Preliminary evidence for green, brown and black worlds in tropical western Africa during the Middle and Late Pleistocene

Abstract

Modern ecological studies indicate that the degree of openness in African vegetation cover is determined, in part, by the presence of herbivores and fire as consumers of vegetation. Where herbivores are the dominant consumer of vegetation the resultant open state is described as a ‘brown’ world. Where fire is the dominant consumer of vegetation the resultant open state is described as a ‘black’ world. While if neither consumer is dominant then a more closed canopy states arises that is described as a ‘green’ world. Here we use palaeoecological data obtained from Lake Bosumtwi (Ghana) to characterize green, brown, and black worlds during two short sections of around 1000 years each, deposited around 200,000 and 100,000 years ago (Middle and Late Pleistocene). We characterize the vegetation cover using pollen and phytoliths, herbivory using Sporormiella and fire using micro-charcoal. We find that during c. 1000 years of the Middle Pleistocene fire was the major consumer of vegetation, while during c. 1000 years in the Late Pleistocene herbivores were relatively more important consumers of vegetation. We therefore suggest that the Middle Pleistocene section represents a black world, while in the Late Pleistocene section we capture a combination of green, brown and black worlds. The duration of these states seems to range from centuries to millennia and transitions are observed to occur in both an abrupt and a stepwise fashion. These preliminary data demonstrate how palaeoecological information can be used to gain insights into past landscape scale processes over thousands of years. Further work is required to test the robustness of these findings and to provide a higher temporal resolution to aid the link with modern ecological studies.