department of Wildlife and National
Parks (DWNP) to improve the preconditions for management of
wildlife resources, the Botswana Norway Institutional Coopera
tion and Capacity Building Project (BONIC) was established in
1998 and ran for 5 years. The project was based on cooperation
between DWNP, the Norwegian Institute for Nature Research
(NINA) and the Centre for International Environment and De
velopment Studies (Noragric) at the Agricultural University of
Norway. The project had a threefold aim:
i) to provide formal
and informal research training to DWNP staff (7 MSc and 4
PhD students have been or are being trained within the project);
ii) to carry out research that improves the understanding of the
ecosystems in northern Botswana and the systematic changes
that are taking place, and
iii) to encourage and facilitate the use
of the improved knowledge and staff capacity by DWNP in the
management of wild natural resources.
There have been many studies on the Chobe elephants (6,
13–17) and their impact on trees (3, 18–20). However, less is
known about the wider ecological implications of the elephant
activities, including the interactions with nutrient availability
and distribution, vegetation dynamics, distribution and behavior
of other mammals and biological diversity and species richness
(9, 21, 22). Therefore, BONIC was not specifically studying el
ephants, but rather elephant – ecosystem interactions (23).
BONIC consisted of seven interrelated studies or subprojects
(Fig. 1), most of them run by one Norwegian researcher and one
DWNP counterpart and/or a PhD or MSc student. The topics of
© Royal Swedish Academy of Sciences 2004
Ambio Vol. 33 No. 6, August 2004
the subprojects were:
To quantify the relationships between elephants: and
1. soil properties and nutrient cycling;
2. herbaceous vegetation composition and grazing;
3. woody vegetation structure, composition, regeneration and
4. distribution and community composition of mammals and
gallinaceous bird
5. population ecology and behavior of impala (Aepyceros
6. behavioral ecology of buffalo (
Syncerus caffer);
7. population ecology of lions (
Panthera leo).
ing the first three years the project also included a socio-
economic component (24, 25).
The project had a common core study area, which was photo
graphed from the air in 1998. Based on the photographs a map of
the six main habitat types was constructed, including floodplain,
riparian forest,
Capparis shrubland, Combretum shrubland,
mixed woodland and
Baikiaea woodland (Fig. 2). Five transects
(old fire breaks) run south from and perpendicular to the river
through the core area. Along each of the 5 transects permanent
sampling sites were established, in total 13 sites in the
shrubland and 15 in each of the other habitat types except the
riparian forest fringe that was too fragmented to sample in this
way. The following is an outline of the project, showing some of
its results and conclusions.
To quantify the relationship between vegetation and large herbi-
vores, we described the woody and herbaceous vegetation and
its current use by elephants and other large herbivores. Present
vegetation was also compared with historical records, in order
to describe the changes taking place primarily since the 1960s,
when elephants were again becoming common in the area. Veg
etation was analyzed in the 73 permanent sample sites along the
five transects. For herbaceous plants and woody plants < 0.5 m
tall, canopy cover per species was recorded in five plots of 1 m
at each site. For the tree layer, including woody plants > 0.5 m
tall, density of trees by species and height and canopy cover for
each tree were recorded in one 400 m
plot at each site. In addi-
tion, in 2003, we re-analyzed the vegetation transects analyzed
by Simpson in 1969/1970 (10) and Addy in 1992 (8).
The seasonally inundated floodplains are today dominated
by the grazing-tolerant, strongly rhizomatous perennial grass
Cynodon dactylon and the grazing-resistant, sharp and stiff Veti
veria nigritana. These obviously grazing-adapted grasses were
not listed by Simpson (10) as dominant floodplain species in
the area in 1969/1970, when grazing pressure was lower than
today. The narrow strip of riparian forest (hardly visible in Fig.
2) along the river was already reduced by the time of Simpson’s
study (10), and only fragments remain today (26). The elevated
alluvial plains between the floodplains and the Kalahari sand
ridge were described by Selous (27) in 1874 as an open flat with
widely scattered clumps of bush and with large
Acacia trees at
the edge of the river (the riparian forest fringe). This descrip
tion is from the time when elephant numbers were declining and
before the rinderpest outbreak. A hundred years later, when ele
phants had increased again, Simpson (10) described the elevated
alluvial plains as a degrading riverine tree savanna with large
Acacia- and Combretum trees and a well-developed shrub layer.
Today, most of the large trees are dead and the area is covered
by Capparis shrubland, dominated by Capparis tomentosa and
Combretum mossambicense
(Fig. 2).
Very limited information is available from old written sources
about the woodland vegetation on Kalahari sand dominating fur
ther away from the river. However, there is evidence (Skarpe
et al. unpubl. data) that some shrub species and the large tree
species, including Baikiaea plurijuga, have decreased within
some kilometers from the river since Simpson’s study (7) in
1969/1970. Instead there has been an increase in some smaller
tree species. A comparative study of aerial photographs from
1962, 1985, and 1998, covering the period of major increase in
elephant numbers, showed a general decline in woodlands and
a corresponding increase in shrub vegetation (26, 28). Today,
Combretum shrublands dominate in the transition zone between
sand and alluvial soils, followed by mixed woodlands with scat
tered large Baikiaea plurijuga and many smaller tree species,
and Baikiaea woodland furthest away from the river. Recent
data from the sampling sites along the transects and from the
aerial photography (Fig. 2) indicate that distribution of habitats
may be governed by interactions between soil resources (Table
1) and herbivore impact.
Figure 1. A schematic picture of the Chobe ecosystem and the
research areas for the BONIC subprojects, referred to by the
numbers from the list in the text.
Table 1. Levels of selected nutrients in four habitat types in Chobe
National Park*. Shrubland includes
Habitat Calcium
(cmol kg
Matter (%)
Floodplain 14,2 9,1 2,4 4,7
Shrubland 3,8 13,1 0,7 6
Mixed woodland 1,1 4,4 0,4 5,1
woodland 1,06 2,2 0,4 5
*Masunga and Dhillion unpubl. data.
Figure 2. Habitat map over the core study area at the Chobe river
front showing the 6 recognized habitat types, major roads and the 5
common sampling transects. The narrow fringe of riparian forest is
barely visible on the map.
© Royal Swedish Academy of Sciences 2004
Ambio Vol. 33 No. 6, August 2004
The herbaceous vegetation in the woodlands is dominated by
grasses of the genera
Aristida and Digitaria, as also recorded by
Simpson (10), but in addition we found Panicum maximum, an
excellent fodder grass, among the dominants. Most grass species
in the woodlands are perennial, and species composition var
ied little with interannual variation in rainfall. Ordination of the
data on herbaceous vegetation using Detrended Correspondence
Analysis (29) shows the gradient in community composition
along one of the transects (Fig. 3).
The concern about the reduction in
woodlands cover has focused
on the visible impact by elephants and fire on mature trees (3,
18, 19). However, to maintain a healthy tree population, a reduc
tion of life span of mature trees is less important than the lack
of regeneration and of recruitment of new individuals into the
reproductive population (30). We found seedlings and saplings
of woody plants to be rare and the soil seed bank small (< 100
viable seeds per m
) in all habitat types (31). In the riparian for-
est fringe the soil seed bank had about five times more viable
seeds in areas with relatively low elephant impact than in heav
ily impacted areas, and the species composition was different
(31). The similarity between the aboveground flora and the seed
bank flora was low, as is often the case. Potential reasons for
the low rate of sexual reproduction could be predation on seeds
and/or seedlings by insects, rodents or larger mammals (31) or
depletion of symbionts, e. g. mychorrizal fungi, in the soil fol
lowing the death of the mature trees.
There has been special interest in the lack of regeneration
of the riverine
Acacia woodlands on the elevated alluvial flats.
Studies at Lake Manyara in Tanzania have shown that
tree populations are unable to regenerate during periods of high
impala density due to intense seedling predation (32). We found
local impala densities higher than 150 animals per km
in some
areas along the Chobe riverfront. To test the idea that seedling
predation by impala is an important reason for the lack of regen
eration of trees on the elevated alluvial plains along the Chobe
riverfront (33), an exclosure experiment was performed. Seed
lings were planted in areas with different impala densities in
complete exclosures, ca. 6 m
; ii) semipermeable exclosures, ca.
6 m
, open below 0.5 m above the ground allowing access by
small animals like birds, rodents and baboons (Papio ursinus
or iii) in unfenced control areas. Seedlings were of four tree spe-
cies, two species that have increased in density in the area since
the studies of Simpson (10) and Addy (8), (Croton megalobotrys
and Combretum mossambicense) and two species that have de-
creased in density during the same period (
Faidherba (formerly
Acacia) albida and Garcinia livingstonei). Growth rate, tissue
loss and mortality of the seedlings were recorded and related to
treatment and density of impala in the area. Preliminary results
show that browsing by large herbivores, e. g., impala, plays an
important role in predation on tree seedlings close to the river
Thus, written evidence shows that the huge
Acacia trees, for
which the Chobe riverine woodlands were famous in the 1960s
(10), established after Selous’ visit to the area in 1874 (27), and
were already old and dying without regeneration in 1969 (7).
Both the written evidence and our experiment suggest that these
Acacia trees were derived from a cohort of seedlings established
some 100 years ago, when populations of elephants, impalas
and other browsers were low. The experiment further predicts
that with high ungulate densities some tree species may not be
able to regenerate in most of the riverine habitat. Such species
may depend on local refugia like steep riverbanks, and on natu
ral dynamics of herbivore densities, caused, e. g., by drought or
disease, for persistence in the area.
Herbivore-induced changes in vegetation composition may
take either of two fundamentally different directions (30), lead-
ing to an increase in fast-growing palatable species or in slow-
growing, often chemically defended, unpalatable species (Fig. 4).
We used both observational studies and controlled experiments
with simulated and real browsing to study the interactions be
tween large herbivores, plant traits and changes in plant species
composition in the relatively nutrient-rich alluvial soil and on
the relatively nutrient-poor Kalahari sand. In the sampling sites
along the transects signs of grazing and browsing were recorded.
For the tree layer the number of twigs of 8 mm diameter (= aver
age elephant bite diameter (16)), current season’s shoots, bites
by ungulates on current season’s shoots and bites/breaks by el
ephants were counted on each tree. Results show a strong gradi
ent with generally increasing herbivore impact from the
woodland to the shrublands. The data also reveal considerable
differences between elephants and ungulates in foraging pattern
and in response to the herbivory-induced vegetation changes.
Whereas elephant browsing pressure (% of trees utilized) was
highest in the mixed woodlands and Combretum shrublands, un
gulate browsing pressure (% of shoots utilized) peaked in the
Capparis shrublands. This pattern may be caused partly by the
low availability of browse for elephants in the shrublands com
pared to the woodlands, but also reflects a difference between
elephants and ungulates in browse-species preference. Some of
the tree species that have increased in density in the
shrublands since the studies by Simpson (10) and Addy (8),
g., Capparis tomentosa and Combretum mossambicense, are
intensely browsed by ungulates like impala and kudu, but are
Figure 3. Ordination (Canonical Correspondence Analysis) of the
herbaceous vegetation in one of the transects showing the gradient
in species composition from the
woodland (green) to the
mixed woodland (red),
shrubland (blue) and
shrubland (yellow). Symbols indicate sampling plots, 5 at each site.
Figure 4. Browsing by ungulates may lead to the dominance
either of fast growing, palatable species (a) or of slow growing,
often defended, unpalatable species (b), with different implica-
tions for nutrient cycling, heterogeneity of resource availability
and further browsing.
© Royal Swedish Academy of Sciences 2004
Ambio Vol. 33 No. 6, August 2004
avoided by elephants. Elephants, on the other hand, selectively
browse some of the Combretum species increasing on the Ka
lahari sand (Skarpe et al.
unpubl.). Elephant impact was also
found to be higher in areas of more undulating terrrain compared
to relatively flat areas (35).
One of the most important ways by which large herbivores in
teract with their environment is by affecting nutrient distribu
tion and availability (30). In most terrestrial ecosystems, the
majority of the net primary production enters the decomposition
subsystem as plant litter. However, in savanna ecosystems fires,
insects or large herbivores may consume considerable amounts
of aboveground biomass.
Selective herbivory may influence species composition of the
vegetation and hence quantity and quality of plant litter. Gen
erally, litter from fast-growing plants or plant parts decompose
rapidly, whereas litter from slow-growing plants decomposes
slowly and may cause immobilization of nutrients in the soil
(36). Thus, the herbivory-induced increase either in fast-grow
ing or in slow-growing species may lead to positive feedback
loops via changes in the nutrient cycling rate (30; Fig. 4).
A pilot study on decomposition of lit
ter from three different tree species was
undertaken in the different habitat types.
Results so far indicate that weight loss in
litter assumed an exponential trend with a
steep rise when monthly average temper
ature and humidity are high. Differences
in decomposition rates between tree spe
cies have been recorded (Fig. 5). Rates of
losses based on total dry matter will be
correlated with loss rates of different nu
trients in the litter and soil. Knowledge
of the different rates of decomposition of
plant species can indicate the spatial pat
tern of nutrient distribution. The results so
far suggest that the different habitats may
represent different nutrient cycling states
within the landscape (Masunga and Dhil
lion, unpubl. data).
By influencing vegetation structure and
composition as well as the fire regime, el
ephants may change the availability and
distribution of food and shelter for other
herbivores (37). We followed communi
ty composition and habitat utilization of
mammals and gallinaceous birds in order
to trace effects of the changing habitats.
A trapping survey of small mammals
(less than 2 kg) in the mixed woodland
(38) showed that the species composition
of the small mammal community varied
greatly over short distances. Apparently,
this is in response to small-scale variation
in vegetation composition and structure,
often created or enhanced by large her
bivore activities like trampling, digging,
defecation and urination.
Density and distribution of mammals
larger than 2 kg and of gallinaceaous birds
(e. g., guinea fowl) were studied using the
methodology described by Buckland et al.
(39) and Motsumi (40). Ten transects, run
ning parallel and perpendicular to the river, were driven during
the wet and the dry season in the morning, evening and night
using spotlights. More ungulate species were present in the dry
season than during the wet season (Fig. 6). Ordination with
Canonical Correspondence Analysis and Redundancy Analysis
(28) showed significant positive correlation between elephant
Figure 5. Decomposition rate of leaves of two different tree species
woodland. Figures are weight loss in g (mean +/- S.E.)
per month.
Figure 6. Canonical Correspondence Analysis (CCA) – triplot diagram of species, sampling sites
and significant environmental variables during the dry season (upper) and a corresponding Re
dundancy Analysis (RDA) triplot for the wet season (lower). Habitat types, sample transects and
environmental variables are explained in the Figure.
© Royal Swedish Academy of Sciences 2004
Ambio Vol. 33 No. 6, August 2004
impact, availability of certain tree species much browsed by
ungulates, ungulate browsing pressure and the distribution of
ungulate species (Fig. 6). Both in the wet and the dry season
about half the ungulate species exhibited a positive correlation
with elephant impact, measured as % of woody plants recently
browsed or broken by elephants (Stokke et al. unpubl.; Fig.
Impalas remained primarily in the heavily elephant-impact
ed Capparis-, and Combretum shrublands the whole year. As
an indicator of impala population performance and to study
juvenile mortality, 80 newborn lambs have been radio collared,
20 per year from 1998 to 2001. Births were highly synchro-
nized in mid-November. The influence of rainfall (amount and
distribution in space and time) on the annual production of im
pala lambs is being investigated. Results indicate that juvenile
mortality is low, and that the numbers of impala are increas
ing along the riverfront (33). One plausible explanation for the
increase in impala densities is the increase in palatable shrubs
like Combretum mossambicense and Capparis tomentosa
Impala population data were also used in a bio-economic
model (24) indicating a high potential value of the animals
for local communities both for nonconsumptive and, outside
the Chobe National Park, consumptive use. Such utilization
would, however, in most cases require improved organizations
for Community Based Natural Resources Management (25).
At the start of the project the Chobe buffalo population was
believed to be in decline (41), and the possibility that competi
tion with elephants was the underlying cause was discussed
by conservationists and management authorities. Buffalo are
the second most important large herbivore in the ecosystem in
terms of biomass, after the elephant, and, like the elephants,
most of the population leaves the river front in the wet sea-
son. To gain a better understanding of the movement pattern
of buffalo, 20 females and 20 males were radio collared during
the first two years of the project. Figure 7 shows a summary
of movement data over 19 months. During the dry season the
buffalo were concentrated along the riverfront in large herds
numbering up to 1300 individuals. In the wet season buffalo
dispersed into the woodlands south of the core area, drinking
at temporary pans. At this time the large herds broke up into
smaller groups of ca. 50 200 animals (42). Although this pop
ulation has been reputed to move as far as Nxai Pan, ca 200 km
south, we found no evidence of seasonal long distance move-
ments. Wet season ranges of the population were confined to
within ca. 60 km of the dry season ranges (Fig. 7).
While it has long been known that male buffalo move in and
out of herds, cows have been considered to remain within their
natal herd all their lives (43, 44). Our study has showed that
a large proportion of the Chobe females switched herds, and
some emigrated long distances (up to 133 km (45)).
During the time of the study, buffalo maintained good body
condition throughout the year, and the population structure
suggested a slow increase in numbers instead of the previ
ously assumed decline (46). This suggests that buffalo were
not limited by competition with elephants or smaller ungulates
for food. In our study, both dung analyses (46) and isotope
analysis (unpubl. data) indicate that buffalo at Chobe browse
very little, limiting potential dietary competition to grazing.
A study in the dry season found elephants avoided areas of
floodplain recently grazed by buffalo, while buffalo preferred
areas of floodplain recently grazed by elephants (46). Thus,
there is no evidence that elephant grazing at Chobe reduces
food resources for buffalo, but some indication that grazing by
buffalo may adversely affect grazing resources for elephants,
a conclusion supported by Prins (44) and Van de Koppel and
Prins (37).
Lion populations are assumed to be regulated by food and so
cial factors. The impact by elephants on vegetation may influ-
ence lion populations directly by changing visibility and ac
cessibility during hunts, and indirectly by changing density,
population structure and/or the behavior of prey animals.
In the project we were able to follow the whole resident riv
erfront lion population by radio tracking eight collared females
in five different groups and the three pride males present in
the area. We monitored data on space and habitat use, food in-
take, social interactions, cub production and loss of individuals
by regularly tracking and observing the lion groups over four
The results show that the riverfront lion population was di
vided in semi-independent groups of females and young, ar
ranged linearly along the heavily elephant-impacted riverfront.
Data on prey selection suggest that buffalo and elephant calves
constituted a significant proportion of the diet both during the
wet and dry season. The lion population size varied consider
ably through our four years of study (> 60%). Even though
birth rate and cub survival appeared to be high, and may have
created short-term increase in population size, it was not large
enough to avoid occasional decreases in population size. In
some periods the lion population was so small that it was
highly vulnerable to stochastic demographic events. The main
reasons for the observed decreases in population size may be
summarized as: i) The litter sex ratio has been significantly
male biased resulting in a high proportion of dispersing sub-
adults; ii) high human-induced mortality among pre-dispersing
sub-adults, e.g. poaching and traffic accidents; iii) emigration
of female groups; iv) lack of immigration; and v) infanticide.
Only one immigrant lion has been observed during our four
years of study. This was an adult male that took over as a pride
male for some of the female groups and killed their cubs. A
female biased sex ratio in the adult segment of the popula
tion and low immigration rate have posed a question regarding
the extent of isolation and inbreeding of the Chobe riverfront
population (47). This question is being addressed by genetic
analyses of tissue samples.
Figure 7. Distribution of buffalo in the dry (red crosses) and wet
(blue dots) seasons. Each point represents one location of a herd
(which may contain more than one collared animal).
© Royal Swedish Academy of Sciences 2004
Ambio Vol. 33 No. 6, August 2004
The changes in vegetation structure and composition recorded
during the last ca. 50 years along the Chobe riverfront are as
sumed by local people, conservationists and scientists to be
caused mainly by the increasing elephant population (7, 8, 10–
12,19). Such changes have been reported from many areas with
increasing elephant activity (21, 48–50), and may involve al-
terations in tree size structure (49) and/or tree species composi-
tion (50). However, while elephants selectively destroy mature
trees, the density and species composition of tree regeneration
may be determined by fire and/or predation on seeds or seed-
lings (31, 32, 34, 48, 51,). While fires were important in the
Chobe riverfront ecosystem following the logging operations
and when herbivore densities were low (3), they are now un-
common within our core study area (26, Skarpe unpubl. data),
although they have significant impact on woodland structure
and composition further away from the river (19, 46).
Other factors that could have contributed to the observed
ecosystem changes include alterations in water availability.
There are indications that the availability of surface water has
decreased in southern Africa during the last ca. 100 years (refs
in 1 and 52). Still, Tyson (52) did not find any regional trend in
rainfall during the period for which records are available (since
ca. 1910; 52). Reduced water availability would, of course,
negatively affect woody vegetation. However, it can not ex-
plain both the establishment of the riverine Acacia woodlands
after the 1870s and their subsequent decline and replacement
with shrubs from the 1960s. Further, the strip of riparian for
est, of which only fragments are left in most places, is still
comparatively intact close to settlements, where animal impact
is reduced. We, therefore, conclude that the declines and later
increases in the elephant and antelope populations are the main
cause for the vegetation changes described.
While changes in structure and composition of vegetation
following herbivory are readily described, we are only begin
ning to understand the impact of large herbivores on ecosystem
processes. Herbivory-induced changes in vegetation composi
tion may lead to dominance of species with different growth
rates and leaf chemistry, compared to the former dominants.
This influences litter quality and quantity and, therefore, nu
trient cycling and resource distribution (30). Eventually, this
is expected to have feedback effects on the herbivores them
selves. How such feedbacks may operate depends on which
direction the vegetation changes take: towards species that are
slow-growing and unpalatable or towards species that are fast-
growing and palatable (Fig. 4). Our data on woody vegetation
and browsing give no evidence of a decrease in palatability of
woody vegetation in the heavily elephant-impacted areas. The
distribution patterns of mammals (> ca 2 kg) and gallinaceous
birds exhibit concentrations in the most impacted areas close
to the river, which for highly mobile species like birds and
larger mammals can not be explained by the vicinity to water
only. Further, we recorded an increase in populations of buf-
falo (46) as well as in impala (34), and older records (3, 7) sug
gest a long-term increase in populations of buffalo, impala and
greater kudu. This is contrary to the findings of Fritz et al. (53),
who analyzed wildlife censuses from 31 conserved African
ecosystems and concluded that elephants negatively affected
populations of browsers and mixed feeders, but had no influ-
ence on grazers. One reason for this Chobe paradox may be
the intermittent pattern of herbivory. Intensity of browsing and
grazing is highest during the dry season, when plants largely
are dormant, and lowest in the wet growing period, when many
animals disperse away from the riverfront. Another contribut-
ing factor may be a net import of nutrients from the wood-
lands to the shrublands and floodplains close to the river, as
elephants forage mostly in the woodlands but spend consider-
able time each day drinking and socializing close to the river,
and, thus, deposit large amounts of faeces and urine there.
The density of elephants in the Chobe area before the large-
scale ivory hunt in the mid- and late-19
century is not known.
Campbell (2) used Parker and Graham's (54) equation for cal
culating elephant densities from rainfall, soil types and human
population. He estimated the number of elephants in the whole
of Botswana in the beginning of the 19
century to be between
200 000 and 400 000 animals, with the highest densities in the
mesic north. Perhaps, there never was an equilibrium between
woodland vegetation and elephants, but instead multiple stable
states, with transitions triggered, e. g., by periodic droughts,
or a stable limit cycle, in which elephant densities increased
while eating down the vegetation, then declined because of a
shortage of food until low elephant density allowed the vegeta-
tion to recover, and elephants again increased (51, 55, cf. 56).
Such dynamics probably cascade through the whole ecosystem,
and may in the long term facilitate the coexistence of species
with different environmental requirements. If this hypothesis
is valid, man can create an artificial equilibrium by locking the
system in a state of low elephant density (55), but this may in
the long term lead to loss of biological diversity.
Much of the Chobe elephant problem has concerned the role
of elephants in the disappearance of the riverine Acacia wood-
lands on the elevated alluvial plains along the Chobe River.
As we have shown, these woodlands were probably a transient
artefact, caused by artificially low densities of large herbivores
following rinderpest and excessive hunting of elephants about
100 years ago, creating a window of opportunity for seedling
establishment. Now that these woodlands have all but disap
peared, their re-establishment would require drastic reductions
in herbivore populations, including not only elephants, but also
smaller browsers like impala (32–34, 37, 57).
Our studies have confirmed that the ecosystem along the
Chobe riverfront has changed profoundly since the 1960s,
probably reverting towards a situation somewhat similar to the
one before the excessive hunting of elephants and the rinder-
pest panzootic. There is, however, little evidence of a reduc-
tion in the carrying capacity for other large herbivores, in fact
the dominating species of browsers, grazers and mixed feeders
have increased in numbers concurrently with the elephants.
This does not, of course, mean that habitat conditions have
not declined for some species, such as bushbuck. We do not,
however, see any ecological reason to artificially change the
number of elephants in Chobe National Park, either through
culling or opening new dry season ranges by providing extra
water from boreholes (58). Nevertheless, there are social and
economic reasons to limit the distribution of elephants in Bo-
tswana, and it may be necessary to draw limits to the permis
sible elephant range, destroying groups that emigrate beyond
those boundaries. This would relieve human-elephant conflict
while providing the opportunity for a sustainable harvest of
such ”surplus” animals and the marketing of various commodi
ties, as far as international rules will allow.
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Corresponding author: Christina Skarpe, PhD,
and associate professor, mainly involved in re-
search on interactions between large herbivores
and their environment. Her address: Norwegian
Institute for Nature Research, Tungasletta 2, N-
7485 Trondheim, Norway.
For other authors addresses contact the corre-
sponding author.
... Furthermore, many studies were once-off or short-term (review in Guldemond & van Aarde, 2008) and do not allow for the assessment of long-term vegetation dynamics. Finally, the few longterm studies that did assess long-term effects of elephants on vegetation in large open landscapes (Conybeare, 1991;Mosugelo et al., 2002;Skarpe et al., 2004) typically focused on periods of drastic changes in elephant densities. These studies are useful to depict vegetation changes during a transition period but are not able to predict future changes of vegetation dynamics when elephant densities remain high in the long term. ...
... <50 cm). This higher abundance of Colophospermum mopane individuals can result from true recruitment with: (a) heavy elephant browsing pressure on adult plants limiting their ability to monopolise resources and hence reducing intraspecific competition with the saplings; or (b) the effect of faeces deposition by elephants with nutrient imports by elephants in areas where they forage, the increased nutrient turn-over rate associated with high elephant abundance and the increased seed dispersal of the more palatable species, which will ultimately favour the growth of fast-growing and more palatable species (Skarpe et al., 2004). The higher abundance of Colophospermum mopane plants could also be apparent and result from resprouting, i.e. the production of secondary trunks as an induced response to injury or to profound changes in growing conditions (Bond & Midgley, 2003; e.g. on other taxa Heisler et al., 2004 for post-fire resprouting, Lewis, 1991 andRogers, 2008 in response to herbivory). ...
... Taken together, these and our results suggest that elephant might have a drastic impact in large ecosystems that have reached an equilibrium, but only close to water sources. An alternative explanation suggested by Skarpe et al. (2004), states that elephant abundances and savanna woodlands might be in perpetual change with multiple stable states suggesting that conservation success should be evaluated by shifts in alternative stable states in addition to elephant-induced changes when they reach high density. ...
Full-text available
Questions Species defined as ecosystem engineers (e.g. elephant) are able to strongly shape their habitat. In African savannas, elephants have often been shown to reduce woody plant abundance and diversity. However, recent studies highlight more complex elephant-induced effects on vegetation. Here, we assessed if long-term high elephant densities (>⁻²) in a large open landscape resulted in the depletion of savanna woodland woody communities or if it led to a new alternative equilibrium. Location Woodland savanna of Hwange National Park, Zimbabwe. Elephant densities at the study site have remained high for the past two decades (>⁻²). Methods We measured long-term (>15 years) elephant utilization of woody plant communities and their effects on vegetation structure, species composition and functional traits (e.g. N leaf concentration, specific leaf area) in twelve vegetation plots. Results We observed opportunistic foraging behaviour by elephants with only a slight temporal shift in species composition, mainly explained by changes in rare species. Further, we did not observe any modification in mean functional trait values, overall height and stem diameters of the woody plant communities. However, we found differential changes in woody plant abundance according to the height layer (decrease in the number of tall plants (>200 cm) and increase in the number of short plants (<50 cm)) and a strong reduction in crown diameter for plants in the 50-200 cm height class. Conclusion Our study strongly suggests that long-term high elephant densities have led to a stable state in savanna woodland vegetation in terms of plant community composition and their functional traits. However, high elephant densities did affect vegetation structure, which would have several important indirect effects on this ecosystem (e.g., predator-prey interactions). We hope that this study stimulates more work on the long-term effects of ecosystem engineers in large and open ecosystems.
... Analyses of historical data including pollen analyses and dendrochronology suggest that trees recruit when herbivore populations are low, resulting in cohorts of even aged trees with distinct missing size classes (Holdo et al., 2009;Prins & Van der Jeugd, 1993;Staver et al., 2011). Browsers such as impala (Aepyceros melampus) are known to intensely predate seedlings (Moe et al., 2009;Prins & Van der Jeugd, 1993;Skarpe et al., 2004). Apart from recruitment, meso-herbivores may also have pronounced effects on adult shrubs and smaller trees. ...
... Apart from recruitment, meso-herbivores may also have pronounced effects on adult shrubs and smaller trees. Skarpe et al. (2004) showed that impala and kudu browsed on species that elephants do not favour, such as Capparis tomentosa and Combretum mossambicense, in riparian woodlands in Botswana. However, there is also substantial overlap between elephant and meso-browsers in niches of species browsed and browsing heights (O'Kane et al., 2011(O'Kane et al., , 2014 and competition with elephant can displace meso-herbivores (Fritz et al., 2002;Lagendijk et al., 2015;Valeix et al., 2008). ...
The ongoing loss of large trees and densification of shrubs are two prevalent processes that take place in African savannas, with profound consequences for their structure and function. We evaluated herbivore impacts on savanna woody communities using a long‐term exclosure experiment in the Kruger National Park, South Africa, with three treatments: the exclusion of large mammals only (i.e. elephant and giraffe), exclusion of all herbivores larger than a hare, and areas open to all herbivores. We asked three questions: (1) How did variable exclusion of herbivores affect woody density and structure across the catena (i.e. riparian, sodic and crest vegetation)? (2) Did the exclusion of herbivores result in unique woody species composition? (3) Did herbivore exclusion result in a higher proportion of palatable species? After 17 years, we found that herbivores mainly affected the heights and densities of existing species, rather than leading to turnover of woody species assemblages. Although densities of individuals increased in the full exclosure (350 ha−1), the change was more moderate than expected. By contrast, mixed mega‐and meso‐herbivores decreased the number of trees and shrubs (decreases of 780 ha−1) via a variety of physical impacts. Meso‐herbivores alone, on the other hand, had less impact on individual density (i.e. no change), but limited average height growth and canopy dimensions in certain habitat types. Where elephants are present, they are effective at reducing the density of woody stems to the point of counteracting woody encroachment, but at the same time are actively preventing the persistence of large trees (>5 m) as well as preventing trees from recruiting to larger size classes. However, the lack of massive recruitment and woody cover increases with elephant exclusion, especially for more preferred species, suggests that factors beyond elephants, such as dispersal limitation, seed predation, and drought, are also acting upon species. We evaluated herbivore impacts on savanna woody vegetation using a long‐term exclosure experiment (17 years) in the Kruger National Park, South Africa, with three treatments: the exclusion of large mammals only (i.e. elephant and giraffe), exclusion of all herbivores, and areas open to all herbivores. Elephants (and giraffe) reduced the density of woody stems to the point of counteracting woody encroachment, and at the same time prevented trees from recruiting into larger size classes. However, removing elephants resulted in non‐linear vegetation responses; partly because smaller herbivores also have pronounced impacts on vegetation dynamics, as well as other factors, such as dispersal limitation, seed predation, and drought, acting upon species.
... However, elephants exert strong influences on woody vegetation (Guldemond et al., 2017) through consumption, tree felling (Asner & Levick, 2012), and seed dispersal (Dudley, 2000;Cochrane, 2003;Bunney et al., 2017). These impacts on woody vegetation can lead to large-scale ecosystem effects on nutrient cycling (Skarpe et al., 2004;Parker, Bernard & Adendorff, 2009), fire regimes (Kimuyu et al., 2014), carbon storage (Davies & Asner, 2019;Berzaghi et al., 2019) and habitat availability for other species (Table 1; Kerley & Landman, 2006;Guldemond et al., 2017). Hippos have comparatively fewer direct impacts on woody vegetation , but their indirect impacts on woody plants via modification of fire spread and extent could have more substantive effects on woody vegetation than currently realised. ...
Megaherbivores perform vital ecosystem engineering roles, and have their last remaining stronghold in Africa. Of Africa's remaining megaherbivores, the common hippopotamus (Hippopotamus amphibius) has received the least scientific and conservation attention, despite how influential their ecosystem engineering activities appear to be. Given the potentially crucial ecosystem engineering influence of hippos, as well as mounting conservation concerns threatening their long-term persistence, a review of the evidence for hippos being ecosystem engineers, and the effects of their engineering, is both timely and necessary. In this review, we assess, (i) aspects of hippo biology that underlie their unique ecosystem engineering potential; (ii) evaluate hippo ecological impacts in terrestrial and aquatic environments; (iii) compare the ecosystem engineering influence of hippos to other extant African megaherbivores; (iv) evaluate factors most critical to hippo conservation and ecosystem engineering; and (v) highlight future research directions and challenges that may yield new insights into the ecological role of hippos, and of megaherbivores more broadly. We find that a variety of key life-history traits determine the hippo's unique influence, including their semi-aquatic lifestyle, large body size, specialised gut anatomy, muzzle structure, small and partially webbed feet, and highly gregarious nature. On land, hippos create grazing lawns that contain distinct plant communities and alter fire spatial extent, which shapes woody plant demographics and might assist in maintaining fire-sensitive riverine vegetation. In water, hippos deposit nutrient-rich dung, stimulating aquatic food chains and altering water chemistry and quality, impacting a host of different organisms. Hippo trampling and wallowing alters geomorphological processes, widening riverbanks, creating new river channels, and forming gullies along well-utilised hippo paths. Taken together, we propose that these myriad impacts combine to make hippos Africa's most influential megaherbivore, specifically because of the high diversity and intensity of their ecological impacts compared with other megaherbivores, and because of their unique capacity to transfer nutrients across ecosystem boundaries, enriching both terrestrial and aquatic ecosystems. Nonetheless, water pollution and extraction for agriculture and industry, erratic rainfall patterns and human-hippo conflict, threaten hippo ecosystem engineering and persistence. Therefore, we encourage greater consideration of the unique role of hippos as ecosystem engineers when considering the functional importance of megafauna in African ecosystems, and increased attention to declining hippo habitat and populations, which if unchecked could change the way in which many African ecosystems function.
... Additionally, this pattern of vegetation browning applies to the southern border with Botswana around the Chobe National Park (Figures 2-4). The Chobe National Park is where the majority of the 200,000 migratory elephant population is located [24,94,95]. Furthermore, it is quite clear that greening is mainly on the opposite side (south) of the northern buffalo fence, where access by elephants is restricted [9,35] (Figure 2c,e) [59]. ...
Full-text available
Human–wildlife conflict in the Zambezi region of northeast Namibia is well documented, but the impact of wildlife (e.g., elephants) on vegetation cover change has not been adequately addressed. Here, we assessed human–wildlife interaction and impact on vegetation cover change. We analyzed the 250 m MODIS and ERA5 0.25° × 0.25° drone and GPS-collar datasets. We used Time Series Segmented Residual Trends (TSS-RESTREND), Mann–Kendall Test Statistics, Sen’s Slope, ensemble, Kernel Density Estimation (KDE), and Pearson correlation methods. Our results revealed (i) widespread vegetation browning along elephant migration routes and within National Parks, (ii) Pearson correlation (p-value = 5.5 × 10−8) showed that vegetation browning areas do not sustain high population densities of elephants. Currently, the Zambezi has about 12,008 elephants while these numbers were 1468, 7950, and 5242 in 1989, 1994, and 2005, respectively, (iii) settlements and artificial barriers have a negative impact on wildlife movement, driving vegetation browning, and (iv) vegetation greening was found mostly within communal areas where intensive farming and cattle grazing is a common practice. The findings of this study will serve as a reference for policy and decision makers. Future studies should consider integrating higher resolution multi-platform datasets for detailed micro analysis and mapping of vegetation cover change.
Most large herbivores require some type of management within their habitats. Some populations of large herbivores are at the brink of extinction, some are under discussion for reintroduction, whilst others already occur in dense populations causing conflicts with other land use. Large herbivores are the major drivers for forming the shape and function of terrestrial ecosystems. This 2006 book addresses the scientifically based action plans to manage both the large herbivore populations and their habitats worldwide. It covers the processes by which large herbivores not only affect their environment (e.g. grazing) but are affected by it (e.g. nutrient cycling) and the management strategies required. Also discussed are new modeling techniques, which help assess integration processes in a landscape context, as well as assessing the consequences of new developments in the processes of conservation. This book will be essential reading for all involved in the management of both large herbivores and natural resources.
Most large herbivores require some type of management within their habitats. Some populations of large herbivores are at the brink of extinction, some are under discussion for reintroduction, whilst others already occur in dense populations causing conflicts with other land use. Large herbivores are the major drivers for forming the shape and function of terrestrial ecosystems. This 2006 book addresses the scientifically based action plans to manage both the large herbivore populations and their habitats worldwide. It covers the processes by which large herbivores not only affect their environment (e.g. grazing) but are affected by it (e.g. nutrient cycling) and the management strategies required. Also discussed are new modeling techniques, which help assess integration processes in a landscape context, as well as assessing the consequences of new developments in the processes of conservation. This book will be essential reading for all involved in the management of both large herbivores and natural resources.
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Gastrointestinal tracts (GIT) of herbivores are lignin-rich environments with the potential to find ligninolytic microorganisms. The occurrence of the microorganisms in herbivore GIT is a well-documented mutualistic relationship where the former benefits from the provision of nutrients and the latter benefits from the microorganism-assisted digestion of their recalcitrant lignin diets. Elephants are one of the largest herbivores that rely on the microbial anaerobic fermentation of their bulky recalcitrant low-quality forage lignocellulosic diet given their inability to break down major components of plant cells. Tapping the potential of these mutualistic associations in the biggest population of elephants in the whole world found in Botswana is attractive in the valorisation of the bulky recalcitrant lignin waste stream generated from the pulp and paper, biofuel, and agro-industries. Despite the massive potential as a feedstock for industrial fermentations, few microorganisms have been commercialised. This review focuses on the potential of microbiota from the gastrointestinal tract and excreta of the worlds’ largest population of elephants of Botswana as a potential source of extremophilic ligninolytic microorganisms. The review further discusses the recalcitrance of lignin, achievements, limitations, and challenges with its biological depolymerisation. Methods of isolation of microorganisms from elephant dung and their improvement as industrial strains are further highlighted.
This study aimed at investigating woody vegetation structure and composition in relation to surface water availability in Mana Pools National Park (MPNP). Two study sites, namely the Zambezi River floodplain and seasonal water pans were selected. Sampling plots were systematically placed along transects from water pans at 100 m, 200 m and 500 m and thirty (30) different sampling plots of size 600 m2 were used. Statistical analysis was done using STATISTICA 7. A total of 192 woody plants from 18 woody species were recorded. More woody tree species (n = 13) were recorded around seasonal water pans as compared to the Zambezi River zone (n = 5). Results obtained from Kruskal Wallis H test showed no significant difference in height, basal area and tree density along a distance gradient from the Zambezi River to the interior (inland). Stem density and diversity showed a significant difference along a distance gradient from the Zambezi River to the interior (p < 0.05). Woody species density and diversity increased as a function of distance from the Zambezi River channel, with species diversity higher around seasonal water pans. A comparison obtained from the Mann Whitney U test between the two sampling sites showed significant difference in all structural variables. Findings of this study showed that the woodlands along the Zambezi River were more degraded as compared to those around seasonal water pans. It is an indication that the concentration of herbivores is impacting woodlands along the Zambezi River. Management should consider establishing artificial water pans around the park to minimise herbivore pressure along the Zambezi River.
This study assessed the relationship between surface water distribution and elephant impacts on the Zambezi River flood plain, Mana Pools National Park woody species ecosystem. Water availability and forage are major requirements for African elephant distribution within an ecosystem landscape in Zimbabwe. Surface water unavailability reduce elephant home range to around peripheries of water bodies and this is intensifying the destruction of wood species around these water bodies. The study adopted a mixed methods research design which combined qualitative and quantitative methods. Field data were collected between 10 January 2017 and 14 February 2019. Questionnaires, interviews and field observations were the major tools used to collect data in Mana Pools National Park. Data were analysed using the Statistical Package for Social Sciences version 20.0. Inferential statistics were employed to determine the relationship between elephant activity and damage of woody species. Chi square test results revealed that there is a significant relationship (P < 0.05; P = 0.001) between elephant activity and woody species damage. This means that woody species damage in the Mana Pools National Park Zambezi Valley flood plain can be attributed to elephant activity. This study recommends that Government and Zimbabwe Parks and Wildlife Management Authourity (ZPWMA) should formulate effective elephant population analysis through periodic surveys in order to continuously update the national data base of elephant population trends in areas such as Mana Pools National Park.
African savanna elephants (Loxodonta africana) have been recognised as ecosystem engineers, where their feeding habits have been shown to alter landscapes. Within small, fenced reserves, studies exploring elephant damage on trees and their recovery have overlooked secondary damages that could be contributing to tree mortality. The aim of this study is to assess the significance of both elephant damage and secondary damage, and the subsequent tree recovery. We identified secondary damage as insects and considered wood borers and termites in this study. This was conducted in in the small fenced Karongwe Private Game Reserve, South Africa. We analysed the level of damage, recovery and insect presence using vegetation transects, where all trees ≥2 m in height were surveyed (n = 1278 trees). Forty tree species were recorded, with 5 species accounting for 77% of the data set and used for further analysis. Termites were found to be more likely to colonise damaged trees without signs of recovery. However, wood borers were more likely to colonise damaged trees showing signs of recovery. Termites and wood borer presence on damaged trees was not dependent on tree height. We suggest carefully considering management approaches for elephant‐induced termite and wood borer damage on trees. Les éléphants de la savane africaine (Loxodonta africana) sont perçus comme les ingénieurs de l'écosystème, car il a été démontré que leurs habitudes alimentaires modifient les paysages. Dans les petites réserves clôturées, les études portant sur les dommages causés par les éléphants aux arbres et leur rétablissement ont jusqu’à maintenant négligé les dommages secondaires qui pourraient contribuer à la mortalité des arbres. L'objectif de cette étude est d'évaluer l'importance relative des dommages directs causés par les éléphants et les dommages secondaires ainsi que le rétablissement ultérieur des arbres. Nous avons identifié les dommages secondaires comme étant dus aux insectes et dans le cadre de cette étude nous nous concentrons sur les xylophages et les termites. Cette étude a été menée dans la petite réserve privée clôturée de Karongwe, en Afrique du Sud. Nous avons analysé le niveau de dommages, la récupération et la présence d'insectes en utilisant des transects de végétation, où tous les arbres ≥2 m de hauteur ont été étudiés (n = 1278 arbres). Quarante espèces d'arbres ont été enregistrées, avec 5 espèces représentant 77% de l'ensemble des données et utilisées pour une analyse plus approfondie. On a constaté que les termites étaient plus susceptibles de coloniser les arbres endommagés qui ne présentent pas de signes de rétablissement. En revanche, les xylophages étaient plus susceptibles de coloniser les arbres endommagés présentant des signes de rétablissement. La présence de termites et de xylophages sur les arbres endommagés ne dépendait pas de la hauteur de l'arbre. Nous suggérons d'examiner attentivement les approches de gestion des dommages secondaires causés par les termites et les xylophages aux arbres endommagés par les éléphants.
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When elephant densities exceed approximately 0.5 per km2, savanna woodlands are generally converted to shrublands or grasslands. The impact of such elephant-mediated habitat change on biodiversity in African game reserves has seldom been measured. We examined species richness of woody plants, birds, bats, mantises and ants in reserves where elephants had destroyed the miombo woodland and in adjacent but intact miombo woodlands outside the reserves. Species richness of woodland birds and ants was significantly lower where elephants had removed the tree canopy. Our findings may have important policy implications for conserving biodiversity in many African reserves in the face of rapidly growing elephant populations (approximately 5% per annum). The problem is further compounded by international public pressures against reducing elephant densities within game reserves while, outside these protected areas, savanna woodlands and their associated faunas are being lost to agriculture. Where then will refugia for habitat-sensitive species exist if not within the region's largest protected areas?.
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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.
Augmenting natural water supplies by providing artificial waterpoints is an intervention commonly adopted by managers of national parks and other large protected areas. Contrasting policies are currently being followed in three of the premier national parks in southern Africa. Some empirical guidelines for waterpoint provision are suggested by case histories of these and other wildlife reserves. A geometric model, based on the rotation of water-dependent herbivores between wet season and dry/season ranges, is outlined to indicate the appropriate spacing between perennial waterpoints. The aim is to apportion vegetation impacts evenly between these ranges, and allow plants a period of recovery from severe grazing pressure. The model suggests that a much wider spacing between perennial water sources is advisable than is currently operative in most conservation areas. Seasonal waterpoints reduce the period of concentration near perennial water, but prolong use of vegetation in the wet season range. Excessive waterpoints (1) favour water-dependent ungulates and elephants at the expense of rarer ungulates, (2) increase predator impacts on prey populations, (3) widen vegetation degradation, (4) worsen animal mortality during droughts, (5) decrease ecosystem stability, and (6) lead to a loss of biodiversity.
The preferred habitats of the African bush elephant, Loxodonta africana, are forestedge, woodland, bushland and wooded or bushed grassland. Increasing amounts of grass in the elephants' diet are correlated with conversion of wood habitats towards grassland, and with increasing elephant mobility, poorer physical condition, and progressively increasing natural regulatory processes leading to decrease in numbers. Elephant occur in discrete unit populations. Each population shows a series of highly contagious instantaneous distributions which, when averaged over a period of time, probably tend, in a uniform habitat, to approach a random or regular distribution. High densities or disturbance by man lead to increase in mean group size and more uneven distribution. The effect on woody vegetation is greater and more lasting than on grass or herbs and usually radiates outwards from the initial centre of damage. The typical cycle begins with destruction of the understory, followed by ringbarking of adult trees, and is accelerated by fire. Several case studies involving forest, moist and dry woodlands, and dry bushland are described which fit this pattern. /// Предпочитаемые местообитания Африканских слонов Loxodonta africana - опушки леса, лесистые местности, кустарник и открытые участки с деревьями или кустами. Увеличение относительного количества травы в диете слонов коррелирует со сменой лесных местообитаний открытыми, увеличением подвижности слонов, ухудшением физических условий и усилением действия естественных регулирующих процессов, ведущих к снижению поголовья. Слоны встречаются отдельными разрозненными популяциями. Каждая популяция образует ряд временных, вступающих в контакт группировок, которые при анализе в среднем за определенный промежуток времени очевидно имеют тенденцию к однородным местообитаниям, и их распределение приближается к рандомическому распределению. Высокая плотность и влияние деятельности человека приводит к увеличению стад слонов и еще более неравномерному распределению. Повреждения древесной растительности более сильные и длительные, чем травянистой. Обычно эти повреждения распространяются вширь от одного исходного очага. Типичный цикл смены местообитаний начинается с уничтожения подстилки и обгрызания коры на деревьях. Эта деятельность усугубляется пожарами. Приведены некоторые случаи исследований во влажных и сухих лесах и сухих кустарниках, которые подтверждают эту схему.
Changes in vegetation cover in northern Chobe National Park (Botswana) were assessed using aerial photographs from 1962, 1985 and 1998, with subsequent ground proofing. In addition, cumulative browsing by elephants and the occurrence of fire scars were recorded on random vegetation sites within shrubland (n = 20) and mixed woodland (n = 20). Coverage of woodland vegetation decreased from 60% to 30% between 1962 and 1998, while shrubland vegetation increased from 5% to 33% during the same period. During the study period, woodland has gradually retreated away from the river front. While riparian forest covered a continuous area along the riverfront in 1962, only fragments were left in 1998. We found a significant decrease in browse use with increasing distance to the Chobe river for Combretum apiculatum, Combretum elaeagnoides, Combretum mossambicense and other woody plants combined (all P < 0.0001). The occurrence of fire (P < 0.0001) and basal area (P < 0.0001) were positively related to distance to the river. Elephant browsing occurred on >70% of available stems within 2 km from the river, while less than 20% of the trees had fire scars in the same zone. Beyond 7 km from the river, elephant browsing was reduced to >50% of available stems, while more than 50% of the trees had fire scars. The density of any of the shrubs was not related to distance to the river neither within shrubland (all P > 0.05) nor within mixed woodlands (all P > 0.05).