The dynamics of elephant populations hold great theoretical and practical interest. The elephant shares many demographic traits with humans—age of sexual maturity, long gestation, single offspring, low death date, and high longevity. It is not only the statistical details of elephant demography, but also the evolution of life history traits and population regulation in this large mammal that are of considerable interest. From a practical viewpoint, an adequate understanding of demographic processes in elephant populations is essential to making sound conservation judgments. Whether it is comprehending how density influences demography or how populations respond to the pressures of poaching, a fundamental consideration in conservation is the trajectory of animal numbers.
Understanding the dynamics of populations requires not only good empirical data from the field, but also the backing of robust mathematical models. Often, very subjective assessments are made about the dynamics of a population. I have, for instance, commonly heard statements to the effect that, "Every elephant group has a calf and thus the population must be doing very well." This could be correct, or it could be completely wrong. The mere presence of a calf in every group does not by itself say anything about the trends in a population. If the population is suffering a high death rate, it could be declining. "Rule-of-thumb" assessments sometimes can be made about the dynamics of a population based on fragmentary data, but this requires additional knowl edge about its demographic attributes. More important, mathematical models allow us to explore the conditions under which a species population is likely to be increasing, stable, or declining. Even if the population itself is increasing, its structure may be getting distorted; for instance, the sex ratio may be skewing in favor of females, as in many parts of southern India, because of selective poaching of male elephants.
Field data on all aspects of demography are not easily available for elephant populations, especially for those living in forests. Even when these have been collected, they are usually snapshots of the population in time. A long-lived species perforce calls for a long-term demographic profile before we can begin to understand its dynamics. Only at Amboseli in Kenya have detailed birth and death records of identified elephants been maintained for longer than one elephant generation. Useful, long-term annual records are also being maintained at Dzanga Bai (Central African Republic), Addo and Kruger (South Africa), eastern Sri Lanka, and the Nilgiris (India). None of these has yet been fully analyzed and presented.
Several methods have been used to estimate demographic variables such as age at first reproduction, interbirth interval, reproductive senescence, and age-specific death rates in the sexes. Postmortem examination of reproductive organs in large samples of culled elephants has provided a wealth of information relating to fecundity in populations such as those in North Bunyoro (Uganda), Luangwa Valley (Zambia), Kruger (South Africa), Hwange (Zimbabwe), and Etosha (Namibia). Field observation methods have been used to derive demographic variables in other places, including Lake Manyara (Tanzania), Amboseli (Kenya), Kasungu (Malawi), Ruhuna-Yala (Sri Lanka [Ceylon]), and Biligiriran-gans-Nilgiris (India). Some very useful demographic data also come from captive elephants held under seminatural conditions, especially in southern India and Myanmar (Burma). The mathematical models needed to make sense of the empirical data have fortunately become progressively sophisticated. These have given us useful insights into demographic factors that could regulate elephant populations. These have also tackled the issue of minimum viable elephant populations for their persistence. Innovative means, such as the use of tusk size in the trade to derive ages of dead elephants, have been used to understand population dynamics in relation to poaching. Models that link population genetics to demography in elephants, however, are still rudimentary. For instance, we have little understanding of the genetics of tusk inheritance in elephants; the modelers, therefore, have relied on the most basic assumptions for linking demography to changes in the tusk trait of elephant populations.
I first provide a comparative account of observed demographic traits across elephant populations and try to relate these to life history evolution in relation to their specific environments. Beginning with the simplest of population models, I then trace the development of the more sophisticated models and the understanding they provide of the dynamics of African and Asian elephant populations.
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