Extinctions: How Many Animal Species Have We Already Wiped Out?

There have been five mass extinction events in the history of Earth’s biodiversity, all caused by natural phenomena. It is believed that the sixth mass extinction is currently underway, this time solely driven by human activities. The Thylacine, the aurochs, and the dodo are just a few of the most well-known examples of extinctions. But how many animal species have humans actually eradicated?

According to the Red List of the International Union for Conservation of Nature (IUCN), 777 animal species have gone extinct since 1500, the beginning of the modern era. Some of these extinctions might be due to natural causes, but humans have played a significant role in almost all, if not all, cases. The human impact on nature, particularly over the last 500 years, has been immense.

Hidden numbers: Unrecorded extinctions

A central problem with the IUCN Red List is that, while a large portion of birds and mammals have been assessed according to conservation criteria, only a tiny fraction of invertebrates have been evaluated. The International Union for Conservation of Nature has only assessed the extinction risk for about five percent of the world’s known species. A 2022 study published in the journal Biological Reviews, led by Robert Cowie, a research professor at the University of Hawaii, estimates that possibly between 150,000 and 260,000 species have gone extinct since around 1500. Even Cowie was surprised by these numbers: “I thought, have I made some mistakes in the calculations?” he told Live Science.

Cowie and his team calculated the extinction rate by studying a random sample of 200 land snails and determining how many had gone extinct. They then applied this extinction rate to all known species over a 500-year period. The resulting extinction rate was between 150 and 260 extinctions per million species per year (E/MSY – Extinctions per Million Species-Years). After comparing with extinction rates from other studies, Cowie settled on an average value of 100 E/MSY. This means that 100 species out of a million go extinct in a year.

Applying the rate of 100 E/MSY to Cowie’s method suggests that around 100,000 of the approximately two million known species have gone extinct over the past 500 years. This estimate does not account for unknown, scientifically undescribed species.

A study published in the journal PLOS Biology in 2011, as well as the World Animal Foundation, estimate that there are approximately 8.7 million species of animals. At a rate of 100 E/MSY, this would correspond to 770 extinctions annually or 385,000 over 500 years. The study’s authors believe that around 86 percent of existing species on Earth and 91 percent of species in the ocean have yet to be described. This implies that many species go extinct unnoticed, without us ever knowing of their existence.

Numbers to be taken with caution

John Alroy, a professor in the Department of Biological Sciences at Macquarie University in Australia, warns against taking these numbers too literally. He emphasizes that to calculate an extinction rate, it is necessary to know the original number of species – information that is often lacking. Many species live in poorly studied regions or belong to under-researched groups. “We should be supercautious about trying to nail a number down based on the existing literature,” said Alroy.

He suggests using museum data to estimate the extinction rate of certain groups and to investigate how many species have been lost over time.

The human impact on extinction rates

Regardless of the exact rate, Cowie and Alroy agree that humans have significantly worsened the extinction rate and the number of extinctions. Both believe that the number of extinctions is far higher than the 777 species listed on the IUCN Red List.

The broad range of estimates clearly shows that all extinction rates far exceed the natural background rate. This rate describes the extinction of species under natural conditions and without human influence. This clearly indicates that humans are significantly harming Earth’s biodiversity. Cowie summarizes: “Whether the extinction rate is 100 E/MSY or 20 E/MSY or 200 E/MSY, it’s still a lot, and it’s still really bad.”

These findings underscore the urgent need for enhanced conservation measures to slow down human-induced species loss and protect the biodiversity of our planet.