Effects of poaching on African elephants
Poaching caused a decline of African elephants from 1.3 million to 600,000 individuals between 1979-1987. Mortality was unusually concentrated among the largest adults with the biggest tusks. Old matriarchs (the oldest adult females that provide the social glue in elephant herds) were particularly vulnerable. Their tusks are large and their groups were easier to find than solitary adult males.
Many family groups lost their matriarchs, compromising their social, competitive and physiological functioning. The youngest offspring often perished with their mothers, causing a disrupted age structure. Many older offspring were orphaned, only to range solitarily or in atypical groups of unrelated females. Documenting the long-term consequences of social disruption caused by poaching on the African elephant is crucial to the conservation and management of this species.
We examined long-term impacts of poaching on elephants of the Mikumi-Selous Ecosystem, Tanzania—one of the largest and most heavily poached elephant populations on the continent prior to the 1989 ivory ban. Our study focused on Mikumi National Park in the northern part of this ecosystem.
We compared group structure, social cohesiveness and physiological health of poached and unpoached elephant groups. Poached groups were identified by their peculiar group structure, based on age and relatedness of their adult females.
The degree of relatedness among group members was determined from DNA extracted from their feces. Hormones extracted from feces were used to quantify physiological stress and reproductive function in these groups. Rates of aggression, affiliative behavior and competitive exclusion from resources were also compared across groups.
A high percentage of single adult female families had an atypically small family size, with an average of only 2.2 adult females. Thirty percent of all adult females were solitary in Mikumi, changing little from what was observed in 1989 despite increased protections since the ivory ban.
There was also high variance in adult relatedness, with some families consisting entirely of non-kin. This is unusual for a species characterized by close family units of several related adult females, and quite consistent with our predictions of long-term impacts on this heavily poached elephant population.
Mean relatedness also corresponded with strength of social association for adult female elephants in Mikumi. Small families tended to be less closely related and some exhibited very fluid, uncohesive social behavior, whereas on average larger families, which are more consistently closely related, tended to form stronger bonds and tighter social units.
Physiological measures of stress and reproduction were also consistent with the disruptive effects of poaching. Stress levels were highest in groups that lacked an old matriarch, had few closely related adult females, and weak social bonds. Stress was particularly high in groups still ranging in areas where poaching was historically high—areas nearest villages, furthest from park headquarters that had high carcass counts during the peak-poaching era of the 1980’s. Females from genetically disrupted groups, many in their reproductive prime, also had fewer young calves.
Our findings suggest that these are long term consequences of poaching since the distribution in group size has changed little since 1989, though the number of families with tusked old matriarchs increased by 14.2%.
Controversy continues to surround management and trade policies for African elephants. Results of this study bear directly on this controversy. The length of time required to recover from extensive poaching detected in our study suggest that African elephants have had insufficient time to recover from the 60% poaching-related decline of the 1980’s. Thus, the impacts of renewed illegal trade in elephant ivory or culling as a means of population control in this highly intelligent, tightly knit social species may be far more grave than predicted by economic models alone.
Regrettably poaching is once again on the rise.
- Gobush, KS and SK Wasser. 2009. Behavioural correlates of low relatedness in African elephant core groups of a poached population. Animal Behaviour 78: 1079-1086.
- Gobush, KS, B Kerr, SK Wasser. 2009. Genetic Relatedness and Disrupted Social Structure in a Poached Population of African Elephants. Molecular Ecology 18: 722-734.
- Gobush, KS, BM Mutayoba, SK Wasser. 2008. Long-Term Impacts of Poaching on Relatedness, Stress Physiology, and Reproductive Output of Adult Female African Elephants. Conservation Biology 22: 1590-1599.
Sokoine University of Agriculture, Tanzania and Tarangire Elephant Project
This work was part of the doctoral disseration of Kathleen Gobush, Ph.D. in our Center. Kathleen is currently the Mortality Mitigation Program Leader, for the Hawaiian Monk Seal Research Program, Pacific Islands Fisheries Science Center- NOAA.
This work was supported by grants from Oracle, Miami MetroZoo, Sophie Danforth Conservation Fund, the Morris Animal Foundation and the Center for Conservation Biology.