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 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.
These are shown in the chart. In more niche taxonomic groups — such as horseshoe crabs and gymnosperms, most species are threatened. When and why did they happen? Extinctions are a normal part of evolution: they occur naturally and periodically over time.
Evolution occurs through the balance of extinction — the end of species — and speciation — the creation of new ones. We can therefore identify periods of history when extinctions were happening much faster than this background rate — this would tell us that there was an additional environmental or ecological pressure creating more extinctions than we would expect. But mass extinctions are defined as periods with much higher extinction rates than normal.
They are defined by both magnitude and rate. Magnitude is the percentage of species that are lost. Rate is how quickly this happens. These metrics are inevitably linked, but we need both to qualify as a mass extinction. At least, since million years ago; we know very little about extinction events in the Precambrian and early Cambrian earlier which predates this. Again, note that this number was never zero: background rates of extinction were low — typically less than 5 families per million years — but ever-present through time.
Finally, at the end of the timeline we have the question of what is to come. Perhaps we are headed for a sixth mass extinction. But we are currently far from that point. There are a range of trajectories that the extinction rate could take in the decades and centuries to follow; which one we follow is determined by us. In the table here I detail the proposed causes for each of the five extinction events.
Some think this new epoch should start at the Industrial Revolution, some at the advent of agriculture 10, to 15, years ago. This feeds into the popular notion that environmental destruction is a recent phenomenon. The lives of our hunter-gatherer ancestors are instead romanticized. Many think they lived in balance with nature, unlike modern society where we fight against it.
The extent of these extinctions across continents is shown in the chart. There is strong evidence to suggest that these were primarily driven by humans — we look at this in more detail later. Humans evolved in Africa, and hominins had already been interacting with mammals for a long time.
But Australia, North America and South America were particularly hard-hit; very soon after humans arrived, most large mammals were gone. Far from being in balance with ecosystems, very small populations of hunter-gatherers changed them forever. A few million killed off hundreds of species that we will never get back. The driver of the QME has been debated for centuries. Debate has been centered around how much was caused by humans and how much by changes in climate. Today the consensus is that most of these extinctions were caused by humans.
Extinction timings closely match the timing of human arrival. The timing of megafauna extinctions were not consistent across the world; instead, the timing of their demise coincided closely with the arrival of humans on each continent.
The timing of human arrivals and extinction events is shown on the map. Humans reached Australia somewhere between 65 to 44, years ago. It was tens of thousands of years before the extinctions in North and South America occurred.
And several more before these occurred in Madagascar and the Caribbean islands. Elephant birds in Madagascar were still present eight millennia after the mammoth and mastodon were killed off in America. Significant climatic changes tend to be felt globally.
If these extinction were solely due to climate we would expect them to occur at a similar time across the continents. QME selectively impacted large mammals. There have been five big mass extinction events, and a number of smaller ones. Large ecological changes tend to impact everything from large to small mammals, reptiles, birds, and fish.
The QME was different and unique in the fossil record: it selectively killed off large mammals. This suggests a strong influence from humans since we selectively hunt larger ones. There are several reasons why large mammals in particular have been at greater risk since the arrival of humans. Islands were more heavily impacted than Africa. As we saw previously, Africa was less-heavily impacted than other continents during this period. We would expect this since hominids had been interacting with mammals for a long time before this.
These interactions between species would have impacted mammal populations more gradually and to a lesser extent. They may have already reached some form of equilibrium. When humans arrived on other continents — such as Australia or the Americas — these interactions were new and represented a step-change in the dynamics of the ecosystem. Humans were an efficient new predator. There has now been many studies focused on the question of whether humans were the key driver of the QME.
The consensus is yes. Our hunter-gatherer ancestors were key to the demise of these megafauna. Human impact on ecosystems therefore date back tens of thousands of years, despite the Anthropocene paradigm that is this a recent phenomenon.
Seeing wildlife populations shrink is devastating. But the extinction of an entire species is tragedy on another level. A complex life form that is lost forever. But extinctions are nothing new. What worries ecologists is that extinctions today are happening much faster than nature would predict. Are we in the midst of another one? Is this really true, or are these claims overblown? 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. The rate is too slow. The magnitude is too low. To be defined as a mass extinction, the planet needs to lose a lot of its species quickly.
Some mass extinctions happen more quickly than this. 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 ].
There are a few things that make this difficult. 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. 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. Calculate extinction rates for the past years or years, or 50 years and ask whether extinction rates during previous periods were as high.
An obvious question to ask is how many species have gone extinct already. Since around 0. 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. Due to the growing illegal trade of the Roti Island snake-necked turtle, the species — endemic to eastern Indonesia — is close to extinction.
Once the most widespread of Asian rhinoceroses, the Javan Rhinoceros ranged from the islands of Indonesia, throughout Southeast Asia, and into India and China. The species is now critically endangered, with only two known populations in the wild, and none in zoos. It is possibly the rarest large mammal on earth. How does biodiversity loss affect me and everyone else?
So what exactly is causing this loss? What can I do? This just seems so BIG! But recent studies have cited extinction rates that are extremely fuzzy and vary wildly. The Millennium Ecosystem Assessment, which involved more than a thousand experts, estimated an extinction rate that was later calculated at up to 8, species a year, or 24 a day. More recently, scientists at the U. But nobody knows whether such estimates are anywhere close to reality. They are based on computer modeling, and documented losses are tiny by comparison.
Only about extinctions have been documented in the past years, according to data held by the International Union for the Conservation of Nature IUCN. Out of some 1. Nor is there much documented evidence of accelerating loss. Helena and a Malaysian snail. Moreover, the majority of documented extinctions have been on small islands, where species with small gene pools have usually succumbed to human hunters.
That may be an ecological tragedy for the islands concerned, but most species live in continental areas and, ecologists agree, are unlikely to prove so vulnerable. But the documented losses may be only the tip of the iceberg. One way to fill the gap is by extrapolating from the known to the unknown. Ceballos looked at the recorded loss since of species of vertebrates.
That represented a loss since the start of the 20th century of around 1 percent of the 45, known vertebrate species. This background rate would predict around nine extinctions of vertebrates in the past century, when the actual total was between one and two orders of magnitude higher. Regnier looked at one group of invertebrates with comparatively good records — land snails. Thus, she figured that Amastra baldwiniana , a land snail endemic to the Hawaiian island of Maui, was no more because its habitat has declined and it has not been seen for several decades.
In this way, she estimated that probably 10 percent of the or so known land snails were now extinct — a loss seven times greater than IUCN records indicate. Several leading analysts applauded the estimation technique used by Regnier. But others have been more cautious about reading across taxa. They say it is dangerous to assume that other invertebrates are suffering extinctions at a similar rate to land snails.
Mark Costello, a marine biologist of the University of Auckland in New Zealand, warned that land snails may be at greater risk than insects, which make up the majority of invertebrates.
The same should apply to marine species that can swim the oceans, says Alex Rogers of Oxford University.
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