Megafauna struggled in the rainforest

In today’s human-dominated world, rampant deforestation is driving many of Southeast Asia’s species towards extinction. But according to research published in the journal Nature, the opposite once occurred – as rainforests replaced grasslands thousands of years ago, megafauna and ancient humans vanished.

“Southeast Asia is often overlooked in global discussions of megafauna extinctions,” says lead researcher Julien Louys from Australia’s Griffith University, “but it once had a much richer mammal community full of giants now extinct.”

For nearly a million years, the region stretching from modern-day southern China to Borneo was covered in grasslands. Many species of megafauna thrived in this vastly different landscape, including giant hyenas, buffalo- and antelope-like bovines, and two species of ancient elephants.

But over the past 100,000 years, these savannahs began to retreat. By the dawn of the Holocene 11,700 years ago, they had been replaced by the lush, dense rainforest we see today.

This new study, resulting from a collaboration between Griffith’s Australian Research Centre for Human Evolution and the Max Planck Institute for the Science of Human History (MPISSH) in Germany, shows that the significant environmental change was too much for many species to cope with: as the savannah environments disappeared, so too did the megafauna.

“Modelling of major climatic systems from around this time suggests it was the changes in the length and duration of ice ages that led to a resurgence of rainforest, to which the megafauna was poorly adapted,” Louys explains.

Species of ancient humans – such as Homo erectus – were also unable to adapt to the more dynamic forest environment and soon disappeared.

“It is only our species, Homo sapiens, that appears to have had the required skills to successfully exploit and thrive in rainforest environments,” says MPISSH’s Patrick Roberts.

The study analysed the chemical make-up of teeth from both contemporary and fossil mammals, specifically looking at stable isotopes of carbon and oxygen.

“Grasses and trees use different photosynthetic pathways, resulting in different ratios of carbon isotopes in their tissues,” Louys explains.

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Modern rainforest in SE Asia. Credit: Julien Louys

Similarly, oxygen isotopes in plants vary depending on evaporation and precipitation. When mammals eat these plants, the different isotopic ratios are preserved in their teeth.

“By examining these isotopes,” Louys says, “we are able to determine whether they ate predominately grasses or leaves, and how wet environmental conditions were.”

This method has been used extensively to study the environmental context of human evolution in Africa, but rarely in Southeast Asia. This new research has now created the largest stable isotope dataset for fossil mammals in the region.

Louys and Roberts collected data for fossil sites spanning the last 2.6 million years, then compared the results to hundreds of baseline measurements from modern Southeast Asian mammals. This allowed them to examine the major trends in environments used by megafauna and determine that environmental changes were the main driver of extinction.

“This contrasts with most megafauna extinction hypotheses, which suggest that modern humans are the primary cause of extinctions,” says Louys.

According to biological anthropologist Michael Knapp, who was not involved in the research, the study makes a convincing argument by combining several lines of evidence, including the distribution and population of numerous mammal species.

“Bringing together multiple different kinds of data is the way forward for reconstructing past environments and understanding how they influenced human migration and evolution,” says Knapp, who is based at the University of Otago in New Zealand.

But even though these past extinctions seem to have been caused by a changing environment instead of humans, we’re not off the hook.

Humans are now intent on clearing the same rainforest that allowed us to become the only surviving hominin in Southeast Asia, critically endangering species such as elephants, tigers, orangutans, leopards and rhinos.

“We’re at risk of losing some of the last megafauna still walking the earth,” Louys says.

Southeast Asia is in the grips of a biodiversity crisis, warns Alice Catherine Hughes, a conservation scientist at the Chinese Academy of Sciences. “The region has a higher proportion of threatened mammals and birds than almost any other region.”

Dramatic forest loss – particularly for palm oil and rubber production – means that many species have increasingly small and fragmented populations and are therefore more vulnerable to hunting, illegal wildlife exports, and ultimately extinction.

“Despite commitments to better conserve the region’s unique diversity, we have failed to see declines in rates of deforestation,” Hughes says.

To stem this, she says dramatic interventions are necessary – including better certifications for rubber and palm oil, comprehensive environmental impact assessments for new developments, and schemes to ensure that native trees are planted as part of climate targets.

If no action is taken, we are in jeopardy of destroying these ecosystems forever, as highlighted by the long-term context of Louys and Roberts’ research.

Ironically, humans now have the dangerous capacity to reverse the vast ecosystem changes of the last 100,000 years. Another paper published this week shows that the combined pressure of the climate crisis and logging is pushing the Amazon towards a tipping point – and it may soon switch from canopied rainforest to open grasslands.

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