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Saving Our World–Appendix A–How Many Species Have we Lost Since Mankind First Appeared?

What’s the first species humans drove to extinction?

Sometime in the late 1600s, in the lush forests of Mauritius, the very last dodo took its last breath. After centuries of untroubled ferreting in the tropical undergrowth, this species met its untimely end at the hands of humans, who had arrived on the island less than 100 years before. With their penchant for hunting, habitat destruction and the release of invasive species, humans undid millions of years of evolution, and swiftly removed this bird from the face of the Earth.

Since then, the dodo has nestled itself in our conscience as the first prominent example of human-driven extinction. We’ve also used the dodo to assuage our own guilt: the creature was fat, lazy and unintelligent — and as popular story goes, those traits sealed its inevitable fate.

But in fact, we couldn’t be more wrong, said Julian Hume, a paleontologist and research associate with the National History Museum in the United Kingdom. He studies the fossils of extinct species, and has devoted a portion of his career to correcting the dodo’s dismal reputation. By digitally modelling the remains of a dodo’s skeleton, he’s produced a 3D digital reconstruction that draws an altogether different picture of a bird that was faster, more athletic and far brainier than popular culture has led us to believe. “It was nothing like this big, fat, bulgy thing that was just waddling around. This bird was super adapted to the environment of Mauritius,” Hume told Live Science. Instead, humans’ unrelenting exploitation was the real culprit behind the dodo’s untimely death.

But that’s not all we’ve gotten wrong. Despite the commonly held belief, the dodo actually wasn’t the first creature that humans drove to extinction — not by a long shot. In fact, humanity was wiping out the world’s fauna thousands of years before we set eyes on the dodo. “There was certainly a lot more going on before and after that event,” said Hume.

So, if the iconic dodo wasn’t the first species we drove to the brink, then which animal gets this disheartening title, instead?

Humans on the move

We’ve grown accustomed to thinking about human-driven species extinction as a relatively recent trend in our history. Yet, researchers have found convincing palaeontological evidence that dismantles that idea.

“The real problem started when we, as humans, started migrating,” Hume said. That starting point is still debated, but most recent estimates suggest that migrations that led to lasting populations of humans spread across the globe began with the movement of hominids — Neanderthals and other ancient human relatives, as well Homo sapiens — out of Africa and southeast Asia, roughly 125,000 years ago. This is where the evidence gets interesting. As humans left their ancestral homes, and over the following tens of thousands of years went on to colonize Eurasia, Oceania, North and South America, the fossil record shows a parallel uptick in the extinction in large-bodied animals — also known as megafauna — across those continents. 

“As hominids migrated out of Africa, you see this incredibly regular pattern of extinction,” said Felisa Smith, a professor of ecology and evolutionary biology at the University of New Mexico, who studies how animals’ body sizes have changed over the course of history. As she and her colleagues explained in a 2018 study published in the journal Science, each time our ancestors set foot in new places, fossil records show that large-bodied species — the humongous prehistoric relatives of elephantsbears, antelope and other creatures — started going extinct within a few hundred to 1,000 years, at most. Such rapid extinction timescales don’t occur at any other point in the last several million years (not since the non-avian dinosaurs were wiped out by an asteroid about 65 million years ago.) “The only time you see it is when humans are involved, which is really striking,” Smith said. 

Some of those early lost species would seem like fantastical beasts if they roamed Earth today. For example, “There was an armadillo-like thing called the glyptodon, which was the size of a Volkswagen bus,” Smith told Live Science. Glyptodons, many equipped with vicious-looking spiked tails, disappeared from the Americas at the end of the last ice age, roughly 12,000 years ago — which is probably connected to the earlier arrival of humans there. The number of gigantic Eurasian cave bears, several hundred pounds heavier than grizzly bears today, went into a steep decline about 40,000 years ago(opens in new tab), around the same time that humans began to spread across their habitat. South America was once home to lumbering giant ground sloths — and humans were also the most likely candidate in their demise, about 11,000 years ago. 

What made large animals, in particular, so susceptible to humanity’s spread? Megafauna likely represented food, or a threat, to incoming humans. What’s more, animals that had never encountered humans before were probably unwary of these strange newcomers migrating into their unspoiled lands, which might have increased their vulnerability to attack. Unlike other smaller animals that breed more rapidly, megafauna also reproduce more slowly and so have smaller populations compared with other species, Hume explained: “So if you take out a big section of [a population] they cannot reproduce quickly enough to build up numbers again.”

It wasn’t just hunting that posed a threat — but also the spread of human-caused fires that would have destroyed swathes of habitat, and increasing competition from humans for food. For instance, it’s thought that by preying heavily on the same herbivores, growing numbers of hungry humans helped drive the extinction of the short-faced bear, a gigantic South American species that once stood at over 10 feet (3 meters) tall, and died out roughly 11,000 years ago. Climate change, paired with human impacts like hunting, also proved to be a lethal combination for some megafauna — most famously, mammoths, which went extinct about 10,500 years ago (except for the dwarf woolly mammoth, which survived until about 4,000 years ago on an island off northern Russia). “If you combine climate change with a negative human impact, it’s a disaster,” said Hume.

An answer?

All of this is to say that humans have systematically wiped out the species around us from almost the beginning of our history. Our migration prompted “a disaster across the world,” said Hume. “We weren’t very pleasant.” Unfortunately, we’ve continued our ancestors’ legacy, with, among thousands of other species, the eradication of Madagascan hippos 1,000 years ago, the loss of moa birds in New Zealand 600 years ago, and the decimation of passenger pigeons 106 years ago. We are also responsible for ongoing extinctions today.

But this still hasn’t answered the question of what species went extinct first. And here’s the catch: the data on human-driven extinction across the planet is only reliable as far back as about 125,000 years— but that doesn’t mean we weren’t driving animals to extinction before that in Africa, too. In fact, there’s compelling evidence to suggest that before humans migrated out, they unleashed their hunting instincts on species there as well. 

Smith’s research has revealed that the average body size of African animals 125,000 years ago was only half that of species that were present on other continents around the world. “Africa is one of the largest continents, so it should have had a mean body size similar to that of the Americas and Eurasia where it was roughly about 100 kilograms [220 lbs.],” Smith said. “The fact that it didn’t suggests that there had already been an effect of hominids on megafauna in Africa, prior to 125,000 years ago.” 

In essence, because the rest of history tells us that humans are good at dispatching the largest creatures in an ecosystem, we can make a fairly safe assumption that hominids in Africa at the time could have been responsible for extinctions going even further back in time.

Still, there’s no way to know for sure what that ‘first’ species would have been — though Smith takes a wild guess: “It was probably some species in the elephant family. But whether that’s palaeomastodon, or stegodon” — the latter being a behemoth with tusks that measured 10 feet (3 meters) long – “I couldn’t tell you.”

Clues for the future

We may not have a clear answer to that original question – but perhaps the more important one to ask is what humanity’s legacy of extinction can teach us about conservation, going into the future. 

Past extinctions have revealed that when animals — especially megafauna — disappear, there are profound ecological consequences. Whole landscapes are transformed in the absence of their shaping effects, with changes to vegetation and species diversity. Smith has even published research showing that the decline of global megafauna in past millennia led to dips in the amount of methane they burped out — with potentially transformative consequences for global climate. What’s more, when animals disappear, whole rafts of dependent species go down with them. The iconic dodo presents one such cautionary tale: when the birds died out, so did a Mauritian dung beetle that relied on dodo feces to survive.

Understanding human-driven extinctions of the past can help us figure out what the environmental consequences have been, explained Smith, and how we can limit those in the future by protecting the species that remain. Even the dodo’s extinction provides clues that are helping us preserve ecosystems today. Hume is working on a project to catalog pollen spores present in the sediments around dodo fossils, to build up a detailed picture of the lush, palm-fringed forests they once roamed. That’s helping conservationists to rewild the island with vegetation that was once there. “We’re actually reconstructing the exact species of plants and trees from the environment the dodo was living in, before humans arrived,” Hume said. 

A bit of paradise was lost when we drove the dodo to extinction — not to mention the thousands of species whose demise came before that. But perhaps with hindsight, and the willingness to learn from our mistakes, some of that can be reclaimed.  

Extinctions have been a natural part of our planet’s evolutionary history. More than 99% of the four billion species that have evolved on Earth are now gone.

At least 900 species have gone extinct in the last five centuries.

Only a small percentage of species have been evaluated for their extinction risk. This means estimates of species threatened with extinction will be an underestimate of the true number.

More than 35,000 species have been evaluated to be threatened with extinction today.

One-quarter of the world’s mammals; 1-in-7 bird species; and 40% of amphibians are threatened.

There have been five mass extinction events in Earth’s history: ‘The Big Five’.

More than 178 of the world’s largest species went extinct during the Quaternary Extinction. Overhunting was likely the main driver.

Extinction rates today are much higher than background rates and rates from previous mass extinctions.

While many species are in danger, conservation has also saved tens of mammal and bird species from extinction.

Extinctions have been a natural part of the planet’s evolutionary history. 99% of the four billion species that have evolved on Earth are now gone. Most species have gone extinct.

But when people ask the question of how many species have gone extinct, they’re usually talking about the number of extinctions in recent history. Species that have gone extinct, mainly due to human pressures.

The IUCN Red List has estimated the number of extinctions over the last five centuries. Unfortunately we don’t know about everything about all of the world’s species over this period, so it’s likely that some will have gone extinct without us even knowing they existed in the first place. So this is likely to be an underestimate.

In the chart we see these estimates for different taxonomic groups. It estimates that 900 species have gone extinct since 1500. Our estimates for the better-studied taxonomic groups are likely to be more accurate. This includes 85 mammal; 159 bird; 35 amphibian; and 80 fish species.

To understand the biodiversity problem we need to know how many species are under pressure; where they are; and what the threats are. To do this, the IUCN Red List of Threatened Species evaluates species across the world for their level of extinction risk. It does this evaluation every year, and continues to expand its coverage.

The IUCN has not evaluated all of the world’s known species; in fact, in many taxonomic groups it has assessed only a very small percentage. In 2021, it had assessed only 7% of described species. But, this very much varies by taxonomic group. In the chart we see the share of described species in each group that has been assessed for their level of extinction risk. As we’d expect, animals such as birds, mammals, amphibians have seen a much larger share of their species assessed – more than 80%. Only 1% of insects have. And less than 1% of the world’s fungi.

The lack of complete coverage of the world’s species highlights two important points we need to remember when interpreting the IUCN Red List data:

  1. Changes in the number of threatened species over time does not necessarily reflect increasing extinction risks. The IUCN Red List is a project that continues to expand. More and more species are been evaluated every year. In the year 2000, less than 20,000 species had been evaluated. By 2021, 140,000 had. As more species are evaluated, inevitably, more will be listed as being threatened with extinction. This means that tracking the data on the number of species at risk of extinction over time doesn’t necessarily reflect an acceleration of extinction threats; a lot is simply explained by an acceleration of the number of species being evaluated. This is why we do not show trends for the number of threatened species over time.
  2. The number of threatened species is an underestimate. Since only 7% of described species have been evaluated (for some groups, this is much less) the estimated number of threatened species is likely to be much lower than the actual number. There is inevitably more threatened species within the 93% that have not been evaluated.

We should also define more clearly what threatened with extinction actually means. The IUCN Red List categorize species based on their estimated probability of going extinct within a given period of time. These estimates take into account population size, the rate of change in population size, geographical distribution, and extent of environmental pressures on them. ‘Threatened’ species is the sum of the following three categories:

Critically endangered species have a probability of extinction higher than 50% in ten years or three generations;

Endangered species have a greater than 20% probability in 20 years or five generations;

Vulnerable have a probability greater than 10% over a century.

How many species are threatened with extinction?

The IUCN Red List has evaluated 40,084 species across all taxonomic groups to be threatened with extinction in 2021. As we noted earlier, this is a large underestimate of the true number because most species have not been evaluated.

In the chart we see the number of species at risk in each taxonomic group. Since birds, mammals, and amphibians are the most well-studied groups their numbers are the most accurate reflection of the true number. The numbers for understudied groups such as insects, plants and fungi will be a large underestimate.

What percentage of species are threatened with extinction?

What share of known species are threatened with extinction? Since the number of species that has been evaluated for their extinction risk is such a small fraction of the total known species, it makes little sense for us to calculate this figure for all species, or for groups that are significantly understudied. It will tell us very little about the actual share of species that are threatened.

But we can calculate it for the well-studied groups. The IUCN Red List provides this figure for groups where at least 80% of described species has been evaluated. These are shown in the chart.

Around one-quarter of the world’s mammals; 1-in-7 bird species; and 40% of amphibians are at risk. In more niche taxonomic groups – such as horseshoe crabs and gymnosperms, most species are threatened.

Are we heading for a sixth mass extinction?

Seeing wildlife populations shrink is devastating. But the extinction of an entire species is tragedy on another level. It’s not simply a downward trend; it marks a stepwise change. A complex life form that is lost forever.

But extinctions are nothing new. They are a natural part of the planet’s evolutionary history. 99% of the four billion species that have evolved on Earth are now gone. Species go extinct, while new ones are formed. That’s life. There’s a natural background rate to the timing and frequency of extinctions: 10% of species are lost every million years; 30% every 10 million years; and 65% every 100 million years.

What worries ecologists is that extinctions today are happening much faster than nature would predict. This has happened five times in the past: these are defined as mass extinction events and are aptly named the ‘Big Five’ [we cover them in more detail here]. In each extinction event the world lost more than 75% of its species in a short period of time (here we mean ‘short’ in its geological sense – less than two million years).

Are we in the midst of another one? Many have warned that we’re heading for a sixth mass extinction, this one driven by humans. Is this really true, or are these claims overblown?

How do we know if we’re heading for a sixth mass extinction?

Before we can even consider this question we need to define what a ‘mass extinction’ is. Most people would define it as wiping out all, or most of, the world’s wildlife. But there’s a technical definition. Extinction is determined by two metrics: magnitude and rate. Magnitude is the percentage of species that have gone extinct. Rate measures how quickly these extinctions happened – the number of extinctions per unit of time. These two metrics are tightly linked, but we need both of them to ‘diagnose’ a mass extinction. If lots of species go extinct over a very long period of time (let’s say, 1 billion years), this is not a mass extinction. The rate is too slow. Similarly, if we lost some species very quickly but in the end it didn’t amount to a large percentage of species, this also wouldn’t qualify. The magnitude is too low. To be defined as a mass extinction, the planet needs to lose a lot of its species quickly.

In a mass extinction we need to lose more than 75% of species, in a short period of time: around 2 million years. Some mass extinctions happen more quickly than this.

Of course, this is not to say that “only” losing 60% of the world’s species is no big deal. Or that extinctions are the only measure of biodiversity we care about – large reductions in wildlife populations can cause just as much disruption to ecosystems as the complete loss of some species. We look at these changes in other parts of our work [see our article on the Living Planet Index]. But here we’re going to stick with the official definition of a mass extinction to test whether these claims are true.

There are a few things that make this difficult. The first is just how little we know about the world’s species and how they’re changing. Some taxonomic groups – such as mammals, birds and amphibians – we know a lot about. We have described and assessed most of their known species. But we know much less about the plants, insects, fungi and reptiles around us. For this reason, mass extinctions are usually assessed for these groups we know most about. This is mostly vertebrates. What we do know is that levels of extinction risk for the small number of plant and invertebrate species that have been assessed is similar to that of vertebrates. This gives us some indication that vertebrates might give us a reasonable proxy for other groups of species.

The second difficulty is understanding modern extinctions in the context of longer timeframes. Mass extinctions can happen over the course of a million years or more. We’re looking at extinctions over the course of centuries or even decades. This means we’re going to have to make some assumptions or scenarios of what might or could happen in the future.

There are a few metrics researchers can use to tackle this question.

  1. Extinctions per million species-years (E/MSY). Using reconstructions in the fossil record, we can calculate how many extinctions typically occur every million years. This is the ‘background extinction rate’. To compare this to current rates we can assess recent extinction rates (the proportion of species that went extinct over the past century or two) and predict what proportion this would be over one million species-years.
  2. Compare current extinction rates to previous mass extinctions. We can compare calculations of the current E/MSY to background extinction rates (as above). But we can also compare these rates to previous mass extinction events.
  3. Calculate the number of years needed for 75% of species to go extinct based on current rates. If this number is less than a few million years, this would fall into ‘mass extinction’ territory.

Calculate extinction rates for the past 500 years (or 200 years, or 50 years)and ask whether extinction rates during previous periods were as high.

How many species have gone extinct in recent centuries?

An obvious question to ask is how many species have gone extinct already. How close to the 75% ‘threshold’ are we?

At first glance, it seems like we’re pretty far away. Since 1500 around 0.5% to 1% of the world’s assessed vertebrates have gone extinct. As we see in the chart, that’s around 1.3% of birds; 1.4% of mammals; 0.6% of amphibians; 0.2% of reptiles; and 0.2% of bony fishes. Due to the many measurement issues for these groups – and how our understanding of species has changed in recent centuries – the extinction rates that these predict are likely an underestimate (more on this later). 

So, we’ve lost around 1% of these species. But we should also consider the large number of species that are threatened with extinction. Thankfully we’ve not lost them yet, but there is a high risk that we do. Species threatened with extinction are defined by the IUCN Red List, and it encompasses several categories:

Critically endangered species have a probability of extinction higher than 50% in ten years or three generations;

Endangered species have a greater than 20% probability in 20 years or five generations;

Vulnerable have a probability greater than 10% over a century.

There’s a high chance that many of these species go extinct in the new few decades. If they do, this share of extinct species changes significantly. In the chart we also see the share of species in each group that is threatened with extinction. We would very quickly go from 1% to almost one-quarter of species. We’d be one-third of the way to the ‘75%’ line.

Again, you might think that 1%, or even 25%, is small. At least much smaller than the 75% definition of a mass extinction. But what’s important is the speed that this has happened. Previous extinctions happened over the course of a million years or more. We’re already far along the curve within only a few centuries, or even decades. We’ll see this more clearly later when we compare recent extinction rates to those of the past. But we can quickly understand this from a quick back-of-the-envelope calculation. If it took us 500 years to lose 1% of species, it would take us 37,500 years to lose 75%. Much faster than the million years of previous extinction events. Of course this assumes that future extinctions would continue at the same rate – a big assumption, and one we will come to later. It might even be a conservative one – there might be species that went extinct without us even knowing that they existed at all.

Are recent extinction rates higher than we would expect?

There are two ways to compare recent extinction rates. First, to the natural ‘background’ rates of extinctions. Second, to the extinction rates of previous mass extinctions.

The research is quite clear that extinction rates over the last few centuries have been much higher than we’d expect. The background rate of extinctions of vertebrates that we would expect is around 0.1 to 1 extinctions per million-species years (E/MSY). In the chart we see the comparison, broken down by their pre- and post-1900 rates.

Modern extinction rates average around 100 E/MSY. This means birds, mammals and amphibians have been going extinct 100 to 1000 times faster than we would expect. 

Researchers think this might even be an underestimate. One reason is that some modern species are understudied. Some might have gone extinct before we had the chance to identify them. They will ultimately show up in the fossil record later, but for now, we don’t even know that they existed. This might be particularly true for species a century ago when much less resource was put into wildlife research and conservation.

Another key point is that we have many species that are not far from extinction: species that are critically endangered or endangered. There’s a high chance that many could go extinct in the coming decades. If they did, extinction rates would increase massively. In another study published in Science, Michael Hoffman and colleagues estimated that 52 species of birds, mammals and amphibians move one category closer to extinction on the IUCN Red List every year. Pimm et al. (2014) estimate that this would give us an extinction rate of 450 E/MSY. Again, 100 to 1000 times higher than the background rate.

How do recent extinction rates compare to previous mass extinctions?

Clearly we’re killing off species much faster than would be expected. But does this fall into ‘mass extinction’ territory? Is it fast enough to be comparable to the ‘Big Five’?

One way to answer this is to compare recent extinction rates with rates from previous mass extinctions. Researcher, Malcolm McCallum did this comparison for the Cretaceous-Palogene (K-Pg) mass extinction. This was the event that killed off the dinosaurs around 65 million years ago. In the chart we see the comparison of (non-dinosaur) vertebrate extinction rates during the K-Pg mass extinction to recent rates. This shows how many times faster species are now going extinct compared to then. 

We see clearly that rates since the year 1500 are estimated to be 24 to 81 times faster than the K-Pg event. If we look at even more recent rates, from 1980 onwards, this increases to up to 165 times faster. Again, this might even be understating the pace of current extinctions. We have many species that are threatened with extinction: there is a high probability that many of these species go extinct within the next century. If we were to include species classified as ‘threatened’ on the IUCN Red List, extinctions would be happening thousands of times faster than the K-Pg extinction.

This makes the point clear: we’re not only losing species at a much faster rate than we’d expect, we’re losing them tens to thousands of times faster than the rare mass extinction events in Earth’s history.

How long would it take for us to reach the sixth mass extinction?

Recent rates of extinction, if they continued, would put us on course for a sixth mass extinction. A final way to check the numbers on this is to estimate how long it would take for us to get there. On our current path, how long before 75% of species went extinct? If this number is less than 2 million years, it would qualify as a mass extinction event.


Earlier we came up with a crude estimate for this number. If it took us 500 years to lose 1% of species, it would take us 37,500 years to lose 75%. That assumes extinctions continue at the average rate over that time. Malcolm McCallum’s analysis produced a similar order of magnitude: 54,000 years for vertebrates based on post-1500 extinction rates. Extinction rates have been faster over the past 50 years. So if we take the post-1980 extinction rates, we’d get there even faster: in only 18,000 years.

But again, this doesn’t account for the large number of species that are threatened with extinction today. If these species did go extinct soon, our extinction rates would be much higher than the average over the last 500 years. In a study published in Nature, Anthony Barnosky and colleagues looked at the time it would take for 75% of species to go extinct across four scenarios.

  1. If all species classified as ‘critically endangered’ went extinct in the next century;
  2. If all species classified as ‘threatened’ went extinct in the next century;
  3. If all species classified as ‘critically endangered’ went extinct in the next 500 years;
  4. If all species classified as ‘threatened’ went extinct in the next 500 years.

To be clear: these are not predictions of the future. We can think of them as hypotheticals of what could happen if we don’t take action to protect the world’s threatened species. In each case the assumed extinction rate would be very different, and this has a significant impact on the time needed to cross the ‘mass extinction’ threshold. The results are shown in the chart.

In the most extreme case, where we lose all of our threatened species in the next 100 years, it would take only 250 to 500 years before 75% of the world’s birds, mammals and amphibians went extinct. If only our critically endangered animals went extinct in the next century, this would increase to a few thousand years. If these extinctions happened much slower – over 500 years rather than a century – it’d be around 5,000 to 10,000 years. In any scenario, this would happen much faster than the million year timescale of previous mass extinctions.

This makes two points very clear. First, extinctions are happening at a rapid rate – up to 100 times faster than the ‘Big Five’ events that define our planet’s history. Current rates do point towards a sixth mass extinction. Second, these are scenarios of what could happen. It doesn’t have to be this way.

The good news: we can prevent a sixth mass extinction

There is one thing that sets the sixth mass extinction apart from the previous five. It can be stopped. We can stop it. The ‘Big Five’ mass extinctions were driven by a cascade of disruptive events – volcanism, ocean acidification, natural swings in climate. There was no one or nothing to hit the brakes and turn things around.

This time it’s different. We are the primary driver of these environmental changes: deforestation, climate change, ocean acidification, hunting, and pollution of ecosystems. That’s depressing. But is also the best news we could hope for. It means we have the opportunity (and some would argue, the responsibility) to stop it. We can protect the world’s threatened species from going extinct; we can slow and reverse deforestation; slow global climate change; and allow natural ecosystems to heal. There are a number of examples of where we have been successful in preventing these extinctions.

The conclusion that we’re on course for a sixth mass extinction hinges on the assumption that extinctions will continue at their recent rates. Or, worse, that they will accelerate. Nothing about that is inevitable. To stop it, we need to understand where and why the world’s species are going extinct. This is the first step to understanding what we can do to turn things around. This is what our work on Biodiversity aims to achieve.

How many species has conservation saved from extinction?

It’s hard to find good news on the state of the world’s wildlife. Many predict that we’re heading for a sixth mass extinction; the Living Planet Index reports a 68% average decline in wildlife populations since 1970; and we continue to lose the tropical habitats that support our most diverse ecosystems. The United Nations Convention on Biological Diversity set twenty targets – the Aichi Biodiversity Targets – to be achieved by 2020. The world missed all of them. We didn’t meet a single one.

Perhaps, then, the loss of biodiversity is unavoidable. Maybe there is nothing we can do to turn things around.

Thankfully there are signs of hope. As we will see, conservation action might have been insufficient to meet our Aichi targets, but it did make a difference. Tens of species were saved through these interventions. There’s other evidence that protected areas have retained bird diversity in tropical ecosystems. And each year there are a number of species that move away from the extinction zone on the IUCN Red List.

We need to make sure these stories of success are heard. Of course, we shouldn’t use them to mask the bad news. They definitely don’t make up for the large losses in wildlife we’re seeing around the world. In fact, the risk here is asymmetric: growth in one wildlife population does not offset a species getting pushed to extinction. A species lost to extinction is a species lost forever. We can’t make up for this loss by simply increasing the population of something else. But we can make sure two messages are communicated at the same time. 

First, that we’re losing our biodiversity at a rapid rate. Second, that it’s possible to do something about it. If there was no hope of the second one being true, what would be the point of trying? If our actions really made no difference then why would governments support anymore conservation efforts? No, we need to be vocal about the positives as well as the negatives to make clear that progress is possible. And, importantly, understand what we did right so that we can do more of it.

In this article I want to take a look at some of these positive trends, and better understand how we achieved them.

Pulling animals back from the brink of extinction

For anyone interested in wildlife conservation, losing a species to extinction is a tragedy. Saving a species is surely one of life’s greatest successes. 

Conservation efforts might have saved tens of beautiful species over the last few decades. The 12th Aichi Target was to ‘prevent extinctions of known threatened species’. We might have missed this, but efforts have not been completely in vain.

In a recent study published in Conservation Letters, researchers estimate that between 28 and 48 bird and mammal species would have gone extinct without the conservation efforts implemented when the Convention on Biological Diversity came into force in 1993. 21 to 32 bird species, and 7 to 16 mammal species were pulled back from the brink of extinction. In the last decade alone (from 2010 to 2020), 9 to 18 bird, and 2 to 7 mammal extinctions were prevented. This has preserved hundreds of millions of years of evolutionary history. It prevented the loss of 120 million years of evolutionary history of birds, and 26 million years for mammals.

What this means is that extinction rates over the last two decades would have been at least three to four times faster without conservation efforts. 

This does not mean that these species are out-of-danger. In fact, the populations of some of these species is still decreasing. We see this in the chart, which shows how the populations of these bird and mammal species that were expected to have gone extinct are changing. 16% of these bird species, and 13% of the mammal species have gone extinct in the wild, but conservation has allowed them to survive in captivity. Across the critically endangered, endangered and vulnerable categories, 53% of bird and 31% of mammal species have increasing or stable populations. This is positive, but makes clear that many of these species are still in decline. Conservation has only been able to slow these losses down.

This only looks at species on the brink of extinction. Many species in serious but less-threatened categories have been prevented from moving closer to extinction. Around 52 species of mammals, birds and amphibians move one category closer to extinction every year. Without conservation, this number would be 20% higher.

There are more examples. Studies have shown that protected areas have had a positive impact on preserving bird species in tropical forests. These are some of the world’s most threatened ecosystems. And while the IUCN Red List usually makes for a depressing read, there are some success stories. This year the European Bison, Europe’s largest land mammals, was moved from ‘Vulnerable’ to ‘Near threatened’ (meaning it’s less threatened with extinction) thanks to continued conservation efforts. We will look at more European success stories later.

Friederike Bolam et al. (2021) looked at what conservation actions were key to saving the mammal and bird species deemed to be destined for extinction.24 For both birds and mammals, legal protection and the growth of protected areas was important. Protected areas are not perfect – there are countless examples of poorly managed areas where populations continue to shrink. We will look at how effective protected areas are in a follow-up article. But, on average, they do make a difference. Clearly these efforts were critical for species that had gone extinct in the wild. Other important factors were controlling the spread of invasive species into new environments; reintroducing old species into environments where they had been previously lost; and restoring natural habitats, such as wetlands and forests.

Restoring wildlife populations across Europe

The European Bison might steal the headlines, but there are many good news stories across Europe. Many of the drivers of biodiversity loss – deforestation, overhunting, and habitat loss – are happening in the tropics today. But these same changes also happened across Europe and North America. Only, they happened earlier – centuries ago. 

Europe is now trying to restore its lost wildlife and habitats through rewilding programmes. The Zoological Society of London, Birdlife International and European Bird Census Council published a report which details how these efforts are going.25 They looked at how the populations of 18 mammal and 19 of Europe’s iconic but endangered bird species had changed over the past 50 years. 

Most had seen an overwhelming recovery. Most species saw an increase of more than 100%. Some saw more than 1000% growth. Brown bear populations more than doubled over these 50 years. Wolverine populations doubled in the 1990s alone. The Eurasian lynx increased by 500%. Reintroduction programmes of the Eurasian beaver saw populations increase by 14,000% – a doubling or tripling every decade.

What were the main drivers of this recovery?

Part of Europe’s success in restoring wildlife populations in recent decades can be attributed to the fact that their development and harvesting of resources came long ago. My European ancestors had already hunted many species to extinction; expanded agricultural land into existing forest; and built cities, roads and other infrastructure that fragments natural habitats. Only in our very recent past have European countries been able to reverse these trends: reforesting; raising livestock instead of hunting; and now reducing the amount of land we use for agriculture through improved productivity.

But there have also been a number of proactive interventions to restore populations. In the chart here we see the main drivers of recovery across European bird species. At the top of the list is habitat restoration – the re-establishment of wetlands, grasslands, forests and other national habitats. Reintroduction of species has also been key. But protecting existing habitats and species has been equally important. Legal site protections and bans on shooting have been the main recovery drivers of almost as many species.

After millennia of habitat loss and exploitation by humans, wildlife is coming back to Europe. Somewhat ironically, humans have played an important role in this.

While most biodiversity trends point towards a barren future for the planet’s wildlife, there are success stories to draw upon. These should not make us complacent, or deflect our attention from the seriousness of these losses. But I think it is important to highlight what we have achieved. Protecting the world’s wildlife is not impossible – we’ve just seen the counter-evidence to this. To commit to wider conservation efforts we need to shout more loudly about these wins. Otherwise policymakers will turn their backs on them and we will lose many beautiful species that we could and should have saved.

Drivers of recovery in European bird populations

The number of European bird species that have seen a significant recovery in their populations in recent decades,categorized by the main driver of their recovery.0 species2 species4 species6 species8 speciesHabitat restoration10 speciesReintroduction9 speciesSite/habitat protection8 speciesProtection from shooting8 speciesCompensation/subsidies6 speciesHabitat shift5 speciesAnti-poisoning campaigns5 speciesSupplementary feeding4 speciesImproved climate3 speciesBan on organochlorines2 speciesControl of problematic species2 species

Source: Deinet, S., Ieronymidou, C., McRae, L., Burfield, I.J., Foppen, R.P., Collen, B. and Böhm, M. (2013) Wildlife comeback in Europe: The recovery
of selected mammal and bird species.

Wait, Have We Really Wiped Out 60 Percent of Animals?

The findings of a major new report have been widely mischaracterized—although the actual news is still grim.

Since Monday, news networks and social media have been abuzz with the claim that, as The Guardian among others tweeted, “humanity has wiped out 60 percent of animals since 1970”—a stark and staggering figure based on the latest iteration of the WWF’s Living Planet report.

But that isn’t really what the report showed.

The team behind the Living Planet Index relied on previous studies in which researchers estimated the size of different animal populations, whether through direct counts, camera traps, satellites, or proxies like the presence of nests or tracks. The team collated such estimates for 16,700 populations of mammals, birds, reptiles, amphibians, and fish, across 4,000 species. (Populations here refers to pockets of individuals from a given species that live in distinct geographical areas.)

That covers just 6.4 percent of the 63,000 or so species of vertebrates—that is, back-boned animals—that are thought to exist. To work out how the entire group has fared, the team adjusted its figures to account for any biases in its data. For example, vertebrates in Europe have been more heavily studied than those in South America, and prominently endangered creatures like elephants have been more closely studied (and have been easier to count) than very common ones like pigeons.

Ultimately, they found that from 1970 to 2014, the size of vertebrate populations has declined by 60 percent on average. That is absolutely not the same as saying that humans have culled 60 percent of animals—a distinction that the report’s technical supplement explicitly states. “It is not a census of all wildlife but reports how wildlife populations have changed in size,” the authors write.

To understand the distinction, imagine you have three populations: 5,000 lions, 500 tigers, and 50 bears. Four decades later, you have just 4,500 lions, 100 tigers, and five bears (oh my). Those three populations have declined by 10 percent, 80 percent, and 90 percent, respectively—which means an average decline of 60 percent. But the total number of actual animals has gone down from 5,550 to 4,605, which is a decline of just 17 percent.

For similar reasons, it’s also not right that we have “killed more than half the world’s wildlife populations” or that we can be blamed for “wiping out 60 percent of animal species” or that “global wildlife population shrank by 60 percent between 1970 and 2014.” All of these things might well be true, but they’re all making claims about metrics that were not assessed in the Living Planet Index.

The uncertainties mount when you consider that the 63,000 species of vertebrates are vastly outnumbered by the untold millions of species of invertebrates—spineless creatures like insects, worms, jellyfish, and sponges, which make up the majority of animal life. Their fates are murkier because scientists have collectively spent less time monitoring them. They are harder to study, and draw less attention, than the allegedly more charismatic vertebrates—although plans are afoot to give them their due.

The average 60 percent decline across populations also obscures the fates of individual species. In the hypothetical scenario above, lions are still mostly fine, the tigers are in trouble, and the bears are on the brink of extinction. And of the species covered in the actual Living Planet Index, half are increasing in number, while only half are decreasing. This means that for those that are actually in decline, the outlook is even worse than it first appears.

None of this is to let humanity off the hook. Since prehistory, humans have killed off so many species of mammals that it would take 3 million to 7 million years of evolution for them to evolve an equivalent amount of diversity. At least a third of amphibians face extinction, thanks to climate change, habitat loss, and an apocalyptic killer fungus. Even invertebrates aren’t off the hook. There might be fewer data for them, but the data that exist paint an alarming picture of rapidly disappearing insects, even in supposedly pristine forests. Meanwhile, in the oceans, coral reefs are bleaching too quickly to recover: Half of the corals in the Great Barrier Reef have died since 2016. All this evidence points to a period of “biological annihilation” that some have likened to the five great mass extinctions of the past. When the reality is this sensational, there’s not much need to sensationalize it even further.

Biodiversity loss: How accurate are the numbers?

Twenty years ago, the Earth Summit in Rio resulted in a Convention on Biological Diversity, now signed by 193 nations, to prevent species loss. But can we tell how many species are becoming extinct?

One statement on the Convention’s website claims: “We are indeed experiencing the greatest wave of extinction since the disappearance of the dinosaurs.”

While that may (or may not) be true, the next sentence is spuriously precise: “Every hour three species disappear. Every day up to 150 species are lost.”

Even putting aside the apparent mathematical error in that claim (on the face of it, if three species are disappearing every hour, 72 would be lost every day) there is an obvious problem in generating any such number. No-one knows how many species exist. And if we don’t know a species exists, we won’t miss it when it’s gone.

“Current estimates of the number of species can vary from, let’s say, two million species to over 30 or even 100 million species,” says Dr Braulio Dias, executive secretary of the Convention on Biological Diversity. “So we don’t have a good estimate to an order of magnitude of precision,” he says.

It is possible to count the number of species known to be extinct. The International Union for Conservation of Nature (IUCN) does just that. It has listed 801 animal and plant species (mostly animal) known to have gone extinct since 1500.

But if it’s really true that up to 150 species are being lost every day, shouldn’t we expect to be able to name more than 801 extinct species in 512 years?

Professor Georgina Mace, who works in the Centre for Population Biology at Imperial College London, says the IUCN’s method is helpful but inadequate. “It is never going to get us the answers we need,” she says. That’s why scientists prefer to use a mathematical model to estimate species loss.

Recently, however, that model has been attacked in the pages of Nature. Professor Stephen Hubbell from the University of California, Los Angeles, says that an error in the model means that it has – for years – over-estimated the rate of species loss.

The model applies something called the “species to area relationship” to habitat loss. Put simply, an estimate is made of the number of species in a given area, or habitat – the larger the area, the greater the number of species are said to be in it.

Then the model is worked backwards – the smaller the area, the fewer the species. In other words, if you measure habitat loss, you can use the model to calculate how many species are being lost as that habitat gets smaller.

The problem, says Hubbell, is that the model does not work in reverse. “The method,” he says, “when extrapolated backward, doesn’t take into account the fact that you need to remove more area to get to the whole range of a species than you need to remove area to find the first individual of a species.”

Hubbell’s point is that if you increase a habitat by, say, five hectares, and your calculations show that you expect there to be five new species in those five hectares, it is wrong to assume that reversing the model, and shrinking your habitat, eliminates five species.

That’s because it takes more area to establish extinction – to show that every individual in a species has been eliminated – than it does to discover a new species, which requires coming across just one individual of that species. Hubbell says when corrected the model shows about half as many species going extinct as previously reported.

Unfortunately for scientists trying to measure species loss, the problems don’t end there. They also need to calculate the ‘background rate’ of extinction. If you want to work out the impact of human life on biodiversity, you need to know how many species would have gone extinct anyway without us being here. Mace says that is difficult.

“Background rates are not constant either,” she says. “If you look back through the history of life on Earth, there have been major periods of extinctions. Extinction rates vary a lot.”

The level of uncertainty faced by researchers in this field means it is perhaps not surprising that no-one can be sure of the scale of species loss. It also means that when a representative of the Convention of Biological Diversity claimed “every hour three species disappear” he must have known it was too precise.

But the fact that the precise extinction rate is unknowable does not prove that the problem is imagined.

Braulio Dias, executive secretary of the Convention on Biological Diversity, says: “We know that the drivers behind species loss are mostly increasing – land conversion and degradation, pollution, climate change. And of course the human population is still growing and consumption is growing – and most of that consumption is not sustainable.”

Professor Hubbell, too, thinks species loss is a serious issue, even though he believes it has been exaggerated.

There is, though, one other problem faced by anyone who wants to call attention to the issue – the fear that people are inclined to care more about so-called charismatic animals (mostly larger animals which we recognise) than the millions of nameless and microscopic organisms which are also included in species loss models.

Hubbell says we should be at least as concerned about such seemingly unimportant species.

“The proportion of the world’s species that are charismatic organisms is really tiny,” he says. “From a biomass point of view, this is a bacterial planet. It’s a very parochial view to assume that we should care only about elephants and zebras.”

But if people do care more about charismatic animals than bacteria, which seems likely, then it might prove difficult to get those people to take the issue seriously unless such animals are threatened.

A number of charismatic species, or sub-species, have become extinct in the wild, but have been kept alive in captivity thanks to the efforts of enthusiasts and campaigners.

Others have gone extinct – like the Pyrenean Ibex or the Baiji river dolphin. But compared to the number of species which exist in the world, even taking the lowest estimates of that number, such known cases are very few.

According to IUCN data, for example, only one animal has been definitely identified as having gone extinct since 2000. It was a mollusc.

Past and future decline and extinction of species

One of the largest effects of humans on the natural world has been to raise the rate of extinction of species far above natural levels. This began many thousands of years ago, and as a result the human-caused loss of global biodiversity was already significant before the modern era. Now, the extinction rate is accelerating, biodiversity is in rapid decline, and many ecosystem processes are being degraded or lost. 

1. History of human-caused extinctions

The effect of humans on global biodiversity first became significant as modern Homo sapiens migrated from Africa to occupy the other continents. Between about 60,000 and 10,000 years ago, a wave of extinctions of giant animals – mammoths, ground sloths, giant kangaroos, and many others – followed the arrival of people in Eurasia, Australia and the Americas. Probably, these megafauna disappeared because of hunting by humans.

Then, between about 5,000 and 500 years ago people discovered and settled oceanic islands. This resulted in extinction of whatever megafauna lived on those islands, such as New Zealand’s moa and Madagascar’s giant lemurs and elephant birds. As well, many smaller vertebrates succumbed to the combined pressures of hunting, forest removal, and impacts of alien species transported by voyaging people; presumably there were extinctions of other components of biodiversity as well, but these are not as well known. Because of the remarkable distinctiveness of biodiversity on islands this second wave of extinctions accounted for a great many species. For example, more than 100 endemic mammal species disappeared from the Caribbean islands alone, and human occupation of Pacific Islands resulted in extinction of at least 1,000 bird species, around 10% of all the world’s birds.

Since 1500 CE a third and still greater wave of extinction has been growing. This third wave is being driven ultimately by growth of the global human population, increased consumption of natural resources, and globalization. It is affecting a wider range of animals and plants than the preceding two extinction waves, in the oceans as well as on land. Our knowledge of which species have gone extinct since 1500 is collated in the IUCN Red List and is most complete for vertebrates, especially birds, mammals and amphibians: 711 vertebrates are known or presumed extinct since 1500, including 181 birds, 113 mammals and 171 amphibians. We know of almost 600 extinctions each of invertebrates and plants since 1500, but given limited basic knowledge, survey, and assessment of conservation status, the true magnitude of losses in these groups is certain to be far higher.

2. Accelerating extinction rates

The list of known recent extinctions is still only a small fraction of all species on the planet. For example, the tally of bird extinctions since 1500 amounts to 1.6% of all bird species that were living in 1500; the figures for mammals and amphibians are 1.9% and 2.1% respectively. What is more concerning than the raw numbers of extinctions is that they represent a rate of extinction far above pre-human levels. The extinction rate for any group of organisms is expressed as the number of extinctions that would occur each year among a million species (or equivalently, the number that would occur in a century among 10,000 species). Standardizing rates in this way allows comparison of extinction rates in different groups of organisms and time periods. Our best estimates suggest that extinction rates in the recent past have been running 100 or more times faster than in pre-human times, and that the pace of extinction has accelerated over the last few centuries (Figure 1). If this continues, the loss of species will soon amount to a large fraction of all species on the planet.

Figure 1. Estimated extinction rates in various animal groups through time, expressed as extinctions per million species per year. The height of each bar represents the range of estimates. Pre-human extinction rates are inferred from the fossil record, recent values from documented extinctions in selected groups, and near-future extinctions are projected from the current rates at which species are transitioning between IUCN categories.

There are two reasons to think that the extinction rate is about to rise still further. The first is that current levels of threat of extinction signal a steep increase in the number of extinctions over the next few decades.

In those groups of plants and animals that have been systematically assessed under IUCN Red List criteria, the proportions classified as threatened with extinction (that is, Critically Endangered, Endangered, or Vulnerable) are typically high, about 25% on average. This figure implies that a total of approximately one million of the world’s species are currently threatened with extinction. Five groups (mammals, birds, amphibians, corals, and cycads) have been comprehensively assessed two or more times since 1980. In all cases the reassessments show an increasing trend in the proportion of species that are threatened.

The most severe category of extinction risk is Critically Endangered. To qualify for this, a species must have some combination of very small total population (250 adults or fewer), extremely restricted distribution (10 km2 or less), and continuing population decline at rates high enough to guarantee extinction within decades. Currently 6,811 species are listed as Critically Endangered (of a total of 120,372 that have been formally assessed, a number still far short of the estimated two million-plus species so far described).

In short, the threat of extinction is now so widespread, and so many species stand on the brink of extinction, it is clear we could be about to lose many more. Even if the future brought nothing worse than the extinction of a significant proportion of all species now listed as Critically Endangered, that would amount to a very large increase in the total number of extinctions since 1500 CE. Because population size is still decreasing in most Critically Endangered species we must consider it likely that many of them will soon be gone. 

The near-future rate of extinction depends not just on current levels of threat, but on the speed with which now-threatened species decline all the way to extinction; it depends also on the rate at which species not currently threatened become so, and how quickly they then travel the full path to extinction. These dynamics of extinction risk are unknown for most groups, but they can be described for a few. The best-studied case is the worlds’ birds, illustrated in Figure 2. 

Between 1988 and 2016 a large cohort of bird species travelled all the way from being Near Threatened (that is, secure, but within sight of one of the markers of Vulnerable) to Critically Endangered (Figure 2). At the same time more than twice as many species joined the ranks of Near Threatened from Least Concern (that is, at minimal risk) and could soon follow the others on the path towards extinction. Another large cohort moved from Least Concern to Vulnerable or Endangered, having crossed the broad territory of Near Threatened in just a few years (Figure 2). The speed of these recent movements from low to high risk suggests that the number of bird extinctions is about to increase much more dramatically than might be suggested by the rather small rise in overall percentage of species listed as threatened (from 12.6% to 13.5% between 1988 and 2016).

Figure 2. Changes of Red List categories for bird species from 1988 to 2016. Numbers of taxa making each change are shown in the circles. Total birds in each category in 2016 are shown in parentheses; the total for extinctions is the number confirmed since 1500 CE. Data from the IUCN Red List, as compiled by Monroe et al 2019.

Recent studies of birds and mammals have used information like that shown in Figure 2 to estimate transition probabilities between IUCN Red List categories (in both directions, for the better or worse) and forecast rates of extinction over the coming decades. These studies suggest that extinction rates for birds and mammals are about to increase by more than tenfold (Figure 1). Similar accelerations in extinction are likely for other groups that are not as well-known; if anything, the increases could be even greater in many groups of organisms that are given less attention than birds and mammals and so are less likely to be helped by specific conservation actions.

3. Intensifying pressure on biodiversity

The second reason to anticipate a steep rise in extinction is that the forces that caused recent extinctions are as strong as ever. The main direct causes of extinction are loss and degradation of habitats due to human use of land and sea; overexploitation of wild populations; and the impacts on populations and ecological communities of invasive alien species, pollution, and climate change. These direct causes are driven ultimately by demographic, economic and societal factors that increase the pressures that human populations place on biodiversity. Most indicators of the direct and ultimate causes of biodiversity decline show that they are continuing to grow stronger. There have been some improvements—most notably, the expansion of protected areas since 2000 (from 10% to 15% of the land surface of the globe, and 3% to 7% of the oceans) and a recent fall in the (still substantial) global rate of deforestation —but they are too small to offset the general increase in pressure. 

There are several other reasons to think that the pressures that have caused extinction in the recent past will have worse effects in the future. Populations of many species are becoming smaller and more geographically restricted, whether or not those species yet qualify as threatened. This makes them more susceptible to threats they might have resisted when abundant and widespread, because for each population affected by some insult such as habitat loss or overexploitation there are fewer others to offset local declines and supply immigrants to replenish losses. Also, the general increase in human impact on nature makes it more likely that remaining natural areas are subject to several different threats at the same time, leading to compounding or synergistic effects with greater total impact. 

The future will also bring an increasingly important overlay of global climate change to the long-standing forces of habitat loss, overexploitation, and so on. The most significant effect of climate change may well be to increase the frequency or magnitude of extreme events. These include many that recent experience shows have great potential to damage biodiversity, such as intense tropical cyclones, marine heat waves, and El Niño and La Niña events. Extreme events that affect large areas can force large, abrupt and unexpected declines of many species at one time.

The climate-driven fires that recently burned much of southern Australia supply an illustration of such an extreme event. During the 2019/20 fire season, 97,000 km2 of southern Australian woodlands, forests and associated habitats were burnt by fires of exceptional intensity. The fires caused extreme damage to habitats that typically experience recurrent fires of lower intensity and extent, while consuming other habitats that normally do not burn at all. In the broad region affected by the fires, 243 vertebrate species or subspecies are listed as threatened. Fires overlapped significant portions (>10%) of the ranges of 46 of these threatened vertebrates. Some had most of their habitat burnt, for example 82% for the long-footed potoroo Potorous longipes and 98% for the Kangaroo Island dunnart Sminthopsis griseoventer aitkeni. Another 49 vertebrates not currently listed as threatened had 30% or more of their habitat burned, 100% in the case of Kate’s leaf-tailed gecko Saltuarius kateae. It is possible that many currently listed vertebrates will move into more severe threat categories, while re-assessment of previously secure species could see the total number of threatened vertebrates in Australia increase by 14%, as a result of this single event. At this stage there is less complete knowledge of impacts on other species, but rapid appraisals have identified 191 invertebrate and 486 plant species as potentially severely affected.

4. Loss of abundance

The figure of 25% of all species threatened with extinction is one measure of a more general decline of populations of wild species. Analysis of aggregated data on population trends of vertebrates from around the world indicates a general decline in abundance of 68% between 1970 and 2016, due both to extirpation of local populations and reduced numbers in those that remain. Roughly similar trends have occurred in most major regions of the world, in freshwater and dryland ecosystems, and in the oceans as well as on land. There is also growing evidence of widespread decline in the abundance of invertebrates, especially insects. This is best studied in Europe, where it is becoming clear that the abundance and diversity of arthropods is declining even in relatively undisturbed habitats, evidently because of spill-over effects from agricultural land use.

So far, conservation action has had little success in reversing the general decline in abundance of wild species. We have prevented some extinctions; for example, interventions between 1993 and 2020 prevented 21-32 bird and 7-16 mammal extinctions, such that extinction rates in both groups would otherwise have been 2.9-4.2 times higher. This is encouraging, but most improvements have been in moving species out of the Critically Endangered category into Endangered (Figure 2), that is, holding the line against extinction for some of the most severely threatened taxa. Few threatened species have recovered their original distribution and abundance against the much stronger tide running in the other direction (Figure 2).

Image caption: the Critically Endangered orange-bellied parrot Neophema chrysogaster by Tiana Pirtle. 

Not all species are being forced into decline; some are becoming more abundant as a result of human disturbance. In general, however, large-bodied and ecologically specialised species are more likely to decline, being replaced by less diverse sets of species that either tolerate disturbance or benefit from it and are capable of invading new or altered habitats. The result of this process is that a great part of the original diversity of nature is being lost from much of the planet. As this happens, ecological communities are being made simpler and some important ecosystem functions are degrading.

5. Species extinction and ecosystem decline

In the places that still have them, very large herbivores such as elephants and rhinos control the structure and pattern of vegetation, promote habitat heterogeneity, limit the extent of wildfire, transport nutrients, and disperse seeds. These various effects combine to promote diversity among smaller animal and plant species; megaherbivores could even influence the climate through alterations to land-surface albedo. The wholesale extinction of mega-herbivores many thousands of years ago damaged ecosystems and diminished biodiversity in ways that are only beginning to be understood. Big predators also sustain biodiversity and stabilize ecosystems by regulating populations of smaller predators and intermediate-sized herbivores. 

Among living mammals, amphibians, birds, reptiles and fish, the very largest species continue to be at highest risk of extinction: 59% of living megafauna are threatened, and 70% are decreasing in numbers. The threat of extinction is also exceptionally high among the smallest vertebrates. That is, human impact is deleting the smallest and largest vertebrates, and thereby confining survivors to a narrower size-range than was produced by evolution. 

This is one instance of a more widespread phenomenon, in which human impact reduces the diversity and range of traits of organisms in natural assemblages. The general result is a reduction of functional diversity as well as total numbers of species, simplification of ecosystems, and in consequence loss or destabilisation of important ecosystem functions. So, for example in forest and woodland ecosystems the loss of mega-herbivores can result in higher incidence of destructive wildfire; in the oceans the loss of large species is reducing connectivity among ecosystems and thereby making them unstable; and declining diversity of insects diminishes many essential ecosystem functions such as pollination and nutrient re-cycling.

6. Conclusions

Our knowledge of past and future extinction rates makes it clear that the problem of extinction is urgent. The problem has two main components. First, the extinction of species is reducing the total diversity of life on the planet. Although the extinction tally does not yet represent a large proportion of all species, it is substantial and ecologically important. The rate of extinction is rising fast, and on current trends a large fraction of all the world’s species could soon be gone. Second, the combination of extinction of some species and declining abundance of many others is causing a general loss of abundance and diversity of wild species and compromising the functioning of ecosystems.

The highest priority for action should be the prevention of extinction, because the extinction of any species is an irredeemable loss. Science-based interventions have a good record in saving species from extinction and recent losses would have been significantly worse without them. The actions that have been most consistently successful are establishment of protected areas, habitat restoration, and intensive management of small populations including reintroduction. The reason that we have not seen more success is primarily that investment in and resourcing of species conservation is in general far too low. 

The problem of preventing broader decline of wild species requires more complex solutions, based on retention of large areas of intact habitat together with rewilding of degraded areas, improvements in the sustainability of exploitation of wild populations, development of strategies for reducing the impacts of invasive species at landscape scales, and mitigation of climate change. Accomplishing these changes will require transformations of the relationship of human communities to nature that will depend on the application of science, development of new socio-economic and governance models, and restoration of indigenous knowledge and practices of environmental management.

Resources

livescience.com, “What’s the first species humans drove to extinction?” By Emma Bryce; ourworldindata.org, “Extinctions.” By Hannah Ritchie and Max Roser; theatlantic.com, “Wait, Have We Really Wiped Out 60 Percent of Animals? The findings of a major new report have been widely mischaracterized—although the actual news is still grim.” By Ed Yong; bbc.com, “Biodiversity loss: How accurate are the numbers?” By Richard Knight; royalsociety.org, “Past and future decline and extinction of species.” By Christopher N. Johnson;

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