Then, as now, the cascading events were originally begun by human disruption of ecosystems, a new 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.
This mass extinction was caused by newly arrived humans tipping the balance of power and competing with major predators such as sabertooth cats, the authors of the new analysis argue. An equilibrium that had survived for thousands of years was disrupted, perhaps explaining the loss of two-thirds of North America's large mammals during this period.
"We suggest that the arrival of humans to North America triggered a trophic cascade in which competition for the largest prey was intensified, ultimately causing the large non-human carnivores to decimate the large herbivores," said Blaire Van Valkenburgh, UCLA professor of ecology and evolutionary biology and a co-author on the paper. "When human hunters arrived on the scene, they provided new competition with these carnivores for the same prey.
"The addition of humans was different from prior arrivals of new predators, such as lions, because humans were also omnivores and could live on plant foods if necessary," Van Valkenburgh said. "We think this may have triggered a sequential collapse not only in the large herbivores, but ultimately their predators as well. Importantly, humans had some other defenses against predation, such as fire, weapons and living in groups, so they were able to survive."
"For decades, scientists 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 lead author on the paper.
In the late Pleistocene, researchers say, major predators dominated North America in an uneasy stability with a wide range of mammals: mammoths, mastodons, ground sloths, camels, horses and several species of bison. The new study cites previous evidence from carnivore tooth wear and fracture, growth rates of prey, and other factors that suggest that there were no serious shortages of food caused by environmental change 10,000 to 15,000 years ago.
The large herbivores seemed to be growing quickly, and just as quickly had their numbers reduced by a range of significant carnivorous predators, including lions, dire wolves and two species of sabertooth cats. Food was plentiful for herbivores, and the system was balanced, but it was dominated by predators.
Humans were the triggering mechanism for the extinction. After that, predators increasingly desperate for food may have driven their prey to extinction over long periods of time and then eventually died out themselves.
"We think the evidence shows that major ecosystem disruptions, resulting in these domino effects, can be caused either by subtracting or adding a major predator," Ripple said. "In the case of the woolly mammoths and sabertooth tiger, the problems may have begun by adding a predator, in this case humans."
The loss of species in North America during the late Pleistocene was remarkable; about 80 percent of 51 large herbivore species went extinct, along with more than 60 percent of large carnivores. Previous research has documented the growth rates of North American mammoths by studying their tusks, revealing no evidence of reduced growth caused by inadequate food, thus offering no support for climate-induced habitat decline.
Rather, the large population of predators such as dire wolves and sabertooth cats caused carnivores to compete intensely for food, as evidenced by heavy tooth wear.
"Heavily worn and fractured teeth are a result of bone consumption, something most carnivores avoid unless prey is difficult to acquire," Van Valkenburgh said.
Trophic cascades initiated by humans are broadly demonstrated, the researchers report. In North America, it may have started with the arrival of the first humans, but continues today with the extirpation of wolves, cougars and other predators around the world. The hunting of whales in the last century may have led to predatory killer whales turning their attention to other prey, such as seals and sea otters — and the declines in sea otter populations has led to an explosion of sea urchins and the collapse of kelp forest ecosystems.
"In the terrestrial realm, it is important that we have a better understanding of how Pleistocene ecosystems were structured as we proceed in maintaining and restoring today's ecosystems," the scientists wrote. "In the aquatic realm, the Earth's oceans are the last frontier for megafaunal species declines and extinctions.
"The tragic cascade of species declines due to human harvesting of marine megafauna happening now may be a repeat of the cascade that occurred with the onset of human harvesting of terrestrial megafauna more than 10,000 years ago. This is a sobering thought, but it is not too late to alter our course this time around in the interest of sustaining Earth's ecosystems."
UCLA is California's largest university, with an enrollment of nearly 38,000 undergraduate and graduate students. The UCLA College of Letters and Science and the university's 11 professional schools feature renowned faculty and offer more than 323 degree programs and majors. UCLA is a national and international leader in the breadth and quality of its academic, research, health care, cultural, continuing education and athletic programs. Five alumni and five faculty have been awarded the Nobel Prize.
For more news, visit the UCLA Newsroom or follow us on Twitter.
Stuart Wolpert | EurekAlert!
Europe’s Demographic Future. Where the Regions Are Heading after a Decade of Crises
10.08.2017 | Berlin-Institut für Bevölkerung und Entwicklung
Scientists reveal source of human heartbeat in 3-D
07.08.2017 | University of Manchester
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
18.08.2017 | Life Sciences
18.08.2017 | Physics and Astronomy
18.08.2017 | Materials Sciences