A new study finds biodiversity loss is slowing, but not enough to celebrate yet.
Amid the doom and gloom we’ve been hearing about the biodiversity crisis, a study came out last week with good news: the loss of species seems to be slowing down globally.
According to the study results, extinction rates over the past 500 years peaked around 100 years ago for all assessed animal and plant species and have generally declined since then.
Don’t get me (or the study authors) wrong: the biodiversity crisis is serious and needs to be addressed immediately and assertively with effective conservation actions to prevent the extinction of more species. While just over 900 species have gone extinct since 1500, nearly 50,000 species are currently believed to be threatened with extinction.
But it does seem that conservation actions taken over the past few decades — from the U.S. Clean Water Act of 1972 and the Endangered Species Act of 1973 to CITES, the Convention on International Trade in Endangered Species of Wild Fauna and Flora, and other international conservation interventions— may be paying off, slowing the rate at which threatened species are going extinct.
The study by Kristen Saban and John Wiens of the University of Arizona appeared in the Proceedings of the Royal Society of London. The researchers analyzed the IUCN Red List, by the International Union for Conservation of Nature, which tracks all known animal and plant species, to determine if current levels of extinction truly match those of the prior five global mass extinctions.
Perhaps I should backtrack to provide some background on what we’re even talking about with the “sixth mass extinction.”
Researchers have determined that in Earth’s approximately 4.5-billion-year history, during which life has existed for about 3.5 or so billion years, there have been five mass extinctions (though truthfully there is some scientific debate about the exact number of extinctions that qualify).
A mass extinction is defined as the loss of at least 75% of species over a “short geological period of time.” “Short” is generally defined as less than 2.8 million years.
As best as scientists can determine from the fossil record, past mass extinction events have lasted from around 50 thousand to just under 3 million years and have killed between 75% and 95% of all animal and plant species alive at the time.
Those mass extinctions resulted in dramatic changes to the trajectory of life on earth. The last mass extinction, during the Cretaceous Period around 145 million years ago, killed off the non-avian dinosaurs and allowed mammals to evolve from tiny rodent-like creatures to the amazing diversity of species we have today. And, ultimately, humans.
In the 1990s Richard Leakey and Roger Lewin argued that humans are causing a sixth mass extinction. A 2023 study by Gerardo Ceballos and Paul Ehrlich evaluated extinction rates of some vertebrates — animals with a backbone, such as mammals, birds and reptiles — and concluded that extinction rates in the past 500 years were much higher than the so-called “background rate of extinction.” That is, the average “natural” extinction rate between the five mass extinction events. They also argued that we are now in a sixth mass extinction.
The current study by Saban and Wiens set out to empirically test that assertion. They examined the IUCN Red List and analyzed the extinction rates over the past 500 years for all assessed species.
What they found surprised them.
“We found that extinction rates are not getting faster towards the present, as many people claim, but instead peaked many decades ago,” Wiens said in a statement.
Digging into the details, the authors found that of more than 160,000 assessed animal and plant species, around 900 were extinct, or 0.6%. The highest percentages of extinct species were among mollusks (which include clams, mussels, snails and octopus) and turtles, each at 2.9%, followed by birds (at 1.5%) and mammals (1.4%).
Species living on islands such as Hawaii and French Polynesia were particularly threatened, mainly because of invasive species such as rats.
It is often difficult to determine the main cause of extinction for many species, but according to the best information from the IUCN data, the main cause of extinction was likely invasive species for the majority, followed by habitat loss, exploitation and pollution. Climate change seems to have played a relatively minor role to date.
Analyzing patterns over time, extinction rates were relatively low before 1800, then dramatically increased in the 1800s and doubled in the 1900s. The highest extinction rates for plants were in the 1920s, for mammals in the 1930s and for birds in the 1890s and 1900s.
The extinction rates for most groups peaked at least 50 years ago, indicating that extinction rates have not accelerated in recent decades, according to the study.
It’s too early to celebrate, however.
While extinction rates have slowed, an alarming number of species are currently threatened with extinction. According to the IUCN, of around 170,000 assessed species nearly 50,000, or 28%, are threatened. That includes 41% of amphibians, 26% of mammals, 34% of conifers and 11% of birds.
Similarly, even among groups with relatively low past extinction rates, such as insects with 0.4%, recent studies have documented dramatic declines in abundance. One Study reported a decline of 20% of butterflies in the past 20 years, including species that were once considered common. Experts are warning of an “insect apocalypse.”
And while that is not close to the official definition of a mass extinction, losing all those species would likely have dramatic consequences for ecosystems and human well-being.
The main causes endangering species have also changed. While invasive species have been the main threat, particularly for islands, most species nowadays are threatened primarily by habitat loss, followed by other causes such as pollution, climate change and invasive species.
The number of assessed species is only a tiny fraction of all species. The exact number of animals and plants is not known, though researchers believe there are at least 2 million species, with estimates as high as 1 trillion.
Still, the study offers a glimmer of hope. We have, apparently, successfully slowed the loss of biodiversity through strategic conservation efforts. We have brought back several species from the brink. It is now incumbent on us to keep investing in our future.
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