The origin and spread of savannas is one of the great mysteries in the earth’s ecological history. A new study published in Proceedings of the National Academy of Sciences suggests a key role for mammal herbivores. Savannas are grassy ecosystems that today cover about a fifth of the world’s land surface. Yet fossil evidence suggests they did not exist 10 million years ago. What triggered the grass revolution? How did grasses replace the ancient forests of earlier times? Climate change is one answer, but most savannas occur in climates that also support forests. Increased fire activity is another possibility. Savanna trees and shrubs are often adapted to frequent fires whereas forest trees are not, and fire suppression can reverse the process causing forests to replace savannas. By studying the evolution of spiny plants, the researchers here suggest that mammal browsers are also part of the story.
Any visitor to one of Africa’s famous game parks will be familiar with the spiny acacias that shape the landscape. Dr Tristan Charles-Dominique from the University of Cape Town (UCT), lead author of the study, commented that “Spines are a peculiar defence; the leaves of spiny trees are often favoured by mammal browsers, yet spiny plants remain abundant”. The new study analysed the distribution of nearly 2000 woody species from southern Africa and found that spiny plants are most common in arid savannas with high densities of mammal browsers. They are rare in humid savannas on infertile soils and absent in forests. Armed with this information, the authors could explore the history of mammal browsers in Africa by using spiny plants as indicators of high herbivory. “We know something of the history of fire from fossil charcoal, but we do not have good fossil evidence for browser/plant interactions” said Professor William Bond, (SAEON and UCT) an author on the study, “Could we use the evolutionary history of spiny plants to examine whether mammal browsing opened up the ancient forests to savannas?”
The study used DNA data for African trees collected by Professor Michelle van der Bank and her team at the University of Johannesburg, to reconstruct the history of spiny plant evolution. In collaboration with Prof Jonathan Davies, an expert in phylogenetic analysis from McGill University, the team were then able to date the evolutionary origins of spines. “We were shocked” said Professor van der Bank “to discover that spiny plants only appeared about 15 million years ago, 40 million years after mammals replaced dinosaurs”. For most of this time, Africa was an island continent dominated by now extinct ancestors of browsing elephants and hyrax. “Apparently spines just didn’t work as a plant defence against these ancient mammal groups” commented Prof van der Bank. But in a remarkable example of apparent coevolution, the diversification of spiny plants coincides with the appearance of antelope. Antelope were latecomers to Africa appearing only after the continent collided with Eurasia. They browsed in novel ways and were highly efficient herbivores. This injection of new types of browsers, argue the authors, demolished young forest trees, opening up forests to the grass invaders. The parallel radiation of spiny plants and the antelope that feed on them initiated the rise of savannas in the drier more fertile regions of Africa. Fire only began to roll back the forests to create wetter savannas several million years later. One implication of the study, according to Professor Muasya (UCT), is that the loss of Africa’s native browsing antelope may threaten the future of drier savannas, and lead to their replacement by dense woody scrub of little ecological or economic value