An introduction to evolutionary ecology: by Andrew Cockburn, Blackwell Scientific Publications, 1991. £17.50 pbk (xii + 370 pages) ISBN 0 632 027290

Abstract

Stable carbon and oxygen isotopes of fossil fauna have become important tools in linking palaeoecology and palaeoclimate. In the case of grazing ungulates, enamel δ13C reflects C3/C4 grass abundance, which in turn is primarily a reflection of minimum growing season temperature. The south-western Cape is today a winter rainfall area where grasses are almost entirely C3. Other parts of the country, in contrast, have summer rainfall and C4 grass flora. In palaeoenvironmental studies, the presence of C3 grasses serve as an important proxy for the presence of a winter rainfall regime, and as a test of regional models of glacial–interglacial climate change. However, evidence for the extent and intensity of the winter rainfall regime beyond the last glacial period is currently thin, and a coherent picture of the long-term evolution of this system has not yet emerged.Here we present a set of δ13C and δ18O values of fauna from paleontological layers at Hoedjiespunt 1 (HDP1), a fossilised brown hyaena (Parahyaena brunnea) lair dating to between 200 and 350 ka on the west coast of South Africa. The assemblage is dominated by large grazing species which were resident during a glacial period when sea levels were lower and grasslands were widespread and productive. Grazers had predominantly C3 diets, an indication of the dominance of C3 grass taxa. Comparison with other sites shows an extended picture of C3 grass dominance, and hence winter rainfall regime on the west coast of South Africa throughout the Pleistocene. However, a positive shift in δ13C values relative to those expected for pure C3 consumers is noted across all species. In part this may reflect fluctuations in the global carbon isotopic content of the atmosphere, but most likely indicates a small but significant intrusion of C4 grasses into the vegetation mosaic. This phenomenon is most consistent with models which predict that C4 grasses should be favoured over C3 species at very low pCO2, and reveals a hitherto unknown subtlety in our understanding of the evolution of this important biome.