Humans are driving global biodiversity loss and ecosystem deterioration. Impacts have led to the loss of hundreds of plant and animal species with an estimated 1 million threatened with extinction today.
But, while the authors of a new study stress the need for urgent actions to address the current biodiversity crisis, their research indicates we are not yet witnessing Earth’s sixth ‘mass extinction event’.
“A recent study suggested that extinctions of animal genera are rapidly accelerating and that these extinctions endanger human survival,” says lead author John Wiens, professor of ecology and evolutionary biology at the University of Arizona in the US.
“We found instead that extinctions of genera are very rare across plants and animals.
“We argue that the reason why future extinctions must be stopped is not because they threaten humans, but because it is morally wrong for humans to drive other species to extinction.”
Throughout the planet’s history, life on Earth has persisted despite 5 mass extinction events in which at least 75% of all species were lost over a short geological period.
The first mass extinction wiped out 86% of all species at the end of the Ordovician period about 443 million years ago (mya). This was followed by the Late Devonian, 375 mya, in which 75% of species disappeared.
The end of the Permian period (about 250 mya) is known as “the great dying” with 96% of species lost. This was followed 50 million years later by the End Triassic (80%). Most recently, 76% of all species went extinct at the end of the Cretaceous period due to a catastrophic asteroid impact 66 mya.
In 2023, a study in PNAS used data from the International Union for the Conservation of Nature (IUCN) to document recent extinctions in tetrapod vertebrates – including birds, mammals, amphibians, turtles, lizards and snakes.
It concluded that genus-level extinction rates in tetrapods are “rapidly accelerating” and that these were evidence of a mass extinction event.
Genus is the taxonomic rank above species and below family when classifying life on Earth. For example, the genus Canis includes multiple species such as grey wolf, domestic dog and coyote.
The extinction of genera and higher families represents a greater loss of diversity and ecological function than individual species.
But as the authors of the new PLOS Biology study point out, “tetrapods encompass [less than] <2% of all known species, and only about half of known vertebrate species”.
They reviewed data on more than 22,000 plant and animal genera assessed by the IUCN to get a better idea of the severity of modern higher-level extinctions. The analysis found that recent extinctions were relatively rare, with 102 known since the year 1500. Among these 79 genera had contained only 1 species.
Genera extinctions occurred in less than 2% of birds and mammals, about 1% in turtles, and less than 0.5% of assessed genera in most others.
The highest extinction rates occurred in the late 1800s and early 1900s.
“They were mostly of genera found only on islands, and … these extinctions actually slowed down over the last 100 years instead of rapidly accelerating,” adds Wiens.
The authors suggest several factors may have contributed to this pattern, including the fact that most genus-level extinctions occurred in birds and mammals in which “conservation action has demonstrably slowed extinctions”.
“There was never any evidence that these extinctions, which peaked [about] ~100 years ago and occurred mostly on isolated islands, endanger human survival,” says Wiens.
Co-author Kristen Saban of Harvard University in the US adds, “Now more than ever, given the widespread mistrust in science, it is important that we conduct conservation research carefully and present it accurately.”
The authors caution their study should not be interpreted as downplaying current threats to biodiversity and stress that there may be many additional genus-level extinctions in the future.
“However, it is unclear if the past extinctions described here are relevant to predicting these future extinctions. After all, most of these past genus-level extinctions were on islands and occurred many decades ago,” they write.
“For example, climate change may be a major cause of future extinctions, but it is not a major cause of recent past extinctions or current IUCN-listed threats among species.
“Similarly, invasive species were the primary cause of the majority of species extinctions on islands, but habitat loss is the greatest current threat to presently endangered species, both on islands and on the mainland.”
Human-related extinctions should be prevented, they conclude, regardless of whether they effect species directly beneficial to humans or are part of a mass extinction event.