Modelling the effectiveness of contraception for controlling introduced populations of elephant in South Africa

Abstract

Re‐introduced African elephant (Loxodonta africana Blumenbach) populations are growing at very high rates in many of southern Africa’s reserves, have attained densities higher than previously thought possible and may be exhibiting irruptive growth. Active management of such populations is necessary to prevent the potentially negative effects on habitat and biodiversity that are associated with elephant overpopulation. One potentially feasible method of elephant management is immunocontraception, but very little is known about the long‐term effectiveness of this method. Using demographic data from three South African elephant populations, we made model projections of the effects of contraception on population growth rates to determine whether contraception may be a feasible management tool for elephant. In comparison with noncontracepted populations, realistic reductions in population growth rate after 20 years of contraception were projected to be up to c. 64%, with 50% being a very feasible target. Through its ability to reduce population growth rates, immunocontraception should be an effective tool for preventing or minimizing irruption in elephants and, perhaps, other introduced ungulate species.

How Immunocontraception Can Contribute to Elephant Management in Small, Enclosed Reserves: Munyawana Population as a Case Study

“…Female elephants may reach sexual maturity as late as 17 years [52] , and will typically produce the first calf two years later [55] , [56] , [57] . However, Mackey [11] , [58] calculated the average age of female sexual maturity in four small, enclosed reserves to be between 8 and 10 years. The average age of sexual maturity of the Munyawana population was previously thought to be 10 years [11] , but additional data up to 2009 indicate this to be nine years.…”

“…The inter-calving interval of cows is between four and five years [55] , [59] , [60] , with estimates as high as four to nine years [61] . However, recent studies in enclosed populations in South Africa determined calving intervals at between three and four years [11] , [58] . Again, newer census data up to 2009 (but before immunocontraception took effect) for Munyawana indicate average calving interval has reduced from four years [11] to three years.…”

Managing Large Herbivores in Protected Areas

“…Hobbs et al [65], found that treating small populations -particularly those in enclosed protected areas -with lifetime contraception magnifies their extinction rate. Conversely, Mackey et al [71], opined that elephant contraception would only be effective on small populations in enclosed reserves, because their population age structures would be unstable enough for contraception to have an impact. In a large and old reserve like Kruger NP the elephant population is so large and stable that halting its growth rate would require sterilizing 75% of all females; and there are more than 7000 elephants in Kruger NP [52].…”

Moss, C. J., H. Croze, and P. C. Lee (eds.).2011. The Amboseli Elephants: A Long-term Perspective on a Long-lived Mammal. University of Chicago Press, Chicago, Illinois, 383 pp. ISBN-13 978-0-226-54223-2 and ISBN-10 0-226-54223-8, price (hardbound), $65.00.

“…To help conserve elephants, Joyce Poole and colleagues make a number of conclusive comments, including the overall condemnation of ivory trade, sport hunting, and capture of wild elephants in any form. In contrast, an emerging question facing many managers of increasing elephant populations is how to reduce the risk of elephant overpopulation within protected areas (Mackey et al 2009). In the 2nd chapter, Harvey Croze and Keith Lindsay argue against the concept of elephant overpopulation in Amboseli, saying that managers should rather take a ''not-at-equilibrium'' ecological perspective.…”