Annals of ivory : perspectives on African elephant Loxodonta africana (Blumenbach 1797) feeding ecology from a multi-decadal record.

Doctoral Thesis


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University of Cape Town

This thesis explores the dietary responses of African elephants (Loxodonta africana) to environmental change by testing the hypothesis that diet switching (from predominantly browse-based to more grass-rich diets) is driven by cyclical patterns of climate and habitat change in a southern African savanna. Elephants are thought to have substantial impacts on their environments, primarily because they consume large amounts of vegetation over sustained periods. However, the woody plant composition of their diet varies considerably across space and through time, so that in some instances they have been found to be almost pure grazers. Tracking these changes by traditional approaches (e.g. field observations) is difficult because of the geographical and temporal constraints inherent to these methods. Stable light isotope tracking of diet allows diet switching to be studied over multiple space/time scales. Here, I use stable isotope data from elephant faeces, tail hair, and ivory to record short- (monthly), medium- (seasonal to annual), and long-term (decadal) ecological variability, respectively, of elephant diets in the Kruger National Park, South Africa. Results from faeces collected at monthly resolution for one year confirm findings of a previous study (based on biannually-collected samples over two years) that elephants generally consume more grass in the more wooded habitats of the northern Kruger Park, but that there is a greater degree of seasonal diet switching in southern Kruger Park habitats. Moreover, diet changes also relate to changes in underlying bedrock across Kruger Park. Isotopic time-series produced by serial profiling of tail hairs confirm patterns observed in faeces. Long-term diet histories of individuals are derived from serial isotope sampling of ivory, yielding records that represent several decades of an animal’s life, at sub-annual (seasonal) resolution. Overlaying individual ivory series in time produces the first, to my knowledge, multidecadal record of African elephant diet, dating from 1903 to 1993. Contrary to expectations, stable carbon, nitrogen, and oxygen isotope records from ivory do not correlate well with cyclical climate trends for the study region. Rather, pronounced diet shifts are observed during extreme climatic events (floods and droughts), and the greatest levels of intra- and inter-annual variability coincide with significant changes in park management policy during the 20th century, i.e. the introduction of water provision programs after the mid 1930s, and the onset of elephant population control in 1967. It is proposed that such direct intervention has played the biggest role in disturbance of elephant-plant equilibria during the 20th century, and further studies to improve our understanding of this phenomenon will be instrumental to development of appropriate management strategies for the 21st century.

Includes abstract.

Includes bibliographical references (leaves 202-256).