Botswana

In 2003 the northern Botswana elephant population was estimated at 123,000 animals (Chase & Griffin 2003; Table 4c). A crude best-fit model for the population suggests that it reached its present level by increasing at a rate of 6.6% from a population of about 8,000 animals in 1960 (Figure 17).

This apparent growth rate raises interesting questions. If immigration into the population is ruled out the question arises whether an elephant population is capable of such a growth rate. With a fecundity of 0.25 and a central mortality of zero, a population model based on a stable age structure suggests a growth rate of 5.7%. Increasing fecundity to 0.33 (i.e. adult females produce one calf every 3 years) with a central mortality of 0.25 (which is about as low as one might reasonably hypothesize) still does not result in a growth rate as high as 6.6%.

If survey techniques had improved over the time span of the estimates this would provide one possible explanation. However, this seems unlikely: from 1993 onwards the standardised techniques of ULG (1995) have been used in all surveys. A model with a starting population of 47,600 animals in 1987 gives a best fit for the years from 1993 onwards with a growth rate of 6.6%.

If immigration were responsible for the high growth rate it would have had to come from Zimbabwe - the Zambian and Angolan populations were already depressed by 1987. Whilst this is theoretically possible, it is unlikely. Martin (1992b) showed that immigration from Botswana was necessary to explain the high growth rates of the Zimbabwe population from 1980-1991.

Increased growth rate through skewed age structure

A growth rate of 6.6% can, however, be caused by a skewed age structure for the population in combination with a reduced central mortality and a slightly increased fecundity.Calef (1988) detected the unusually high growth of the northern Botswana elephant population but failed to recognise the importance of a stable age structure in the population and attributed the growth rate mainly to a very high fecundity. Using the population model of Martin (2000b), a skewed age structure was simulated by overhunting the adult male segment of the age pyramid so that there were very few males older than 30 years in the population. With fecundity set at 0.28 (i.e some 12% higher than normal) and central mortality reduced to 0.1 % (which implies one natural death per year for every 1,000 animals between the ages of 5 and 45 years), when the hunting quota is reduced to a sustainable level the population growth rate immediately rises to a level of 6.6% and decreases slowly over about 20 years as the population assumes a stable age distribution.

A skewed age structure is the most plausible explanation for the history of population estimates. There is evidence to suggest that prior to the inception of sustainable quotas in the 1990s, the northern Botswana population had suffered a long history of overhunting for trophy males both from citizen hunting and international sport hunting. The absence of large trophy animals in the population has been noted by numerous observers (P. Becker, pers.comm., G.F.T. Child, pers.comm., I.S.C. Parker, pers. comm.).

If the population curve is projected to the year 2005, a population over 150,000 animals is predicted. However, the number of animals resident in Botswana is likely to be less than this. Firstly, the population growth rate should by now have decreased to under 5% as a result of achieving a stable age structure. Secondly, there is evidence of emigration from the population (Chase & Griffin 2004, Chase et al 2004, G. Owen-Smith pers.comm. and population models for Caprivi and Khaudum in this report).

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Zimbabwe

The north-western elephant population in Zimbabwe has a record of standardised sample surveys dating back to 1980. As the elephant population has expanded into forest reserves, communal lands and private farms in the last 15 years, the survey area has increased slightly. However, it is unlikely that because the new areas were not surveyed in the 1980s that the earlier estimates were incomplete: 20 years ago there were very few elephants outside Hwange National Park and the Matetsi Safari Area. Rigorous annual surveys were carried out until 1995. There have been no surveys since 2001.

Taking into account major culling operations in the 1980s, two alternative methods of modelling the population both yield the result that the population is increasing at a rate similar to the Botswana elephant population (more than 6.5% per annum). The explanation for this high rate of increase is likely to be the same as for Botswana - only with a highly skewed age distribution are such growth rates possible.

The north-western elephant population suffered a high level of problem animal control for perhaps 30 years prior to 1980 (D.H.M. Cumming pers.comm.). This could well have been responsible for a population with a low number of adult males and the necessary skewed age structure to produce the high growth rates demonstrated by the population. From 1980 onwards there was an increasing trend towards wildlife becoming the predominant form of land use in Matabeleland North and this allowed the expansion of the elephant population into Forest Areas, communal lands and commercial farms. At the same time, sustainable quotas for elephant sport hunting were being set. These two factors acting together are likely to have produced the apparently high growth rates.It is important to note that a high population growth rate does not necessarily mean any increase in the annual production of elephant calves. Without a stable age distribution, it is simply an arithmetic artefact caused by dividing the same crop of calves by a smaller adult population.

The combined effect of these two very large elephant populations (northern Botswana: 150,000 animals; north-western Zimbabwe: 50,000 animals) has major implications for the management of elephants in north-eastern Namibia.

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Zambia

Elephant populations throughout Zambia were severely depleted by a wave of illegal hunting which began in the late1970s. Between 1981 and 1985 Zambia may have lost 100,000 elephants (Martin 1986). Despite a hunting ban in 1981 and the listing of the Zambian elephant population on Appendix I of CITES, elephants continued to decline in most parts of Zambia. AfrESG (1998) estimated the population as 15,873 animals ('definite' estimate). The south-west corner of Zambia was not exempt from this holocaust.

Chase (et al 2004) carried out a survey of Sioma Ngwezi National Park and its immediate environs and estimated 1,212 elephants, of which the majority were in the national park (1,099). The authors remark that the population does not appear to have increased since the last survey which estimated 1,187 elephants in 1991 (Tembo 1995). They attribute the status quo to high levels of illegal hunting, human settlement along the Kwando River which is preventing Botswana's dispersing elephants from reaching Sioma Ngwezi and to veterinary fences also constraining movements.

DG (2004) shows a discontinuous elephant range in south-western Zambia with no links between Sioma Ngwezi and the nearby Kafue National Park. Recently large numbers of elephant are being seen along the Zambezi from Livingstone westwards to the point where the borders of Botswana, Namibia, Zambia and Zimbabwe meet.

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Angola

The elephant population in Angola was heavily hunted during the UNITA occupation of southern Angola after independence in the 1970s and, although the civil war has ceased, wildlife law enforcement continues to be problem due a lack of manpower and resources.

Chase & Griffin (2004) surveyed the Luiana Partial Reserve in the south-east corner of Angola and estimated 372 elephants. Most of the animals were within 50km of the Namibian border and away from human settlements. Radio tracking studies have shown movements of elephants from Botswana into the area despite the presence of landmines. The authors stress the importance of the Luiana Partial Reserve as a dispersal area for the irrupting elephant population in Botswana.

The status of elephants in Iona National Park is uncertain. They were present here in the 1970s but there are no recent reports which confirm their presence or absence.