Managing the elephants of Kruger National Park

The elephant population in Kruger National Park, Republic of South Africa, is growing rapidly. To prevent damage to the Park’s ecosystems, the management has culled about 7% of the population annually. Such culls are very controversial. At first glance, contraceptives seem an attractive alternative means of control. We examine contraception as a management option, review the relevant aspects of elephant reproduction, physiology and demography and conclude that this optimism is probably misplaced. First, contraceptives have a wide range of physiological and behavioural side-effects that may prove to be damaging to the individual female and those around her. Second, the elephants in the Park have near-maximal growth rates with inter-calving intervals of less than four years. To achieve zero population growth, about three-quarters of the adult female elephants would need to be on contraceptives. There are no simple alternatives. The smallest numerical target for controlling population numbers is to kill or sterilize females about to become pregnant for the first time. Such a solution is unlikely to appease those who consider killing elephants to be unethical. It may, however, be the one closest to the natural patterns of elephant mortality. All correspondence to: Stuart L. Pimm at Department of Ecology and Evolutionary Biology, The University of Tennessee, Knoxville, TN 37996, USA. Tel: 423–974–1981; Fax: 423–974–0978; E-mail: [email protected] the first place? The second is how do we go about doing it? ‘Playing God’ is the frequent criticism – including by an anonymous reviewer of this paper – of those wildlife managers who would intervene in the natural course of things. The elephant problem in KNP is self-imposed: the decision to control elephants is based on speculation about what would happen if the elephants were to increase. Yet such speculation may be unavoidable. The obvious experiment of letting the elephants increase could have unfortunate consequences to the ecosystem and the species that depend on it that persist for decades or longer. These consequences raise more questions. Did elephants effect such ecosystem changes in the past? Even if they did, should they do it now? What were the natural processes that regulated elephant numbers in the past? And what is ‘natural’ today in an Africa with a large and rapidly growing human population and few well-protected parks? Given the choice of controlling elephant numbers, does KNP’s management have alternatives to killing elephants that will still keep their numbers low? The use of contraceptives has been touted as a promising solution (Short, 1992; The Economist, 1996). Consequently, much effort has been spent to investigate the endocrinology of elephants and to evaluate the potential use of substances such as RU3486 that may block implantation (Greyling, van Aarde & Potgieter, 1997; Greyling, Ford et al., in press). Other methods based on either immunocontraception or slow-releasing estrogen implants are presently under trial in KNP. We outline the serious and unpredictable consequences to the individuals of these methods. These include changes in behaviour, social status within the breeding herd, and in the steroidogenic activities of the ovary and uterus. Females treated with estrogen implants may remain in permanent estrus. We cannot predict either their behaviour or the consequences that this may have for the park’s male elephants. Moreover, contraception is impractical. We will present models of elephant demographics to show that perhaps 75% of female elephants would need to be treated. Even at this level, the number to be treated would grow for well over a decade before we would achieve zero population growth. Finally, we ask what is the smallest number of animals one should kill or sterilize if the goal is to control the population. In this answer is more than just a practical solution to controlling the population. For it identifies the weak link in the chain of life-history – the age and sex group where natural processes would make the largest difference to whether the elephant population increased or decreased. WHY CONTROL ELEPHANT NUMBERS? In at least some dry forests, increasing numbers of elephants progressively destroy the forest and convert it into more open habitats (Laws, 1970; Barnes, 1980, 1983; Douglas-Hamilton, 1987; Dublin, Sinclair & McClade, 1990; Jachman & Croes, 1991; Dublin, 1994; Leuthold, 1996; see Lawton & Gough (1970), Guy (1982) and Ben-Shahar (1996) for alternative explanations). In KNP, elephants also suppress the rejuvenation of selected woodland trees (Viljoen, 1988; Trollope et al., in press). By comparing ecosystems on either side of an elephant fence, Cumming et al. (1997) demonstrated significant reductions in the species richness of birds, ants and other taxa, where elephants have removed the tree canopy. There are several reasons why we may wish to prevent these changes. We might justify the low elephant numbers from assessments of biodiversity. Naturally high elephant numbers may have made woodlands and their inhabitants scarce or local. On a continent where so little protected habitat remains, there may be the need to conserve a set of species more localized and so more vulnerable to extinction than elephants (Hoeft & Hoeft, 1995). This justification explicitly recognizes that restoring nature to its original state on its original scale may now be impossible. Alternatively, the target size that the population must not exceed may be historically justified. Phrased another way, conditions may be different now and we need to restore them to their former, natural state. We are reasonably sure that elephant densities are now different from the recent past. Evidence suggests that KNP held only low numbers of elephants at the turn of the century (Vaughn Kirby, 1896; Whyte, in prep.). Pienaar, van Wyk & Fairall (1966) estimated the 1947 population to be 560 elephants. During the 1960 (first) aerial survey the population comprised 1186 individuals (Pienaar et al., 1966). Whether these were natural low numbers or, as Spinage (1973) has argued, the consequences of the ivory trade is still debatable. If the latter explanation holds, then elephants would naturally have been abundant. So why were there originally any areas with extensive woodlands? Elephants might increase over century long time scales, destroying the woodlands in the process, and thus causing their own decline (Caughley, 1976). Elephants and forests might co-exist only over very large spatial scales that permit large mosaics of soon-to-be-destroyed woodland, about-to-recover near-treeless ecosystems and everything in between. Finely resolved palynological data might elucidate such changes, but we are not aware of such studies over sufficiently large areas. In any case, even parks as large as KNP’s 20 000 km2 may be too small to allow such cycles to operate. The idea of trans-frontier conservation areas (‘peace parks’) – huge protected areas that stretch across international barriers – may take care of some of these problems of geographic scale. The realization of such parks is still in the future. If the explanation that elephants were always at low numbers is correct, then what were the reasons why elephants remained scarce and what has changed? To answer these questions we need to consider elephant demography. 78 I. WHYTE ET AL.