Thursday, September 1, 2022

The Ancient Lost Savannas of Southeast Asia (Sunda Grasslands)


Stegodon trigonocephalus was a grazer that depended on ancient savannas (By I, Vjdchauhan)

The countries of Southeast Asia today are draped in closed canopy forests (where development has not cut them down), but new studies have found that vast areas had been grass biomes for a time in the recent past.

The Pleistocene epoch lasted from 2.6 million years ago (mya) to 11,700 years ago, when repeated glaciation events caused massive changes in the landscape. During the last glacial period, the lowering of sea levels caused the emergence of (now submerged) land masses that formed the continental shelf Sundaland, which included insular Indonesia and Malaysia today. Coupled with  a reduction in precipitation across the area, grasslands expanded to form savannas that stretched all the way from continental Asia to parts of the Philippines (Bird et al, 2005; Louys and Roberts, 2020, and see yellow area in image below), 

Sunda Grasslands in Yellow (Louys and Roberts, 2020)

The expansion of the grasslands in the mid-Pleistocene caused the extinction of megafauna browsers that depended on the forest. One such species that declined and disappeared was Gigantopithecus blacki, a gigantic forest dwelling ape that could have stood up to 3 meters high. Another was an extinct giant panda called Ailuropoda wulingshanensis.

Gigantopithecus blacki was a browser that went extinct after the forests decreased (By Concavenator)

This rise of the Southeast Asian savannas also resulted in the evolution of grazer megafauna that lasted until the late Pleistocene, when sea levels again rose and the grasslands were replaced by the closed canopy forests that hold sway over the area today. Animals that thrived in the grass savannas included the spectacular extinct elephants Elephas hysudrindicus and Stegodon trigonocephalus, as well as Bubalus palaeokerabau, an amazing water buffalo with enormous horns.

Modern water buffalo (Da - Own work, CC BY 2.5)
In order to help prove the existence of these ancient and forgotten savannas, a recent study examined the diet of modern and historical mammalian specimens (Louys and Roberts, 2020). They were able to determine whether the animals had a diet rich in C3 plants or C4 plants by looking at the ratio of carbon-13 to carbon-12 in the specimens. Both carbon-13 and carbon-12 are stable isotopes of the element carbon, although carbon-12 makes up the vast majority of the carbon on earth (around 99%).

If you remember, C4 and C3 refer to 2 different types of photosynthesis in plants, with the vast majority of plants using C3 photosynthesis. Grasses make up the bulk of C4 plants, and it turns out C4 plants take in more carbon-13 relative to C3 plants. The ratio of carbon-13 and carbon-12 in specimens can thus be used to determine whether the animal had a diet rich in C3 or C4 plants.

Their research showed (see image below) that in the Early Pleistocene, there were both C3 and C4 eaters, but by the Middle Pleistocene, most of the animals had a diet rich in C4 plants (ie. C4 grasses). The pendulum swung back by the Late Pleistocene such that by the Holocene, almost all the animals were eating C3 plants.

They interpreted this to mean that C4 grasslands expanded significantly in the Middle Pleistocene, but then gave way again to C3 forests by the Holocene.

(Louys and Roberts, 2020)

The short rise and fall of these tropical ancient savannas in Southeast Asia was something that I had never heard about before. That region of the world in its "natural" state today is so forested that there is a tendency to always think it has been that way since time immemorial.

Finding out that there were once vast savannas in these lands, with astounding creatures like the gigantic elephant Stegodon trigonocephalus roaming in large herds across the open landscape, truly boggles the imagination.

Literature Cited

Bird, Michael I, David Taylor, Chris Hunt (2005). Palaeoenvironments of insular Southeast Asia during the Last Glacial Period: a savanna corridor in Sundaland? Quaternary Science Reviews Volume 24, Issues 20–21

Louys, J., Roberts, P. Environmental drivers of megafauna and hominin extinction in Southeast Asia. Nature 586, 402–406 (2020).

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