Mass extinction: Contribution of climate, man and fire – 08/22/2023 – Science

In the last decade, wildfires have become increasingly common due to human-caused climate change and destructive land management practices. Southern California in the United States has been particularly affected by this problem.

Southern California also had a wave of wildfires about 13,000 years ago. These fires permanently transformed the region’s vegetation and contributed to Earth’s largest mass extinction event in over 60 million years.

As paleontologists, we have a unique perspective on the causes and long-term consequences of environmental changes, both those linked to natural climate fluctuations and those caused by man.

In a new study, published this August, we seek to understand the changes that were happening in California during the last major extinction event, at the end of the Pleistocene, known as the Ice Age.

The event wiped out most of Earth’s large mammals between about 10,000 and 50,000 years ago. It was a time marked by dramatic climate change and rapidly expanding human populations.

The Last Great Extinction

Scientists often call the last 66 million years of Earth’s history the Age of Mammals. During that time, mammals took advantage of the extinction of the dinosaurs to become the planet’s dominant animals.

During the Pleistocene, Eurasia and the Americas were full of huge animals like woolly mammoths, giant bears and huge wolves. Two species of camels, three species of ground sloths, and five species of big cats lived in what is now Los Angeles.

Then, abruptly, they began to die. Around the world, the large mammals that characterized global ecosystems for tens of millions of years have disappeared.

North America has lost more than 70% of mammals weighing more than 44 kg. South America lost over 80%, Australia nearly 90%. Only Africa, Antarctica and some remote islands maintain what could be considered animal communities like those that existed before.

The reason for these extinctions remains unknown. For decades, paleontologists and archaeologists have debated the possible causes. What complicates the scientists’ analysis is not that there are no obvious culprits—on the contrary, there are many.

With the end of the last ice age, a warmer climate led to altered weather patterns and the reorganization of plant chains. At the same time, human populations were rapidly increasing and spreading across the globe.

One or both of these processes may have resulted in the extinction event. But the fossil record from any one region is generally too sparse to know exactly when large mammal species disappeared from different regions.

This makes it difficult to determine whether habitat loss, resource scarcity, natural disasters, human hunting or a combination of these factors are to blame.

a deadly combination

Some records offer clues. The La Brea Tar Pits in Los Angeles are the most ice-age fossil-filled site in the world. They preserve the bones of thousands of large mammals that have been trapped in bitumen over the past 60,000 years.

Proteins in these bones can be precisely dated using radioactive carbon, giving scientists unprecedented insight into an ancient ecosystem and an opportunity to shed light on the timing — and causes — of its collapse.

Our recent study at La Brea and nearby Lake Elsinore unearthed evidence of a dramatic event 13,000 years ago that permanently transformed Southern California’s vegetation and caused the iconic La Brea mega-mammals to disappear.

Lake bottom sediment archives and archaeological records provide evidence of a deadly combination — a warmer climate punctuated by decades of drought and rapidly growing human populations. These factors have brought the Southern California ecosystem to a tipping point.

Similar combinations of climate warming and human impacts were responsible for ice age extinctions elsewhere, but our study found something new. The catalyst for this dramatic transformation appears to have been an unprecedented increase in wildfires, likely caused by humans.

The processes that led to this collapse are familiar today.

As California warmed out of the last ice age, the landscape became drier and forests receded. At La Brea, herbivore populations have declined, likely due to a combination of human hunting and habitat loss. Species associated with trees, such as camels, have completely disappeared.

In the millennium leading up to the extinction, average annual temperatures in the region reached 5.5 degrees Celsius and the lake began to evaporate. Then, 13,200 years ago, the ecosystem entered a 200-year drought. Half of the remaining trees died. With fewer large herbivores to eat, dead vegetation has accumulated in the landscape.

At the same time, human populations began to expand across North America. And as they spread, people brought with them a powerful new tool—fire.

Humans have used fire for hundreds of thousands of years, but fire has different impacts on different ecosystems.

Records from Lake Elsinore reveal that, before humans, the presence of fire was low on the southern California coast. But between 13,200 and 13,000 years ago, with the growth of the human population, fire in the region increased very quickly.

Our research suggests that the combination of heat, drought, loss of herbivores and human-caused fires has brought this ecosystem to a tipping point.

At the end of that period, southern California was covered in chaparrals (low vegetation biome that can recover after a fire), which thrived after the fires. A new fire regime took hold and the iconic megafauna of La Brea disappeared.

Lessons for the future

Studying the causes and consequences of the Pleistocene extinctions in California can provide valuable context for understanding the current climate and biodiversity crises. Today, a similar combination of climate warming, expanding human populations, loss of biodiversity and man-made fires that characterized the ice age extinction gap in Southern California is happening again.

The startling difference is that temperatures today are rising ten times faster than at the end of the ice age, mainly because of the burning of fossil fuels.

This human-caused climate change has contributed to a five-fold increase in the frequency and intensity of wildfires and the amount of area burned in the state of California over the past 45 years.

Although California is now famous for extreme fires, our study reveals that fire is a relatively new phenomenon in this region.

In the 20,000 years before the extinction, the Lake Elsinore record shows a very low incidence of any fires, even during comparable periods of drought. Only after human arrival does fire become a regular part of the ecosystem.

Even today, downed power lines, bonfires and other human activities start more than 90% of coastal California wildfires.

The parallels between late Pleistocene megafaunal extinctions and current environmental crises are striking.

The past teaches us that the ecosystems we depend on are vulnerable to collapse when pressured by multiple pressures. Redoubling efforts to eliminate greenhouse gas emissions, prevent reckless fires, and preserve Earth’s remaining megafauna could help avert another, even more catastrophic transformation.

*Emily Lindsey is curator of the La Brea Paleontological Site and a professor at the University of California UCLA

*Lisa N. Martinez is a doctoral candidate in geography at the University of California

*Regan E. Dunn is a professor at the USC Dornsife College of Letters, Arts and Sciences

**This article was published on the scientific dissemination website The Conversation and reproduced here under license Creative Commons. Click here to read the original English version.