Detection of anthropogenic mortality in elephant Loxodonta africana populations: a long-term case study from the Sebungwe region of Zimbabwe

Abstract

A survey of the Sebungwe elephant Loxodonta africana population in Zimbabwe in 2006 revealed a large rise in the number of dead elephants. The estimated number of carcasses increased >16-fold from 1989 to 2006 and the carcass ratio (number of all elephant carcasses as a percentage of the number of all elephants) rose from 1.25 to 15.4%. The ratio for fresh or recent carcasses, which reflected the mortality rate during the survey year, increased from 0.19 to 1.70% during 1995-2006. Records of elephants killed before 1995 were supplemented with estimates of the numbers killed after 1995, with these estimates increasing exponentially, as did the observed number of fresh or recent carcasses. A maximum likelihood analysis to compare population models revealed that the best fit to the survey estimates of this closed population was a model that started with 9,500 elephants in 1979 and that each year increased at 4.02% and decreased by the number killed, with the number killed annually increasing at 23.5% per year after 1995. A rise in anthropogenic mortality, mostly due to poaching, caused the increase in carcass numbers observed after 1999. Since 1997 the mortality rate of elephants in the National Parks and Safari Areas in the Sebungwe has been positively correlated with the observed number of poachers’ camps. Anthropogenic mortality is now great enough to keep the elephant population approximately constant at 14,000-16,000 animals. The population number was also constant (at a lower level) during the 1980s, when elephants were culled and the sale of meat, hides and ivory covered the costs of elephant management but there have been no recent culls, partly because the ivory trade ban prevents tusks from culled elephants being sold to offset the costs of management. This study illustrates the value of a long-term data set collected with consistent techniques, and including data on other species and the environment collected at no extra cost under the financial umbrella of a charismatic species.