History of An Invasive Species in South America

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According to a new study, when humans first came to South America, they had a wonderful time, eating up the wildlife. Then, apparently when they'd managed to drive some of their main food sources into extinction, they went into decline. Somewhat later, they settled down a bit and once again began to thrive, surpassing their previous levels of population.

"Populations of early human settlers grew like an 'invasive species,' Stanford researchers find" | Stanford Report

The paper, titled "Post-Invasion Demography of Prehistoric Humans in South America," is the first in a series on the interaction of local animal populations, humans and climate during the massive changes of the last 25,000 years in South America. The series will be featured at the Latin American Paleontology Congress this fall.

The study lays a foundation for understanding how humans contributed to the Pleistocene era's largest extinction of big mammals, such as ground sloths, horses and elephant-like creatures called gomphotheres. It reconstructs the history of human population growth in South America using a newly assembled database of radiocarbon dates from more than 1,100 archaeological sites. Unlike many archaeological studies that look at environmental change in one particular site, the Stanford research's continental approach provides a picture of long-term change, such as climatic fluctuations, fundamental to human populations rather than a single culture or ecosystem.

The researchers found strong evidence for two distinct phases of demographic growth in South America. The first phase, characterized by logistic growth, occurred between 14,000 and 5,500 years ago and began with a rapid spread of people and explosive population size throughout the continent.

Then, consistent with other invasive species, humans appear to have undergone an early population decline consistent with over-exploitation of their resources. This coincided with the last pulses of an extinction of big animals. Subsequent to the loss of these big animals, humans experienced a long period of constant population size across the continent. The second phase, from about 5,500 to 2,000 years ago, saw exponential population growth. This pattern is distinct from those seen in North America, Europe and Australia.

The seemingly obvious explanation for the second phase – initial domestication of animals and crops – had minimal impact on this shift, the researchers wrote. Instead, the rise of sedentary societies is the most likely reason for exponential population growth. Practices such as intensive agriculture and inter-regional trade led to sedentism, which allowed for faster and more sustained population growth. Profound environmental impacts followed.

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The full paper is available free (for now) from Nature, if you go to the Australian Broadcasting Corporation and click on the link in the story there.

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An interesting story about a study looking at this period of prehistory, which may modify the narrative given above:

"Giant jaguars, colossal bears done in by deadly combo of humans and heat" | Science

If you're fossil hunting in Patagonia you might find some weird creatures: giant jaguars, 3-meter-tall sloths, and bears 10 times the size of grizzlies. The southern part of South America was once crawling with these great beasts, collectively known as megafauna. But around 12,000 years ago, they suddenly disappeared from Patagonia and many other parts of the Americas. What caused the mass extinction? A new study suggests it was a one-two punch: rapid climate warming and humans.

For many years, scientists believed that people were responsible. When humans arrived in the Americas shortly before 13,000 years ago, the story went, they almost immediately hunted the New World's megafauna to extinction. Archaeologists turned up many butchered bones, so a "blitzkrieg" of hunting seemed to make sense. But now, most researchers think that people were already in the Americas by at least 15,000 years ago. That means that people and megafauna actually coexisted for a few thousand years before the mass extinctions began. What changed to drive these animals out of existence?

To find the missing ingredient, Alan Cooper, an evolutionary biologist at the University of Adelaide in Australia, radiocarbon dated nearly 100 fossils from Patagonia and sequenced their mitochondrial DNA, genes found in the power plants of cells and passed down only from the mother. When he lined up their ages with global climate records, he noticed a pattern: Many species of megafauna seemed to disappear during a period of extreme warming around 12,300 years ago, Cooper and his team write today in Science Advances.

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[OldGit]

Pity about the gomphotheres.