More animals have joined the list of species on the brink of extinction in a new study published in the journal Proceedings of the National Academy of Sciences.

In the updated list, there are now 515 animal species with less than a thousand individuals remaining. This serves the start of Earth’s sixth mass extinction.

Researchers evaluated about 29,374 land-based vertebrates using the data given by the International Union for Conservation of Nature (IUCN) Red List and BirdLife International. They identified 75 mammal, 335 bird, 41 reptile and 65 amphibian species on the brink of extinction. More than half of these species have fewer than 250 individuals left.

“The ongoing sixth mass extinction may be the most serious environmental threat to the persistence of civilization, because it is irreversible,” the authors wrote.

Humans are to blame for this current extinction crisis. Animal species are continuously depleting due to habitat destruction, habitat fragmentation, poaching, illegal trade, overharvesting, the introduction of non-native and domesticated species into the wild, pathogens, pollution and climate disruption.

The researchers said the extinction rate is expected to be 117 times higher by 2050.

“Predictions that one-fifth of all species would be in danger of extinction by midcentury and half or more by the end of the century begin to make sense,” they said.

They added that the extinction of one species can set off another species to decline in population, noting that it can lead to a domino effect of extinctions. “Extinction breed extinctions,” the authors wrote.

“What we do to deal with the current extinction crisis in the next two decades will define the fate of millions of species,” Gerardo Ceballos, study lead author and senior researcher at the National Autonomous University of Mexico’s Institute of Ecology, said in a statement. “We are facing our final opportunity to ensure that the many services nature provides us do not get irretrievably sabotaged.”

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