Ecological and anthropogenic drivers of giraffe (Giraffa camelopardalis tippelskirchi) population dynamics in the Serengeti

Abstract

Considerable changes in the abundance and demography of Serengeti National Park giraffes (Giraffa camelopardalis) provide insight into the drivers of ungulate population dynamics. When last systematically studied in the 1970s, the giraffe population was growing at a rate of 5-6% per year, in response to an increase in the biomass of palatable browse species. During three field seasons (2008-2010), I collected data on a significantly reduced population on giraffes: 5-7 times smaller than the 1970s peak. Interestingly, this drop in abundance corresponds to an increase in the density and cover of woody vegetation across the Serengeti. Using the Serengeti giraffe population as a case study, I explore the role of food supply, predation, parasites, and poaching on population dynamics. This investigation is facilitated by contemporary field studies, including vegetation surveys, aerial counts of giraffes, and regular observations of >900 individually identified giraffes. Recent observations are compared to results from the 1970s and combined with a range of other long-term data from the Serengeti (including data on lion kills from a >40-year dataset and data on poaching) to provide a multifaceted picture of the giraffe population and its environment. (I also devise a novel technique of studying lion claw marks on the skin of live giraffes as a measure of predation attempts.) My population estimates confirm that giraffe abundance has fallen significantly since the 1970s. Moreover, I find that recruitment and adult survival are lower now than in the 1970s. The data suggest that giraffe abundance in the Serengeti is currently limited by poaching and by a reduction in the relative dominance of palatable plant species brought about, in part, by the giraffe's own browsing behavior. Using a population model, I investigate how survival and fertility affect giraffe population growth and find evidence that population growth is most sensitive to adult survival. I recommend reducing the targeted poaching of adult giraffes as a conservation measure.