There have been five mass extinctions in the history of Earth, all caused by dramatic but natural phenomena, the last one 66 million years ago when an asteroid struck the Yucatan Peninsula and wiped out the dinosaurs.

Today, a growing number of experts fear that a sixth mass extinction is in progress, this time caused entirely by human activity.

A new paper by biologists with the University of Hawaii at Manoa and the Museum National d’Histoire Naturelle in Paris offers further evidence to suggest the crisis is real.

“Humans are the only species able to manipulate the Earth on a grand scale, and they have allowed the current crisis to happen,” said Robert Cowie, lead author of the study and research professor at the UH-Manoa Pacific Biosciences Research Center.

The paper, published last month in the journal Biological Reviews, declares that “the biodiversity that makes our world so fascinating, beautiful and functional is vanishing unnoticed at an unprecedented rate.”

It even goes so far as to suggest that scientists should start collecting and documenting as many species as possible before they disappear.

“It’s so people one day can say, ‘Wow, this is what they had back then,’” Cowie said.

Cowie, a biologist who has documented the widespread decline of native snails in Hawaii, said even though accelerating rates of species extinctions and declining abundances of animal and plant populations are well documented, some deny the mass extinction trajectory.

“This denial is based on a highly biased assessment of the crisis which focuses on mammals and birds and ignores invertebrates, which of course constitute the great majority of biodiversity,” he said.

In their study, Cowie and his co-authors extrapolated estimates obtained for land snails and slugs to estimate that since the year 1500, Earth has lost between 7.5% and 13% of the 2 million known species. That represents a whopping 150,000 to 260,000 species.

The number is orders of magnitude greater than the 882 species listed as extinct on the International Union for Conservation of Nature’s Red List, which is heavily biased toward birds and mammals while having only a minute fraction of invertebrates evaluated against conservation criteria.

Incorporating estimates of the true number of invertebrate extinctions, Cowie said, leads to the conclusion that the rate of extinctions vastly exceeds the background rate (the rate of extinction in Earth’s history before the influence of humans) and that it is likely we are witnessing the start of the sixth mass extinction.

But rates of extinction are not the same everywhere. The paper points out there is no evidence the crisis is affecting the biodiversity of the oceans to the same extent as on land, where the rates of extinction for plants seem to be lower than those of animals, while island species, such as those of the Hawaiian islands, are much more affected than continental species.

Scientists consider a mass extinction to have occurred when 75% of the planet’s species become extinct. In the past, some extinctions have occurred over millions of years.

While the last mass extinction was due to the effects of a huge asteroid hitting the planet, others were thought to be caused by volcanic eruptions that changed the Earth’s carbon cycle and led to hyper- global warming and related effects.

If the Earth is in the throes of another mass extinction, it’s because of what humans are doing, including things such as habitat destruction, the spread of invasive species and the acceleration of global warming.

Cowie said that while some deny a sixth extinction has begun, others simply accept it as a new and natural evolutionary trajectory, as humans are just another species playing their natural role in Earth’s history. Some even consider that biodiversity should be manipulated solely for the benefit of humanity.

The paper rejects those assertions, saying they merely pave the way for Earth to continue on its sad trajectory toward another mass extinction.

“We are the only species that has conscious choice regarding our future and that of Earth’s biodiversity,” Cowie said. “We have a moral responsibility and ability to choose to preserve the Earth.”

To fight the crisis, various conservation initiatives have been successful in fighting off extinction for certain high-profile creatures. But these initiatives cannot target all species, and they cannot reverse the overall trend of species extinction.

“It’s overwhelming,” he said. “There’s not enough money and political will to turn things around.”