The five previous mass extinction events in our planet’s history have all been caused by dramatic natural phenomena, yet the authors of a new study claim that a sixth great die-off is now in progress and that this time, humans are to blame. Writing in the journal Biological Reviews, the researchers estimate that up to 13 percent of all invertebrate species may have become extinct in the last 500 years, and warn that the onus is now on us to take action in order to avoid a catastrophic decline in biodiversity.

According to the authors, the true extent of the current predicament is somewhat masked by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, which gives the impression that the present rate of species loss remains in line with the background rate. “However, the Red List is heavily biased,” write the researchers. “Almost all birds and mammals but only a minute fraction of invertebrates have been evaluated against conservation criteria.”

“Incorporating estimates of the true number of invertebrate extinctions leads to the conclusion that the rate vastly exceeds the background rate and that we may indeed be witnessing the start of the Sixth Mass Extinction,” they say.

To prove their point, the study authors reference a 2015 study on molluscs which concluded that around seven percent of land snail species have become extinct since 1500. Assuming that this figure represents extinction rates for all non-marine invertebrates and that such species outnumber marine invertebrates by about three to one, the researchers calculate that between 7.5 and 13 percent of the two million known species have now disappeared.

In absolute figures, this equates to between 150,000 and 260,000 extinctions, which is significantly higher than 882 species listed as extinct by the Red List. Admittedly, this finding is based on a “bold assumption” and has not been definitively verified, yet the researchers nonetheless conclude that a sixth mass extinction event “has surely begun, and is being caused by human activities.”

While this statement may sound alarming, the authors go on to explain that there is no evidence to suggest that marine species are being affected in the same way as land-based invertebrates, while plants also appear to have been spared from the present crisis.

Summing up the need for mankind to assume responsibility for averting this impending disaster, study author Robert Cowie explained that “humans are the only species capable of manipulating the biosphere on a large scale,” and that “we are the only species that has conscious choice regarding our future and that of Earth’s biodiversity.”

The argument that our actions and their consequences are simply part of the natural evolution of life on Earth, therefore, doesn’t wash with the researchers. On the contrary, Cowie warned that “denying the crisis, accepting it without reacting, or even encouraging it constitutes an abrogation of humanity’s common responsibility and paves the way for Earth to continue on its sad trajectory towards a Sixth Mass Extinction.”