Ecological Change in the Turkana Basin Over the Past 4 Ma

Abstract:

The fossil record of C₄ grasses is extremely sparse so that much speculation on their importance through time is based solely on stable carbon isotope ratios in derived materials, such as tooth enamel, soil carbonate, leaf waxes. I present stable isotope analyses of modern large mammals from different 25 sites in East and Central Africa, primarily from national parks and national reserves, that show patterns of dietary strategies in environments ranging from closed forests to open grasslands. This uses isotope values from each locality for each taxon to estimate diet, rather to assume a constant diet for a taxon across all localities. Each taxon can be represented on the dietary continuum from pure C3-browsing to pure C4-grazing. Fossil assemblages from the Turkana basin are evaluated in the same way over the past 4.2 million years. The fossil record shows patterns with no modern analogue in the national parks and reserves of East and Central Africa for significant periods of time. Significant patterns shifts occur between 2.0 and 2.5 million years ago, and also in the past million years. These dietary changes at the community level occur during the evolutionary backdrop of C₄ grasses, which have gone from less than 1% Net Primary Productivity (NPP) at 10 million years ago to more than 50% NPP in the tropics today. It is likely that C₄ plants have also undergone important changes in nutrient content, defense against herbivory, fire adaptation, and propagation strategies in the past 10 million years; such issues could signficantly influence dietary strategies at the community level through time.