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Biological Conservation 82 (1997) 129-136 © 1997 Published by Elsevier Science Ltd

All rights reserved. Printed in Great Britain 0006-3207/97 $17.00 + 0.00

H U M A N SETTLEMENTS A N D WILDLIFE DISTRIBUTION IN THE S O U T H E R N K A L A H A R I OF B O T S W A N A

Alex Verlinden

Botswana Department of Wildlife and National Parks, PO Box 131,Gaborone, Botswana and Institute of Nature Conservation, Kliniekstraat 25, B 1070 Brussels, Belgium

(Received 12 March 1996; accepted 17 August 1996)

Abstract Relationships between selected wildlife species and •e- stock distributions obtained from aerial surveys in the wet season o f 1992 were examined in the southern Kalahari of Botswana. Borehole locations were used to identify main human and livestock activity. Multivariate statistics were used to analyze the influence of environmental variables, including vegetation condition and vegetation structure, on wildlife and livestock distribution in the wildlife core area of the Kalahari. Grass greenness and distance to boreholes were found to be the main variables influencing the distribution of larger wildlife species and livestock. Many wildlife species selected green areas. The northern portion o f the southern Kalahari has a higher average rainfall and was consequently greener; a further conse- quence was that more humans and boreholes were found there. It appeared that wildlife species most dependent on moisture occupied only a relatively small range of avail- able habitat, selecting the mainly unprotected greener areas but avoiding areas very close to boreholes, possibly because they are occupied by livestock and close to humans. It is postulated that increasing human impact in the wet-season wildlife ranges may contribute to a decrease of wildlife numbers in the Kalahari, especially those species highly dependent on patches o f green grass. © 1997 Published by Elsevier Science Ltd

Keywords: Kalahari, human settlements, ungulates, greenness, livestock.

I N T R O D U C T I O N

The southern Kalahari is a vast semi-arid region con- sisting mainly of infertile red sand overlying discontin- uous calcrete layers (Knight et al., 1988). The calcrete is exposed within pans and dry riverbeds and has a vege- tation of better nutritional quality (Leistner, 1967). No natural permanent water exists within the region (DHV Consulting Engineers, 1980), but due to deep borehole technology the last two decades have seen a notable expansion of livestock production into the Kalahari (Cooke, 1985; Arntzen & Veenendaal, 1986). The

increased pressure of man on the Kalahari has reduced the natural ranges of the indigenous ungulates, resulting in the provision of artificial water for wildlife in the neighbouring Kalahari Gemsbok National Park of South Africa (Knight et al., 1988) since the 1930s. Some Kalahari ungulates are generally able to survive without drinking water because of their physiological and beha- vioural adaptation (Taylor, 1968; Williamson, 1987; Skinner & Smithers, 1990; Knight, 1991). The wildlife populations are characterised by high mobility in search of better quality food patches to satisfy water and energy demands. Key elements in the survival of these animals may be the ability to move over vast distances without being hampered by barriers, and to frequent pans for nutrients (Parris & Child, 1973; Knight et al., 1988). It has been suggested that die-offs during droughts in dry-season refuges are related to these being fenced off (Campbell, 1981) or increasingly occupied by humans and livestock with a reduction in available sur- face water for wildlife (Williamson et al., 1988).

As the possibilities to move over vast areas in sur- rounding countries are increasingly hampered by fen- cing and by the proliferation of human settlements and associated developments, the studied area is one of the few left in southern Africa where natural movement patterns still occur over vast distances. As more infor- mation is needed on the effect of man's increased pres- sure on wildlife population and distribution in the southern Kalahari of Botswana, the aim of the present study was to determine how livestock and wildlife dis- tributions vary in relation to each other and selected environmental factors, such as vegetation condition and distance to artificial waterpoints and, in particular, to determine the effects of pastoralism in the wet season range of major wildlife species of the Kalahari.

129

S T U D Y AREA

An important wildlife core area of the Kalahari during the wet season is the Schwelle (Fig. 1) (Bonifica, 1992b). This contains numerous pans and has a thorn tree savanna vegetation type characterised by Acacia erioloba

130 A. Verlinden

and A. luederitzii. The present study was conducted in the Kgalagadi district of Botswana, an area of c. l l 0 0 0 0 k m 2 between the settlements of Ukwi, Kang, Werda, and Bokspits (Fig. 1). The average rainfall lies b e t w e e n 200 m m in Bokspits, 300mm in Tsabong and 350mm in Kang (Bhalotra, 1985). The total rainfall for the wet season of 1991-1992 in Tshane, situated in the centre of the study area, was 197mm, ~>40% below average (Meteorological Services, Botswana). Although more than 50% of this area is protected as either National Park, Game Reserve or Wildlife Management Area (WMA), there is growing concern that increasing human impact could disturb wildlife movements and prevent accessibility to key areas now or in the future. Mass die-offs of up to 80% of the populations due to a severe drought in 1982-1986 occurred in the area (Spinage

NAMIBIA

ZIMBABWE

BOTSWANA

I/o SCHWELLE~o Kang ~ ~ M a t ~

GALAGADI GEMS" .......... KBO ~ NATIONAL~ ~ . = L PARK ~C~,~ Ir . • ~ j

]~,'J Tsa bo ng

SOUTH AFRICA

100km

Bokspits

Fig. 1. Map of Botswana, showing the Schwelle; ~ Kgala- gadi district; ~ the protected areas and settlement~ referred

to in the text.

& Matlhare, 1992) while game counts suggest no signif- icant recovery after this drought (Bonifica, 1992a; DWNP, 1995). This resulted in a selective hunting ban for most species in effect from the hunting season of 1992 in the study area. Only hunter-gatherers were allowed to hunt.

M E T H O D S

Since 1986, the Depar tment of Wildlife and National Parks (DWNP) of Botswana has conducted aerial sur- veys to monitor wildlife and livestock populations in the country following the methodology of Norton-Griffiths (1978). These surveys cover districts degree square by degree square, using 6min of longitude as the basic spacing. Sampling strips were demarcated by rods and streamers attached to the lift struts of a Cessna 206 air- craft and calibrated trigonometrically. The mean strip width was calibrated to 200 m at a flying height of 300 ft (90m). The wet season survey of Kgalagadi District 1992 (Fig. 1) was conducted during 5-20 February to assess vegetation condition and structure in addition to the animal census. (Table 1) presents the number of animals observed during the census, the maximum number seen in 6 x 6 rain grids, and the number of grids where each species was recorded.

At 2-min latitude intervals the following vegetation characteristics were estimated: (1) tree and large shrub cover, excluding shrubs at herbaceous layer level; (2) herbaceous layer (mainly grass) cover; (3) greenness of grass cover; and (4) observations on fire occurrence. The mean value of three observations was used in the calcu- lations on 6 × 6 min grids. Woody plant cover was esti- mated because of the importance of shade in a hot climate and to detect areas characterised by shrub encroachment, while grass cover, fire and grass green- ness are mainly related to food availability and quality. Melons are found to be an important dry season moisture source (Williamson, 1987; Knight, 1995) and therefore good indicators, but they were not seen during the survey as they are scarce in drought years.

Table 1. Total number of observed animals during the census, the maximum number seen per grid and the number of 6 × 6 m i n grids where the species was recorded

Species Total observed Maximum per grid Recorded grids

Gemsbok 1791 75 204 Eland 516 150 28 Hartebeest 976 100 93 Wildebeest 385 58 24 Springbok 1859 428 44 Kudu 21 5 11 Steenbok 97 5 79 Duiker 53 4 41 Ostrich 337 21 109 Cattle 1639 300 74 Sheep and goats 1056 143 19 Donkey 127 23 25 Horse 47 6 18

Set t lements and wildlife in the Kalahari 131

Grass greenness was identified as a variable influen- cing wildlife movements by Mills and Retief (1984) and Knight (1991), the latter author finding that higher greenness values correspond with higher moisture and protein content in the southern Kalahari. Greenness was visually assessed on a five point scale: (0) no grass, (1) grass without green colour or moribund, (2) grass slightly green, mainly brown or yellow, when in burned area a very slight green flush, (3) grass colour yellow-green, when burned a slight green flush, (4) grass colour green-yellow, when burned a marked green flush and (5) grass colour predo- minantly green, when burned a bright green flush.

Observations on fire occurrence included an estimate of when they had occurred--ei ther dry season 1991 or early wet season 1991-1992. The latter showed burn signs on shrubs and trees. It is generally thought that most dry-season fires are started by people. Large areas were burned. Older burn scars were not evident.

In this area, artificial waterpoints are centres of human activity since virtually all former hunter-gather- ers are settled and other people are mainly pastoralists. The distance from the centre of every grid to the nearest waterpoint was measured using the results of a recent waterpoint survey of the district (Natural Resources Services & Knight-Piesold Botswana, 1991) using a Global Positioning System. In the analysis, all boreholes were included except those exclusively designed for wildlife use, i.e. boreholes for cattle were included with boreholes for settlements. To simplify the calculations the many boreholes in settlements were pooled into one unit. Figure 2 shows the distribution of livestock bio- mass in Botswana derived from the dry-season aerial surveys in 1994 (ULG, 1994) and calculated according to Bonifica (1992a). The map is a useful illustration of the current distribution of humans and their livestock in the study area and shows that only a few livestock-free corri- dors now exist between the southern and central Kalahari.

The actual numbers of animals seen inside the trans- ects and all the above-mentioned environmental data were imported into a Geographical Information System (GIS) (IDRISI) for further analysis. Multiple linear regression techniques and multivariate analysis were used to analyse the influence of the estimated and mea- sured environmental factors on wildlife and livestock distribution. The latter included Detrended Correspon- dence Analysis (DCA), based on reciprocal averaging using D E C O R A N A (Hill, 1979) and Canonical Corre- spondence Analysis (CCA) using CANOCO (ter Braak, 1986, 1987). DCA is an indirect ordination technique for species and samples, assuming a unimodal model for the relationship between the responses of each species and the ordination axes. CANOCO is also based on correspondence analysis (Greenacre, 1984), an eigen- vector ordination technique for generating ordination axes which are constrained to be linear combinations of environmental variables. Canonical coefficients were derived from the least squares fit of the multiple regres- sion model which was calculated for each axis sepa-

rately. T-values above 2.1 are considered to indicate significant environmental variables (ter Braak, 1987). The significance of the canonical analysis was tested with Monte Carlo Permutation tests (ter Braak, 1987). To be valid, besides this test, the ordination of the spe- cies in the CCA has to be similar to the ordination in the DCA, taking into account that the orientation of the axes might be reversed.

RESULTS

Relations between environmental factors

Tree cover in relation to borehole distance High woody plant cover was associated with proximity to boreholes (Fig. 3(a)). A second order polynomial regression between woody plant cover and borehole distance showed this to be significant (r = 0.42, p < 0.01).

Grass cover in relation to borehole distance A first order asymptotic model fitted to the data showed a significant relationship between grass cover and bore- hole distance (r=0.47, p<0-001). Maximum cover occurred between 15 and 60 km from the nearest bore- hole, and below average cover within 10 km (Fig. 3(b)).

[ ] 1-64 LU / lOOkm 2

[ ] 64-512 LU / lOOkm 2

[ ] 512-2048 LU / lOOkm 2

• >2048 LU / lOOkm 2

Fig. 2. Livestock biomass distribution in 1994 in Botswana. Metabolic biomass is expressed in livestock units (LU). The

distribution map is derived from ULG (1994).

132 A. Verlinden

This was attributable to heavy grazing by livestock which were concentrated around boreholes. Low grass cover also occurred in the south and southwest which not only generally receive lower average rainfall than

(a) .<.~<..~7-~:.~

i f i l i ........ 7."1jr

i ~ 1.1 ilit-t921111(ii~!11 4o i i.it,7 i771111111 1111. 32 i i ' !.. L ' ~ ' ~ l l l l l f i

8 " : ! , :. ~ ; - = 1 0 ~,~ ?.; ::; 20

the north, but also had more burned areas (Fig. 4). Burned areas have lower grass cover, but might show increased greenness and therefore the relationship between greenness and borehole distance might differ from the grass cover~listance relationship.

Greenness in relation to borehole distance Figure 3(c) shows that intermediate greenness values (scores 2-3) predominated and were most prevalent between 20 and 30 km from boreholes, i.e. they followed a similar pattern to grass cover. There were relatively few values with maximum scores of 5, possibly a result of the drought.

Species ordination A D C A of 758 grid data was produced for actual num- bers of animals seen (Fig. 5) . The eigenvalue of axis 1 was 0.9 and of axis 2 was 0-6. Figure 5 shows that live- stock species were distributed as a distinctive cluster with high values along axis 1. Wildlife comprised a group consisting of wildebeest Connochaetes taurinus, harte- beest Alcelaphus buselaphus and eland Taurotragus oryx

%

'¢~0~<<. (/C4 ~/ T1

(c) ~<- ~ i ~ : ~

, ~ ~ .... . I .......... I. i "~ I i .... • "I ~ " -4. ' " -

5 6 , , ~ "1

1 ~ t ; I i ~- ri-7--i~lll-i-~J 'i i ~ 'i

.:; i l l 7Wl11/12 24 !. ! i .J<'(~;lfi"3~ I.I7774 I ~ I J I II II I'

t , <... ;:- Fr l[7111ll llllf II II If II

O / ' S ' ~ I 4 , , C , " , ' ~ ' , P , ~ ~ 4 a

Fig. 3. (a) Tree and shrub cover (%) in 758 grids in relation to borehole distance; (b) grass cover (%) in relation to borehole distance in unburned grids; (c) grass greenness degree of 758

grids in relation to borehole distance.

Fig. 4. Map of burned areas in Kgalagadi District recorded during aerial surveys in 1992.

1 0 0

.co -1 O0

-200

- 3 0 0

gemsbok

steenbok

ostrich duiker

c a h ° d o sh springbok

eland hartebeest

wildebeest

i = =

0 1 O 0 2 0 0

1st axis

- 4 0 0 '

- 1 0 0 3 0 0 4 0 0 5 0 0

Fig. 5. Detrended correspondence analysis of animals recor- ded in 758 transect grids: ca, cattle; ho, horse; do, donkey; sh,

sheep and goats.

Settlements and wildlife in the Kalahari 133

with low values along axis 2, gemsbok Oryx gazella with a high value and four other species with intermediate values along both axes.

As the water dependence of wildebeest in the Kala- hari is well documented (Knight et al., 1988; Williamson et al., 1988), while gemsbok is considered to be the most drought-resilient (Taylor, 1968), the second axis appears be related to moisture dependence of these ungulates. Steenbok Raphicerus campestris and common duiker Sylvicapra grimmia are widespread and considered to tolerate human impact.

distance, grass greenness, fire and tree cover were found to be statistically significant environmental parameters to explain wildlife/livestock distribution in the CCA, the relationships between individual wildlife/livestock spe- cies and these three parameters were further analysed using the results of multiple linear regression tests (Table 3). While the CCA suggested significant correla- tions between livestock and shrub encroachment, this was not confirmed with the multiple linear regressions, perhaps because of a non-linear relationship between livestock and tree cover.

Relations between animal distribution and environmental factors

Canonical correspondence analysis The environmental variables distance to borehole, grass greenness, grass cover and tree cover were used in the analysis. Preliminary runs of the program showed that grass cover did not contribute significantly to explaining the percentage of variation in animal distribution. Grass cover was then excluded, improving the eigenvalues of the first two axes to, respectively, 0.66 and 0.24, and Monte Carlo Permutat ion tests revealed that both were significant (p < 0-01). Grass greenness, borehole distance and fire were negatively correlated with the first axis, while tree cover was significantly correlated with the 1 st axis ( t>2.1) , which also separated livestock species from wildlife species (Table 2).

Figure 6 shows the resulting species-environment biplot. This graph suggests that high tree cover (indi- cating bush encroachment) was closely related with livestock. High greenness values were negatively corre- lated with the first axis, indicating that they were not common in livestock areas. The graph further suggests that gemsbok especially was found away from bore- holes. Eland, wildebeest, springbok and hartebeest had an opt imum in green areas with a low tree cover at medium distances from boreholes.

The results of the CCA are in general agreement with the results of the DCA. The majority of larger mammals prefer green areas but also try to avoid areas character- ised by livestock. In 1992, areas with higher greenness values were found relatively close to livestock areas, indicating a possible conflict over resources between wildlife and livestock in the wet season in the Schwelle.

Fires were not common in livestock areas but wildlife species did not seem to avoid burned areas. As borehole

Gemsbok. Table 3 shows not only a significantly positive correlation (p < 0-001) between gemsbok num- bers and increasing distance to boreholes, but also with greenness and fire. The preference for green grass was not obvious from the ordination results.

E/and. Although far less abundant than gemsbok, the results in Table 3 show that eland have a similar preference. Wildlife distribution data suggest that eland tend to avoid areas affected by human impact. There are, however, exceptions to this impression. In 1992, eland were found more frequently in areas that received higher rainfall and consequently show higher grass greenness values. These areas were confined to the Schwelle, possibly forcing eland to come closer to set- tlements than usual.

{0

o4

400

200

-200

I ~ g borehole distance

e du

~ h a ! sp

donkey sheep + goats

horse

cattle tree cover

greenness

-400 ' ~ ' -500 -300 - 100 100 300 500

1st axis

Fig. 6. Canonical correspondence analysis of 400 grids in the Schwelle with environmental variables greenness, distance to borehole, fire and tree cover: ge, gemsbok; ha, hartebeest; el, eland; wi, wildebeest; sp, springbok; du, duiker: st, steenbok;

os, ostrich.

Table 2. Canonical coefficients (C J) and t-values of the first two axes derived from a canonical correspondence analysis of 400 aerial survey grids located in the Schwelle

Environmental variable CJ axis 1 t axis 1 CJ axis 2 t axis 2

Greenness 4)-95 -15.7* 4)-35 -5-8* Borehole distance 4)-5 -1 l* 0.5 10.9* Fire 4)-32 -7* 4)- 13 -2-8* Tree cover 0-26 3-9* 4)-09 -1.0

*Significant at p < 0.05.

134 A. Verlinden

Hartebeest. In contrast to eland and gemsbok, har- tebeest were more regularly found in higher numbers closer to settlements and boreholes. Hartebeest were quite abundant between 10 and 4 0 k m from the bore- hole because of their preference for green grass during the wet season. The regression analysis suggests that hartebeest did not only select areas with high greenness values but were also more frequently found in burned areas than elsewhere.

Wildebeest. Although less abundant now than har- tebeest, the wildebeest distribution along the borehole gradient showed similarities with hartebeest, suggesting an avoidance of overgrazed areas but a preference for green habitat, especially in northern Kgalagadi during the wet season. As wildebeest are now protected and are generally a less desired species for meat, they undergo low hunting pressure. The narrow distribution along the borehole gradient suggested a high vulnerability towards drought and deteriorating grazing conditions due to increasing livestock numbers in their wet season range. There was no indication that wildebeest selected burned areas.

Springbok. In February 1992, springbok were found regularly between 2 and 40 km from boreholes. In con- trast to many wildlife species, springbok appear to be attracted to areas close to boreholes (p<0.001), sug- gesting a tolerance of overgrazed areas. They also show an opt imum between greenness degrees 3 and 4, sug- gesting that springbok outside livestock areas prefer green forage.

Ostrich. The presence of boreholes had no apparent influence on ostrich. This suggests that livestock have no impact on ostrich distribution. They clearly preferred green areas and are therefore more common in northern Kgalagadi. This preference for green areas and high occurrence in burnt areas suggests a higher dependence on moisture than is revealed by the results of the DCA.

Livestock. Given their dependence on boreholes, the distribution of livestock along the borehole gradient was as expected. Goats and sheep remain closest to the borehole with cattle moving further away in search of good grazing. Horses, donkeys, and goats and sheep are positively correlated with low greenness values as a result of very low grass cover. Cattle might be more dependent on two factors which are hard to combine over a short distance in the southern Kalahari, namely water from a borehole and green grass associated with high grass cover.

D I S C U S S I O N This study explored relationships between human set- tlements and wildlife distribution during the wet season of 1992. In the southern Kalahari, the area called the Schwelle, which is characterised by numerous pans and a mainly thorn savanna vegetation, can be considered as the core area for Kalahari wildlife (Williamson et al., 1988; Bonifica, 1992b; Verlinden, in press).

Although the estimated values of some of the envir- onmental factors assessed here are observer-biased, the data of the studied area are internally consistent. The applied method is useful in cases where the analysis of a large number of aerial photographs is not feasible, in less known remote areas, or when quick range assess- ments are required. In such a vast, inhospitable and remote region as the southern Kalahari, more objective methods are at present unworkable.

Livestock are confined to boreholes, and distance to borehole appears to be a highly useful index of human impact in the Kalahari. The relationship between tree and shrub cover with borehole distance suggests that bush encroachment as a result of livestock grazing is a common phenomenon in communal rangelands in this port ion of the Kalahari. Bush encroachment was detec- ted up to 25 km away from the nearest waterpoint in communal grazing areas but this could be the result of pooling the boreholes around settlements. In the wet season, areas with low grass cover are confined to a radius

Table 3. Standardised multiple regression coefficients of censused animals with environmental factors grass greenness, grass cover, fire, tree cover and distance to borehole in 758 grids

Species Grass greenness Grass cover Fire Tree cover Borehole distance

Gemsbok 0-19"** -0-11 0-18"** -0.02 0.18"** Eland 0-17"* 4). 14"* 0.03 0-02 0.13"** Hartebeest 0.23"" 4).09 0.13"** 0.09 -0-04 Wildebeest 0.18*** -0.08 0.01 0.02 -0.02 Springbok 0.11"* 4)-03 0.11"* -0.01 -0.11"** Kudu 0.10 -0-02 -0.02 4).01 4).02 Steenbok 4)-01 0-05 0.04 0.05 0.01 Duiker 4).06 4)-02 0-01 0.01 4).07 Ostrich 0.17*** 0-01 0.16*** 0.05 4).01 Cattle -0.08 -0.23"*" -0.20"* 0.04 4). 13*** Sheep and goats -0-18*** -0.20"*" -0.22"** 0.03 4). 13*** Donkey -0-14"* -0-18*** -0-19*** 4)-03 4). 10"" I-to~'se 4). 19*** -0-07 4)- 12"" -0.02 -0-14"**

"'Significant at p < 0.01; "'*significant at p < 0.001.

Settlements and wildlife in the Kalahari 135

o f < 10km from the borehole. This is in line with the results of other studies in the Kalahari (Perkins & Tho- mas, 1993; Bogrnan & Lammers, 1995). Ostrich, common duiker, steenbok and springbok are less influenced by human settlements than other wildlife species. Spring- bok in the Kalahari are attracted to vegetation types characterised by short grass and/or dwarf shrubs, which occur mostly in and around pans or ancient riverbeds but also in areas close to boreholes that are grazed by cattle. Springbok, steenbok and common duiker are considered to be preferential browsers (Skinner & Smi- thers, 1990), and browse is generally still abundant or even locally increased in highly grazed areas.

The analysis suggests that gemsbok and eland can be counted among the animals that move out, or are removed by hunting. For both species hunting and/or poaching seems to be the main reason why they con- centrate in protected areas. Gemsbok are relatively easily hunted by making use of dogs and spears, the only hunting technique officially allowed to be used by hunter-gatherers in the area (Wildlife Conservation and National Parks Act, 1992), while their meat is also much sought after. Gemsbok and eland seem to be able to fulfil all their requirements despite the current human impact, although the latter, being highly mobile and having a higher preference for green areas, may be more susceptible.

The proximity to each other of eland, wildebeest and hartebeest in the DCA and CCA was probably accen- tuated by their occurrence in high numbers in a small number of grids that had a high greenness degree. This suggests a high mobility and could confirm the general idea that some Kalahari ungulates are nomadic during the wet season and move towards greener areas in order to meet their water and nutrient requirements (Smithers, 1971; Verlinden, in press). It appeared that during the wet season of 1991-1992 grass greenness was highest between 10 and 40 km from the nearest borehole due to higher rainfall averages in the northern area (Bhalotra, 1985) where there is a widespread occurrence of boreholes. The explanation is that not only do wildlife prefer green areas in the wet season, but also people probably have selected green areas for their livestock. The results of this study suggest that humans preferred species, cattle, might be especially more dependent on green grass than the others.

Past studies (Williamson et al., 1988; Bonifica, 1992b) were mainly concerned with mass die-offs of wildlife during droughts in dry-season refuges. The present study suggests that a gradual decline of habitat availability in wet seasons due to human encroachment in the Kala- hari Schwelle might contribute to a decline in the num- bers of the studied wildlife species, wildebeest, hartebeest and eland. The general avoidance of live- stock areas by gemsbok, wildebeest, hartebeest and eland suggests that populations of these species occur- ring in the southern and central Kalahari might become separated when the area occupied by livestock increases along the boundary between the two areas. The need to maintain the integrity of the protected areas by pre-

venting livestock encroachment into them, if the Kala- hari is to retain significant herds of large herbivores, cannot be overstated.

Acknowledgements I gratefully acknowledge J. Perkins, J. Matlhare and two anonymous referees for reading and commenting on earlier drafts of this paper. D. Gibson (ULG) is thanked for providing the 1994 survey data. Professor E. Kuijken from the Institute of Nature Conservation, Belgium is thanked for making this study possible and the Depar tment of Wildlife and National Parks, Bots- wana for permission to publish. The aerial survey team, consisting of C. Craig, B. Pelekekae and B. Moagele is thanked for their excellent cooperation in the field. Without the support of Jill and Keith Thomas I would not have enjoyed desert life as I did.

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