How \”dark extinctions\” are silently erasing life on Earth

They say what you don’t know won’t hurt you. They’re wrong.

There’s something missing on our planet, but we don’t know what it is. Like trees that fall in the silence of an empty forest, entire species are vanishing with no one to witness or record their existence – or their demise. It’s a phenomenon known as “dark extinction”, and it severely undermines our ability to catalog the world’s biodiversity or to fully understand our own impact on the web of life.

What Is Dark Extinction?

The term “dark extinction” refers to the loss of species that we don’t even know exist. In other words, it applies to life forms that become extinct before they have been discovered or scientifically described, meaning they have not received a taxonomic classification. 

Typically, the label is reserved for those plants and animals that have exited the building as a direct result of human-induced habitat change, which means undiscovered dinosaurs don’t count as dark extinctions. However, these unknown anthropogenic extinctions are by no means limited to the modern era and extend all the way back to prehistoric times when our ancestors first colonized the world’s continents and hunted certain megafauna to oblivion.

Many of these long-lost giants are well represented in the fossil record, yet experts believe there are probably a fair few megafaunal extinctions from the past 10,000 years that we don’t even know about. 

We know what we’ve lost and described, but the big unknown is what disappeared before description, and in some cases disappeared before the science of taxonomy even started.

Dr Alexander Lees

A major wave of human-driven extinction then began to ripple across the planet from about the 14th century, when advances in European maritime technology enabled Westerners to colonize remote oceanic islands, bringing with them rats, cats, and other non-native mammals that absolutely obliterated the local wildlife. However, because this predated the so-called taxonomic period (which began at the start of the 19th century), many of the species that perished during this era were not scientifically described or logged in any sort of catalog, meaning we have no idea exactly how many creatures were extinguished.

Even since the start of the taxonomic period itself, an unknown number of species have gotten off the dancefloor before anyone had a chance to log their existence. Many of these life forms simply disappeared without a trace; others left fossils that have either been discovered after they became extinct or are yet to be unearthed; others still exist only in museums or collections, waiting for a taxonomist to posthumously describe them.

“We know what we’ve lost and described, but the big unknown is what disappeared before description, and in some cases disappeared before the science of taxonomy even started,” Dr Alexander Lees from Manchester Metropolitan University tells IFLScience. “The post-Linnaean world – in which we’ve been describing species – has really only been moving forward for a few centuries. So anything prior to that age of Linnaean discovery could be completely unknown,” he adds.

To fill in all the blanks, we’d need to find the remains of every species that has been driven to extinction, yet it goes without saying that this is pretty unlikely. “For many species, we have recourse to fossils and sub fossils. So species which fossilize well may have a better record of historical extinctions – but not all species,” says Lees. “Birds, for instance, don’t fossilize as well as mammals, and most invertebrates are very rare in the fossil record.”

In other words, we don’t really know how many dark extinctions have occurred throughout human history, although scientists have made a few guesses – and the figures are pretty shocking.

How Many Species Have Become Extinct?

The official tally of extinct plants and animals is kept by the International Union for Conservation of Nature (IUCN) Red List, and currently stands at just 909 species. However, by the organization’s own admission, this figure is probably a “significant underestimate” as it doesn’t account for unknown extinctions or undiscovered species.

Researchers have therefore made several attempts at developing statistical models to calculate the number of dark extinctions that have accompanied these confirmed extirpations. For instance, the results of one study indicated that around 1,430 species of bird may have been killed off by human activities, while another suggests that up to 60 percent of all extinctions within certain taxonomic groups may be undiscovered species.

Yet another paper concluded that we may have lost up to 260,000 invertebrate species over the past 500 years, highlighting the inadequacy of the Red List when it comes to logging dark extinctions. However, all of these estimates are pretty rough and somewhat speculative, which means we don’t really have any solid figures to represent all the organisms that have vanished from the face of the Earth.

And while there’s a growing body of theoretical work on dark extinctions, every now and then scientists find actual physical evidence for the phenomenon – most of which only serves to show us how wrong our numerical estimates have been. For instance, in 2013, a lineage of extinct land snails was discovered in French Polynesia, instantly increasing the total number of confirmed mollusk extinctions by around 2 percent. Given that most dark extinction estimates are based on extrapolations of known extinctions, findings like this can massively influence the statistical modeling.

To complicate the matter further, Lees explains that each newly confirmed historical extinction opens up endless possibilities for the loss of “commensal species” that may have been associated with these defunct creatures. For instance, while ancient megafauna may be easy to spot in the fossil record, Lees says that when it comes to “the parasites which lived in these species and all the things which foraged their dung, we have a very poor idea of what might have been lost.”

“The potential then for lots of continental extinctions during this loss of megafauna is also huge, and we really don’t have a good idea about the nature of those losses,” he laments.

How Can Dark Extinctions Be Prevented?

Extinction is an inevitable part of the natural selection process, and it’s thought that even without the help of humans, the world would lose about 0.1 organisms per million species per year. This is known as the background extinction rate.

However, current estimates indicate that the world is emptying thousands of times faster than this background rate, leading to the suggestion that we may even be in the midst of the sixth mass extinction in our planet’s history. It’s therefore painfully obvious that we’re failing to protect Earth’s biodiversity, and massive investment in habitat conservation is now needed if we want to have any hope of arresting the slide.

Nowhere is this more urgent than in Brazil’s Atlantic Forest, which has the highest concentration of threatened vertebrates in the Americas and also happens to be the focus of Lees’ research. Having once covered 1.2 million square kilometers (463,000 square miles), the forest has now been reduced to “patches that are often too small to support viable populations of species,” he says.

“You might have one pair living [in a patch], but there’s no sharing of genes between forest patches and you only need one individual to disappear and that’s your species gone,” explains Lees. As a result, he says that conservationists have had to simply “watch birds disappearing from all those patches.”

The problem is now so extreme that Lees believes many of the region’s threatened species are unlikely to survive without captive breeding programs. Even those efforts, however, will be “irrelevant if there’s not a contingency plan to restore those [forest] patches, to restore connectivity between patches, and to increase the size of those patches.”

[U]nless you describe a new dinosaur, stuff doesn’t get to [journals] Nature or Science. It goes into parochial publications… – Dr Alexander Lees

Of course, ex situ breeding programs are designed to rescue known species from annihilation and therefore don’t directly help to ease the dark extinction crisis. By restoring habitats, however, we could unknowingly bring huge numbers of as-yet-undiscovered species back from the brink, in addition to saving those we are actually trying to save.

Then again, dark extinction would cease to exist as a concept if we had a more complete inventory of the world’s inhabitants. We may not stop many of these species from disappearing, but if only we could describe them all then at least we’d know what we’re losing.

This, too, is likely to be a mammoth task. It’s thought there are around 8.7 million animal species on Earth, and in more than two centuries of taxonomic classification, we’ve only described about 1.2 million of these. According to some calculations, 86 percent of all terrestrial creatures and 91 percent of those living in the ocean have still not been officially discovered.

Clearly, then, we need to significantly ramp up our taxonomic output. Unfortunately, however, things seem to be going backwards, as the science of taxonomy is finding fewer and fewer devotees.

“There’s this mission to go and catalog Earth’s biodiversity before it goes extinct, but that process is really slowed up because taxonomy is massively underfunded these days,” says Lees. Part of the problem, he explains, is that “unless you describe a new dinosaur, stuff doesn’t get to [journals] Nature or Science. It goes into parochial publications, and it’s a huge amount of work.”

In other words, the pay-off doesn’t justify the time and effort of describing unspectacular new species that few people really care about. To prevent taxonomy from going extinct itself, then, increased investment is badly needed.

It goes without saying, though, that even an army of well-funded taxonomists would fail to catalog every single species on the planet. Yet the more organisms we can classify, the less extinctions will fall into the “dark” category,

“You will never ever describe everything,” says Lees. “But hopefully we can get well above 10 percent. That should be the goal.”