Animals going extinct so fast that recovery may take 3-5 million years

The
indri of Madagascar — the largest living lemur — is critically
endangered and highly evolutionarily distinct. If the indri goes
extinct, we will lose 19 million years of unique evolutionary
history.
Aarhus
University

• The Earth is undergoing a mass extinction, which is caused by
  human activity that destroys habitats and kills off species.
• In a new study, Danish researchers concluded that so many
  mammal species will go extinct in the next 50 years that the
  planet’s evolutionary diversity won’t recover for at least 3
  million years. 
• The scientists said their findings could be used to help
  determine which endangered species are the biggest priorities for
  conservation efforts.

Humans will cause so many mammal species to go extinct in the
next 50 years that the planet’s evolutionary diversity won’t
recover for 3 to 5 million years, a team of researchers has
found.

The Earth may be entering its sixth mass extinction: an era in
which the planet’s environments change so much that most animal
and plant species die out. The International Union for the
Conservation of Nature predicts
that 99.9% of critically endangered species and 67% of endangered
species will be lost within the next 100 years.

The five other times a mass extinction has occurred over the
past 450 million years, natural disasters were to blame. But
now, human activity is killing mammal species.

In a study
published Monday in the journal PNAS, scientists from Aarhus
University in Denmark calculated how fast extinctions are
happening, and how long it would take for evolution to bring
Earth back to the level of biodiversity it currently has.

The scientists concluded that in a best-case scenario,
nature will need 3-5 million years to get back to the level of
biodiversity we have on Earth today. Returning to the state
Earth’s animal kingdom was in before modern humans evolved
would take 5-7 million years.

Evolution can’t keep up

Evolution is the planet’s defense mechanism against the loss of
biodiversity. As habitats and climates change, species that can’t
survive die, and new species slowly emerge. But it takes a long
time for new species to fill the gaps — and that process is far
slower than the rate at which humans are causing mammals to go
extinct.

For their calculations, the Aarhus University researchers used a
database containing existing mammal species and mammals that
already went extinct as humans spread across the planet. They
combined that data with information about extinctions expected to
come in the next 50 years, and used advanced simulations of
evolution to predict how long recovery would take.

Their estimates are based on an optimistic assumption that people
will eventually stop ruining habitats and causing species to die
out, and the extinction rate will go back down. But even in
that best-case scenario, the timeline depends on how quickly
mammals start recovering. If the extinction rate doesn’t start
falling for another 20-100 years, more species will likely
disappear, causing greater diversity loss, the study said.

Litopterns,
like this one discovered by Charles Darwin, were a
strange-looking group of prehistoric South American mammals that
were not closely related to any species alive today. When they
went extinct at the end of the Ice Age, the mammal Tree of Life
lost one of its deepest branches.
Robert Bruce Horsfall via Wikimedia
Commons

The researchers noted that in their model, certain species were
given more importance than others. Matt Davis,
a paleontologist at Aarhus University who led the study,
cited the shrew as an example. There are hundreds of
species of shrew, so if one or two go extinct, that would not
kill off all shrews on Earth.

But there were only four species of sabre-toothed tigers on the
planet. So when they all went extinct, many years of evolutionary
history disappeared with them. 

“Large mammals, or megafauna, such as giant sloths and
sabre-toothed tigers, which became extinct about 10,000 years
ago, were highly evolutionarily distinct,” Davis said in a press
release. “Since they had few close relatives, their extinctions
meant that entire branches of Earth’s evolutionary tree were
chopped off.”

The research could help conservationists  

Today, other large animals like the black rhino are facing
extinction. Asian elephants’ chance of making it to the 22nd
century is less than 33%, the study found. These elephants are
one of only two remaining species from a group of mammals that
once included mastodons and mammoths.

“We now live in a world that is becoming increasingly
impoverished of large wild mammalian species,” Jens-Christian
Svenning, an Aarhus University professor who researches
megafauna, said in the press release. “The few remaining giants,
such as rhinos and elephants, are in danger of being wiped out
very rapidly.”

He noted that the planet no longer boasts giant beavers, giant
deer, or giant armadillos.

Though the researchers’ findings are dire, the scientists said
their work could be used to figure out which endangered species
are evolutionarily unique, which might help conservationists
decide where to focus their efforts to prevent the most
devastating extinctions.