Oct. 16 (UPI) -- Evolution can't keep pace with human-caused ecological damage.
According to a new study, the rate of extinction among mammals is accelerating, limiting nature's natural defense mechanism against ecological damage, evolution.
In the wake of Earth's five mass extinctions, evolution allowed Earth's animal population to regenerate and diversify, filling in the gaps vacated by extinct species.
Earth's sixth mass extinction is ongoing, but humans have already done considerable damage -- damage that won't soon be undone. According to the latest research, published Tuesday in the journal Proceedings of the National Academy of Sciences, it will take 5 to 7 million years for evolution to restore mammalian diversity to pre-human levels.
Scientists at Denmark's Aarhus University surveyed mammal fossil records to accurately quantify humans' impact on mammal diversity. For thousands of years, humans have targeted Earth's largest species. Larger species tend to have fewer relatives. When they're wiped out, Earth's biodiversity is significantly reduced.
"Large mammals, or megafauna, such as giant sloths and saber-toothed tigers, which became extinct about 10,000 years ago, were highly evolutionarily distinct," Matt Davis, Aarhus palaeontologist, said in a news release. "Since they had few close relatives, their extinctions meant that entire branches of Earth's evolutionary tree were chopped off."
"There are hundreds of species of shrew, so they can weather a few extinctions," Davis said. "There were only four species of saber-toothed tiger; they all went extinct."
Over the last million years, wherever humans went, mammals shrunk. Today, Earth's largest mammal species remain under threat. To better understand the ecological damage caused by Earth's ongoing mass extinction, scientists calculated how long it will take mammalian biodiversity to recover.
Researchers used powerful evolutionary simulations to estimate the recovery time based on past and future extinction rates. The models accounted for evolutionary relationships and body sizes among living and extinction mammal species.
Under a best case scenario, evolution will restore pre-human mammal diversity levels in 5 to 7 million years. The scenario assumes humans will stop destroying habitats and eradicating species.
Models showed extinctions expected during the next 50 years will require 3 to 5 million years of recovery time.
"Although we once lived in a world of giants: giant beavers, giant armadillos, giant deer, etc., we now live in a world that is becoming increasingly impoverished of large wild mammalian species," said Aarhus professor Christian Svenning. "The few remaining giants, such as rhinos and elephants, are in danger of being wiped out very rapidly."
Humans can help evolution do its job by protecting habitat and prioritizing the conservation of endangered, evolutionarily distinct species.