]Over 10,000 years ago in the Americas, there were many more large mammals than today epitomized by the mammoth. The extinction of woolly mammoths and other large mammals more than 10,000 years ago may be explained by the same type of cascade of ecosystem disruption that is being caused today by the global decline of predators such as wolves, cougars and sharks, life scientists report July 1 in the cover article of the journal Bioscience.
The mammoths are members of Elephantidae, the family of elephants and mammoths, and close relatives of modern elephants. They lived from the Pleistiocene epoch around 4.8 million to 4,500 years ago. They were part of a number large mammal species called megafauna.
They all lived on Earth during the Pleistocene epoch and became extinct in an extinction event. These species appear to have died off as humans expanded out of Africa and southern Asia, the only continents that still retain a diversity of megafauna comparable to what was lost. The Americas, northern Eurasia, Australia and many larger Islands lost the vast majority of their larger and all of their largest mammals. Four throries have been given for these extinctions: hunting by the spreading humans, climate change, spreading disease, and an impact from an asteroid or comet. A combination of those explanations is also possible.
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."
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.
There was an ecological stability dominated by large predators. The new human predator imbalanced the ecosystem leading to its ultimate collapse.
This pattern of extinction has been seen more recently on isolated islands. Many islands had unique megafauna that went extinct upon the arrival of humans (over the last few millennia and continuing into recent centuries). These included the giant bird forms in New Zealand such as the moas; giant lemurs, two species of hippopotamus, and a giant tortoise in Madagascar; giant geese and moa-nalo (giant ducks) in Hawaii.
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.
"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."
For further information: http://www.eurekalert.org/pub_releases/2010-07/wsu-wru070210.php