In each case the cascading events were originally begun by human disruption of ecosystems, a newly released study concludes, but around 15,000 years ago the problem was not the loss of a key predator, but the addition of one human hunters with spears.
In a study published recently in the journal BioScience, scientists propose that this mass extinction was caused by newly-arrived humans tipping the balance of power and competing with major predators such as saber-toothed cats. An equilibrium that had survived for thousands of years was disrupted, possibly explaining the loss of two-thirds of North America's large mammals during this period.
"For decades, researchers have been debating the causes of this mass extinction, and the two theories with the most support are hunting pressures from the arrival of humans, and climate change," said William Ripple, a professor of forest ecosystems and society at Oregon State University, and an expert on the ecosystem alterations that researchers are increasingly finding when predators are added or removed.
"We believe humans indeed may have been a factor, but not as most current theory suggests, simply by hunting animals to extinction," Ripple said. "Rather, we think humans provided competition for other predators that still did the bulk of the killing. But we were the triggering mechanism that disrupted the ecosystem."
Here's a hint of what might've happened in North America:
Mesopredator release hypothesis
This is a potential death knell for the theory blaming the Paleo-Indians for slaughtering the megafauna. Even if the Paleo-Indians were present during the extinction era, even if they killed some animals, they may not have been responsible overall.
Scientists who have disparaged Paleo-Indians as killers have relied mainly on the coincidence: people arrived and megafauna died. Now we're learning that this correlation isn't enough to prove causation. Unless these scientists find large numbers of dead animals with spear points in them, they'll have trouble disproving the trophic cascade theory.
No doubt this isn't the final word on the subject. In fact, the number of theories is growing. The cause could be climate change, a comet or meteor strike, or something more subtle such as ecosystem disruption.
What we do know is that the theory blaming the Paleo-Indians is losing traction. I don't think I've seen new evidence supporting it in years. That's tough luck for the pundits who have used it to attack today's Indians as hypocrites.
For more on the subject, see:
Lucayans in Extreme Cave Diving
Megafauna died before Clovis Indians arrived
The "black mat" theory
Comet killed megafauna?