You may have read in the news recently of a new study concluding that the extinction of animal species is more severe than we originally thought. The study found that of the over 150,000 species studied, 28% are at risk of extinction. Almost half of all species studied are experiencing population decline, with only 3% experiencing population growth. Even among species not considered endangered, 33% are experiencing population decline. We’ve known for a while that human activities are having a detrimental effect on the global ecosystem, but these statistics show it’s a much worse situation than originally thought. You may have even read the phrase ‘sixth mass extinction’ in some of these articles. So today, let’s talk about the concept of mass extinctions and the extent to which we are within one.
The Big Five
A mass extinction is defined as a rapid and widespread decrease in the Earth’s biodiversity. There have been numerous instances throughout Earth’s history where a significant portion of the planet’s species died out in a short period of time. It's also important to note that the definition of what counts as a mass extinction is fairly arbitrary. In 1982, a paper by Jack Sepkoski and David Raup cataloged all marine fossil species and identified five periods where the planet’s biodiversity dropped precipitously, therefore these events have since been labeled the Big Five mass extinctions. However, determining biodiversity loss from the fossil record is extremely difficult due to how incomplete the fossil record is. Some time periods are better cataloged than others due to there being more fossil sites. More fossils makes it easier to determine the number of species which makes it easier to get a proper sample size for determining biodiversity. In recent decades techniques for analyzing biodiversity from the fossil record have improved, so other extinction events have been identified that come close to and perhaps surpass the big five, so the Big Five is more of a legacy concept than the result of a hard definition of mass extinction. And even if we could settle on a single threshold that makes an event count as a mass extinction, there would still be numerous smaller extinction events worth studying. But regardless of how messy our definitions are, the big five are still a useful sample for studying mass extinctions as they do include the biggest extinction events in Earth’s history.
Timeline of Earth’s History to Provide Scale
You’ve almost certainly heard of the most recent mass extinction event. The Cretaceous-Paleogene extinction event occurred roughly 66 million years ago and resulted in the extinction of 75% of all species, including all (non-avian) dinosaurs. The widely supported theory* is that an asteroid the size of Mt Everest hit the Earth at 20 kilometers per second (Mach 50). We believe it happened like this because all around the world, wherever there is rock old enough to contain dinosaur fossils, paleontologists have found a thin layer of sediment called the K-Pg boundary. Every dinosaur fossil ever found has been found below this boundary, meaning there were no dinosaur fossils ‘created’ after this layer of sediment was layed down. Everywhere it is found, this boundary consists of sediment rich in iridium, an element common in asteroids. The smoking gun for this theory was discovered in 1991; a ten kilometer (six mile) wide crater submerged off the coast of the Yucatan peninsula dated to be as old as the K-Pg boundary. Mathematical models of asteroid impacts show what happened next; the force of this impact would have created a fireball the size of a continent, supersonic winds, tsunamis, and earthquakes. The debris of the impact would then have rained down on the Earth over the course of several hours, igniting global wildfires. Dust and ash would then have filled the upper atmosphere, blocking sunlight and thus photosynthesis for over two years. (Video from Kurzgesagt for more details) Most species over 25 kilograms (55 pounds) died out, with the species that survived being small animals that reproduced quickly. While mammals had existed for as long as the dinosaurs, they consisted of small rodent-like species that didn’t compete directly with dinosaurs but were able to weather the mass extinction. The dinosaurs had existed for almost 200 million years up to this point (almost four times as long as they’ve been extinct) and were one of the most successful clades of animals in Earth’s history; so the degree to which this impact altered the Earth’s biosphere shouldn’t be understated.
Another mass extinction you might have heard of is the Late Permian extinction event, often called the Great Dying. It’s a fitting name; with over 90% of all species going extinct, it is the single largest mass extinction event in Earth’s history. It occurred roughly 252 million years ago, by which point reptiles had evolved and the earliest ancestors of mammals had begun to differentiate themselves. Unlike the Cretaceous extinction event, the Permian extinction is believed to have occurred rather slowly over around 60,000 years (for perspective, what we call human civilization is only about 12,000 years old). While there is more mystery surrounding this one, the leading theory is an eruption of the Siberian Traps. Traps are regions where the ground is primarily igneous rock, i.e. dried lava, and the Siberian Traps are one such region in Siberia that is the size of Western Europe. 252 million years ago, this entire region would have been covered in lava due to the sheer amount of volcanic activity. These kinds of massive, sustained eruption events have occurred elsewhere in Earth’s history, though what triggered this one is still being studied. Just to add insult to injury, this region would have been home to massive coal deposits**, so the erupting volcanoes would have burned several times more coal than the human race has since the Industrial Revolution. The carbon dioxide added to the atmosphere by these volcanoes and coal would have raised global temperatures by between 8 and 14 °C (to compare, the absolute worst case scenario for human-made climate change is only 4-5 °C). The chemicals ejected into the atmosphere would have destroyed the ozone layer and caused enough acid rain to significantly acidify the oceans. It’s hard to determine exactly what else happened due to how far back this all was, but possibilities include the warming climate melting undersea ice that trapped significant amounts of methane, further warming the planet, and the acidified ocean causing chemical reactions that deoxygenate the water, creating a perfect environment for bacteria that produce hydrogen sulfide. There are still other theories as to how this happened as we still don’t have enough surviving evidence to tell the full story. And it’s worth noting that Earth’s biodiversity was already lower than normal before the Great Dying; a smaller extinction event occurred 10 million years earlier, one that is now being debated to be an equal event to the Big Five instead of the first phase to the Permian extinction. But according to the fossil record, it took over 10 million years for Earth’s biosphere to completely recover, the longest recovery from any such extinction event.
I mention these historic disasters to showcase what extinction events tend to look like. They can be triggered by asteroid impacts or extreme volcanic activity, but these events then trigger or worsen other disasters such as climate change, sea level rise or fall, and changes to the atmosphere and ocean chemistry in a massive cascading failure of numerous systems. They happen very quickly on a geologic timescale, less than a million years, so species cannot adapt to the new environment. But they can still take a long time on a human timescale; unless you were in Siberia during the Permian extinction, you might not notice how bad the situation was unless you were taking measurements over the course of centuries. But above all, they represent significant shifts in the Earth’s geologic history that can be measured and observed for eons to come.
Sixth Mass Extinction
Geologists have split Earth’s timeline into standardized subdivisions in order to better catalog and conceptualize these massive stretches of time. (See image above) We currently live in the Holocene Epoch, roughly translated as “Entirely Recent Epoch.” This refers to the period of roughly 12,000 years ago (the end of the last ice age) to the present. In recent years, there has been a push by some scientists and academics to rename this period (or at least part of it) the Anthropocene Epoch, translating as “Human Epoch.” This would be done out of acknowledgement that the presence of the human species is the defining aspect of this epoch. This video by Be Smart lists many of the artifacts humanity will leave behind that will show a future geologist or paleontologist that we were here, even hundreds of millions of years after our extinction. We have had one of the biggest effects on the Earth’s biosphere of any organism in history, and it has not been for the better.
The environmental impact human civilization has had on the planet is multifaceted and all-encompassing. The most obvious effect is certainly climate change. While the amount of carbon dioxide we’ve added to the atmosphere is far dwarfed by that emitted during the Permian extinction, we’re emitting our CO2 about 200 times faster. Since the big killer in mass extinctions is the world changing faster than organisms can adapt, this can’t be good. Terrestrial habitats can be made unsuitable for species via warming, drought, or storms. Glacier loss results in changes to river flow, temperature and salinity which affects river and wetland habitats. Warmer, wetter environments can cause an increase in disease-causing species, such as the chytrid fungus that affects frog species. The oceans absorb carbon dioxide from the atmosphere and become more acidic, which lowers oxygen levels and inhibits shell formation in shell-making species. On top of climate change, about 70% of all the land on Earth has experienced some amount of land degradation with 30% being severely degraded. Land degradation can be the result of deforestation, urban sprawl, mining, livestock overgrazing, overuse of groundwater, improper farming that depletes soil nutrients, and desertification caused by climate change. Not only does this damage habitats, but the habitat left behind can be non-contiguous and too small to sustain larger animals. Species that humans keep around (pets, livestock, crops, even pests) have seen their habitats radically expanded around the world. These human-associated species can displace local species by hunting them or by competing with them for space, thus harming overall biodiversity. I could continue listing specific ways that wildlife have been put in danger by human activity for a while so I’ll stop for now, but do notice how all of these numerous ways species are dying out are the downstream results of only a few actions (CO2 pollution, urban development, agricultural development, etc).
Since the 1500s, 680 species of vertebrates have been driven to extinction due to human actions. This is just the extinctions we know about; scientists are continuing to discover new species to this day, so it stands to reason some could go extinct without us ever noticing they were there. The rate of species extinction for the past century is 100 times higher than the average extinction rate of the past 65 million years. The number of extinctions that occurred in the past century would’ve taken between 800 and 10,000 years to occur before now. A UN report shows that over a million plant and animal species are at severe risk of going extinct in the next few decades, out of the 8.7 million species believed to currently exist. The severity of this crisis comes in part from how extinction begets extinction due to the interconnectedness of ecosystems. When one species goes extinct, every species that preyed on them is affected. If those predators only ate the extinct species, they would go extinct too. If those predators had other prey, those prey species will see population decline due to being hunted more. Whatever species the extinct species ate or preyed on will see their population explode, which would cause declines in the species that were eaten by those former prey or which competed with them for space. And every species whose population waxes or wanes due to that initial extinction will cause downstream effects for their predators, food sources, competitors, and symbiotic partners. While ecosystems can find a state of equilibrium again with enough time, multiple extinctions in a short period of time make it more likely that more species will be knocked off the rocking boat. And humans can be affected by these extinctions. For example, studies have suggested the extinction of the passenger pigeon in 1911 may have significantly increased the prevalence of lyme disease. These pigeons ate the same seeds as the white-footed mouse, whose population has now exploded and which is a significant carrier of lyme disease.
Among those scientists who designate our current era as the sixth mass extinction, there is debate as to where we are currently within it. Start dates for this mass extinction range from the end of the ice age when the earliest humans over hunted already endangered species to the industrial revolution when climate change and mass urbanization really began to change the planet. Therefore, it’s debated if we are currently at the very beginning or in the middle of such an extinction event.The choice to designate our current situation the Sixth Mass Extinction is more of an argument of semantics and the definition of mass extinctions; these events will continue to transpire regardless of what we chose to name them. But the fact that extinction rates have risen dramatically and will continue to rise even further is not up for debate. This situation isn’t completely hopeless, though fixing it will require significant effort. Over 100 nations have signed on to the 30 by 30 initiative, a plan to designate 30% of the Earth’s land and oceans to conservation by 2030. Conservation biologists continue to identify the endangered species whose extinction would cause the most downstream effects in order to prioritize their conservation. Smaller actions we could take include making fisheries more sustainable and using agricultural methods which allow animals to move through farm and ranchland. There are larger things that need to happen too; getting a handle on climate change will still be necessary and it’s questionable if the 30 by 30 initiative will be enough to guarantee enough biodiversity is protected to ensure long-term stability. Figuring out what changes would need to be made economically and politically to ensure these outcomes is outside my purview to write about, so I won’t go down that rabbit hole. So instead, I will point out the biggest difference between the sixth mass extinction and the previous five; the Chicxulub asteroid and the Siberian Traps volcanoes did not need a healthy biosphere in order to continue existing. We humans are far more dependent on our planet’s ecological health than it is on us, so preventing a mass extinction is within our best interests.
For More Details
*I feel I should mention the Deccan Traps here, a region of extreme volcanic activity that was erupting around the same time as the Chicxulub impact. While the asteroid impact is universally agreed to be the primary cause of the Cretaceous Extinction, the fact that this volcanic activity was occurring at the same time is noteworthy to scientists. Many have suggested both caused the mass extinction, that neither alone would have been as severe as both together. It’s also been suggested that since the Deccan Traps are nearly on the other side of the planet to the Chicxulub crater, it might be possible the asteroid impact was severe enough to induce the volcanic eruptions or to at least significantly worsen them.
**The creation of these coal deposits is an interesting story in-and-of itself. By 350 million years ago, the first large trees and forests had emerged, made possible by a protein called lignin that made up bark. But lignin was so tough that it would take millions more years for bacteria and fungi to evolve that could break lignin down, meaning that trees couldn’t rot for almost 100 million years. Instead, dead trees fell over and were over time buried until they fossilized into coal. Almost all coal originated from this time period . The fact that so much CO2 was being pulled out of the atmosphere and trapped underground caused a 100 million year long ice age and an atmospheric oxygen concentration that was 50% higher than it is today. If you’ve ever seen images of long-extinct giant insects, they came from this time period, able to grow so large due to the abundance of oxygen.
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