Spiny Plants, Ungulates and the Savannah Habitat

The Evolution of Spiny Plants Holds Key to the Establishment of the Savannah

The continent of Africa contains a wide diversity of habitats, dominating the south, central part of Africa are the grasslands, the extensive savannahs that are home to a great diversity of iconic animals.  A team of international scientists writing in the academic journal “The Proceedings of the National Academy of Sciences United States”, have mapped the origins of the African savannah and concluded that the emergence of this ecosystem is, at least in part, down to the grazing habits of antelopes and their kin.

Ancient Bovids Influenced Habitat Formation in Africa

Rusingoryx illustrated

Honking to communicate in the hot savannah.

Picture Credit: Todd Marshall

The picture above shows an illustration of Rusingoryx atopocranion a wildebeest from the Pleistocene Epoch.  Grazing bovids and antelopes may have had a remarkable impact on the evolution of plant communities.

In a study that plotted flora/faunal relationships on a continental scale, the researchers identified which mammal browsers are most closely associated with spiny communities of trees.  The team were able to show that over the last sixteen million years or so, plants from unrelated taxa developed spiny defences against being eaten a total of fifty-five times.  This pattern of convergent evolution suggests that the arrival and diversification of bovids in Africa changed the rules for persisting in woody communities.  Contrary to current understanding, this new data indicates that browsers predate fire by millions of years as agents driving the origin of the African savannah.

The study was conducted in an unorthodox manner.  The researchers, which included biologists from McGill University (Montreal, Quebec, Canada), started by observing fauna and flora relationships in Africa today and then working backwards in time to the middle of the Miocene Epoch.

An Arms Race Between Plants and Animals

Many browsers like gazelles, delicately pick leaves off branches full of wicked-looking spines that are several centimetres long.  The scientists were able to uncover what happened in the past by mapping the distribution and evolution of the spiny plants on which gazelles and their relatives like to feed today.

A Gerenuk Antelope Browsing

A Gerenuk Antelope Browsing

A Gerenuk Browsing on Trees.

Picture Credit: Everything Dinosaur

Research team member, Jonathan Davies, (McGill University) commented:

“It’s been difficult to get a picture of how savannah ecosystems evolved because the conditions needed to preserve animal and plant fossils are very different from one another.  By working with the African Centre for DNA Bar-coding at the University of Johannesburg, we were able to sequence and compare DNA from nearly two thousand trees, and show that African plants only developed spines about fifteen million years ago. That was about the same time that a new type of mammal, antelope and their relatives, spread across the continent following the collision between the continental plates of Africa and Eurasia.”

Prior to this collision, the African continent had been dominated by the large, now extinct, ancestors of browsing elephants and hyrax.  These large herbivores would have bull-dozed trees and trampled vegetation, so spines were an ineffective defence against them according to the lead author of the study, Tristan Charles-Dominique (University of Cape Town).  However, antelopes and their relatives that arrived in Africa after the continental plate collision were highly efficient browsers, often using their delicate lips and prehensile tongue to remove leaves from branches.  It is likely that plants developed spines to defend themselves against these new plant “predators”.

Evolving a Spiny Defence Against Browsers

The study suggests a remarkable “arms race” between the trees and plant-eaters.  The arrival of new and efficient herbivores on the continent of Africa led to the evolution of more and more elaborate defences, including longer and longer spines.  One of the implications of this research is that the loss of large mammals like antelopes today, through human activities such as the bush meat trade, may have a substantial impact on the African landscape, with present day open savannahs being converted into thicket or brush.  Extensive forests may also make a comeback.

The Paper:  “Spiny Plants, Mammal Browsers and the Origin of the African Savannahs”.

Everything Dinosaur acknowledges the support of McGill University in the compilation of this article.

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