Impact of severe drought-related mortality on the subsequent dynamics of a population of African savannah elephants

Abstract

Drought-induced mortality can have major impacts on the numbers of African savannah elephants (Loxodonta Africana) in a population, with juveniles and older adults often disproportionately represented among the elephants that die. During the severe drought in south-eastern Zimbabwe in 1992, hundreds of elephants perished, especially young elephants under eight years of age. Notably, more male juveniles than females succumbed to the drought.  The impacts on the age and sex structure of an elephant population may persist for many years following a drought period. This manuscript utilises data from frequent aerial surveys and a simple population model based on age and sex, including the unusual age and sex structure of the female herds recorded a year after the drought, to explore these impacts over a 30-year period in a population of several thousand elephants in southern Africa. Immediately after the drought, the number of elephants in female herds increased rapidly, but there was no corresponding increase in the number of elephants in bull groups. Male recruitment—when young male elephants transfer from their natal female herds to bull groups—decreased to low levels during the ~16 years after drought. Only when males born after the drought transferred to bull groups did the predicted and observed numbers of bulls increase. Model-predicted trends in the numbers of elephants in female herds and bull groups were a credible fit with survey estimates. The number of elephants in Gonarezhou National Park increased three-fold after 1993.  Since 2013, the population has numbered ~11,000.  This levelling off coincided with the movement of some elephants into areas bordering the Park.