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The Sixth Mass Extinction: Why Species are Disappearing 1000x Faster
Right now, life on our beautiful planet seems normal and calm. The trees are green and blossoming. The birds are flocking on branches and power lines. The fish are jumping in the rivers and creeks. The summers are hot, and the winters are cold. Surely, we have it good, and it cannot possibly get better, right? Not exactly. We are in the middle of a cataclysmic event heralded as the sixth mass extinction, and although you may not notice the effects, species are disappearing 1000 times faster than normal.
So why is this happening, and why should you care? Well, in the past few centuries alone, hundreds of species have disappeared worldwide, and that should be a cause for alarm. When the asteroid struck at the end of the Cretaceous, the dinosaurs did not vanish right away. They steadily died out over the course of 30,000 years. Since modern humans have only been around for about 50,000 years, 30,000 years may seem like a long time, but on the geological timescale, they are milliseconds. Thus, if you compare today's extinction rate (where a dozen species can go extinct in one day) to past extinction rates, our current extinction rate has reached unprecedented levels. Let's analyze this ecological disaster in greater detail.
Once common on the US eastern seaboard, the Carolina parakeet disappeared in 1918 due to deforestation.
"The Big Five" Mass Extinctions
Let's turn back the clock a bit and look at previous extinction events. Throughout Earth's 4.6 billion years of existence, extinctions have been a norm. Like the changing of seasons, organisms have appeared and disappeared. When a lake dries up, species disappear. When a volcano erupts, species disappear. When an ice age ends, species disappear. Extinctions are as natural as the transition from fall to winter.
Yet, on occasion, extinctions occur on a massive, global scale, and they are called mass extinctions. So far, there have been five mass extinctions, colloquially known as "the Big Five". Here's a little background on each:
1.) The Ordovician-Silurian Extinction
This was Earth's first mass extinction and the second most devastating. Occurring between 450 and 440 million years ago, 27% of all families, 57% of all genera and 60% to 70% of all species went extinct.
2.) The Late Devonian Extinction
This was the second mass extinction, and it lasted 20 million years. 70% of all species perished during this event. Notably, the a class of armored fish called placoderms disappeared from the world's oceans.
3.) The Permian-Triassic Extinction
Known as "the Great Dying", this was the world's most devastating mass extinction. Compared to this, the extinction of the dinosaurs doesn't even come close. Between 90% and 96% of all species died out, including the trilobites, which had dominated the world's oceans for over 270 million years. As a comparison, dinosaurs ruled the Earth for 165 million years, so the reign (and overthrow) of the trilobites was nothing to scoff at.
4.) The Triassic-Jurassic Extinction
In this event, most of the archosaurs died out in an extinction that killed off 70% to 75% of all species, paving the way for the dinosaurs to dominate the planet.
5.) The Cretaceous-Paleogene Extinction
The most famous (and most recent) of all mass extinctions is the Cretaceous-Paleogene extinction. This extinction event wiped out all non-avian dinosaurs, from Tyrannosaurus to Triceratops. It also eliminated the flying reptiles like the pterosaurs and the marine reptiles like the mosasaurs. Overall, the event caused the extinction of 75% of the world's species.
Humans Enter the Picture
Between the Cretaceous-Paleogene extinction and the rise of humanity, several more extinctions occurred, but they were nowhere near the scale of previous mass extinctions. Once humans gained a foothold, everything changed. Fueled by their big brains and adaptable bodies, humans took the powers of natural selection into their own hands. Never before had a single species so rapidly adapted to the land, the sea, and the air. Never before had a species developed methods of preventing and curing illness and disease. Our adaptations and impact on the planet have been so far-reaching that some scientists and activists have proposed a new epoch: the Anthropocene.
Cro-Magnon artists painting mammoths in Font-de-Gaume by Charles R. Knight, 1920
The Extinction of Pleistocene Megafauna
Despite humanity's triumphs over natural selection, other species fell victim to its throes. When humans migrated out of Africa, they hunted down the massive prehistoric beasts that roamed the ancient lands. Not long after humans expanded, the megafauna began to disappear.
Some scientists speculate that humans' hunting wiped out the Pleistocene megafauna. Yet, others speculate that the changing climate and changing landscapes had a more dramatic effect. Whatever the cause, the majority of megafauna disappeared between the Pleistocene and Holocene epochs. 10,000 years ago, you could see elephants and rhinoceroses on every continent except Australia and Antarctica. Today, Africa and Southeast Asia are their last bastions.
Ice Age megafauna in prehistoric Spain.
The Collapse of Ecosystems on Fragile Islands
Once humans began building boats, they started crossing oceans and expanding onto islands. This spelled bad news for the native inhabitants. Small and isolated from the rest of the world, islands harbor highly specialized species that have evolved separately from those on large continental landmasses. After all, the unique species on the Galapagos Islands inspired Charles Darwin's theory of evolution.
However, this isolation and specialization led to many island species' downfall. Island ecosystems are like eggshells: they are fragile and can very easily crack under pressure. On islands around the world, tortoises and flightless birds are common residents, and as you might suspect, these animals are not well-equipped to deal with hunting pressures or predation.
Less than 100 years after humans arrived in Mauritius, the dodo disappeared.
The tale of the ill-fated dodo is one of the most infamous extinctions. First encountered by Europeans on Mauritius in 1598, the dodo was flightless and readily approached humans. This made the dodo easy prey for hunters and introduced species such as cats and rodents. Less than 100 years after humans arrived on Mauritius, the dodo went extinct.
Yet, the dodo was not alone. After the arrival of humans on various islands around the world, other native species disappeared for good. Here are a few examples:
While wooly mammoths went extinct on all the world's continents around 10,000 years ago, they existed on Wrangel Island until 4000 years ago. Their disappearance coincided with the appearance of humans on the island.
Of all island ecosystems affected by human colonization, New Zealand was among the most devastated. Once populated by giant, flightless birds called moas and their equally large predators known as the Haast's eagle, New Zealand lost both the moas and the eagles less than 200 years after the Māori arrived. Even today, New Zealand is still reeling from the effects, which were worsened when European colonists cleared much of the native forests for farmland and introduced predatory animals such as cats and rodents.
Even more devastated by human colonization, Hawaii has been dubbed "the extinction capital of the world". Over half of Hawaii's historically recorded bird species are extinct. Like New Zealand, Hawaii's native species suffered from the introduction of cats and rodents, and they are still suffering today.
Today, Madagascar is known for tiny chameleons and cute lemurs, but long ago, it was also home to gorilla-sized lemurs, cougar-sized fossas, terrestrial crocodiles, and gigantic flightless birds--not unlike New Zealand's moas--called elephant birds. Once humans arrived on Madagascar around 2000 years ago, all of these island giants disappeared.
The moa disappeared less than 200 years after the Polynesians arrived in New Zealand.
In comparison to the rest of the world, the devastation and collapse of fragile island ecosystems is only a microcosm. The loss of a dodo or a giant lemur does not affect the global environment, but it does demonstrate what can (and is) happening on a grander scale. Even today, introduced species like crazy ants on Christmas Island and brown tree snakes on Guam are ravaging islands and causing even more extinctions.
Causes of the Sixth Mass Extinction
Now, with this brief history of extinctions and humanity fresh in your mind, let's consider the causes for the Sixth Mass Extinction. Do you think humans have played a significant role in the demise of species? Do you think we will lose more species in the future? Well, let's examine the roots behind this issue.
Until the mid-19th Century, people did not hold the preservation of species in high regard. In fact, nobody even believed extinction was possible until the early 19th Century when Georges Cuvier proposed that species can and do go extinct. Mass extinctions were similarly derided. Even Charles Darwin himself scoffed at the idea, believing all extinctions were gradual (and most were). Yet, here we are amid the Sixth Mass Extinction.
A pile of American bison skulls ready to be ground into fertilizer in the 1870s.
Overhunting and Overfishing
Whether or not humans hunted the Pleistocene megafauna to extinction, we are definitely responsible for overhunting and overfishing species in more recent millennia, especially in the past 4000 years. With the invention of weapons and the advent of commercial fisheries, we developed more effective methods for gathering food and resources, and the Industrial Revolution only exacerbated this. Birds were blasted. Fish were gutted. Bison were butchered. Whales were harpooned. Wolves were slaughtered.
Today, there are more protections in place, but in the less developed parts of the world, the process continues. In Africa and Asia, the last remnants of the megafauna are being decimated. In all oceans, irresponsible commercial fisheries are wiping out fish stocks and ensnaring unwanted marine life. In China, various animals are being collected and abused for medicinal purposes.
After decades of overhunting in the early 1900s, the Japanese sea lion went extinct in the 1970s.
Introduced and Invasive Species
Before humans, most animals had no means of crossing oceans. Sure, some could fly, some could cross ice or land bridges, and some could hitch a ride on a piece of driftwood, but those cases were few and far in between. When people started building boats and ships, not only did they carry their livestock and pets, but they also carried stowaways and hitchhikers such as rats and fire ants.
In many cases, the extinction of island species can be attributed to introduced species, rather than the humans themselves. While humans did hunt the dodo, the domestic animals they brought with them were the dodos' ultimate killers. Dogs, cats, pigs, and other animals raided the dodos' nests and competed for food.
Worse yet, some invasive species were intentionally introduced. In Australia in October 1859, an English settler released 24 rabbits for hunting purposes. By the 1920s, there were over 10 billion rabbits in Australia. Since their introduction, the invasive Australian feral rabbits have been eating seedlings, preventing native plants from germinating.
Originating from South America, the invasive red imported fire ant (RIFA) plagues many countries.
When a species' population rises, and predators are amiss, disease act as a natural form of population control. Yet, just like deadly, infectious diseases can be transmitted between humans in different countries, they can also be transmitted between species in different continents thanks to human transportation. Right now, we are experiencing massive amphibian die-offs, and diseases are partially to blame. One disease in particular called Chytridiomycosis has had a dramatic effect on amphibian populations worldwide. Seemingly overnight, various frog and toad species have vanished from their native habitats. The crisis is so severe that naturalists are collecting species from the wild and keeping them in captivity in an "ark" of sorts.
In 1987, 1500 golden toads were counted in Costa Rica. On May 15, 1989, only one male was found (pictured).
Habitat Destruction and Fragmentation
Habitat destruction and fragmentation has been ongoing since the first forests were cleared, the first dams were built, and the first cities were founded. Today, with the propagation of roads and metropolitan areas, it is worse than ever. Lizards crossing roads are flattened into roadkill, and bears shopping for food venture into suburban neighborhoods. With less and less land available, many species are experiencing population reductions.
While common and adaptable, deer suffer from habitat fragmentation, often leaping across busy roads.
Since the Industrial Revolution, harmful CO2 has been unleashed into the atmosphere via fossil fuels. Today, there is debate over whether or not climate change is real or not. Yet, despite what some people might claim, scientists have proven that climate change is happening, and global temperatures are rising at an ever increasing rate. NASA has an excellent time machine map that demonstrates the changes in temperature, sea ice, and CO2 levels over the past century.
This spike in climate is warming the oceans, and many species will suffer. Coral are among the most vulnerable, since they require very specific water temperatures and do not grow or reproduce quickly. If this trend continues, the world's coral reefs will be reduced to lifeless rock.
Due to disease and temperature rise, staghorn coral populations have declined by 98%.
Ocean acidification is, by far, one of the most alarming and wide-reaching issues, and it ties into climate change and CO2 emissions. Life on this planet started the seas, and now, the increasingly acidic seas are choking it. 30-40% of carbon dioxide emissions released into the atmosphere dissolve in the oceans, decreasing their pH and increasing their acidity. This is already affecting calcifying organisms such as coral and some plankton (in addition to the warming waters).
Cephalopods like this reef cuttlefish are highly susceptible to ocean acidification.
This one is a no-brainer. Of course, pollution is a factor in the Sixth Mass Extinction. With all of our fossil fuels and products that are "built to last", we did not think about the environmental repercussions. In April 2010, the Deepwater Horizon oil spill caused tremendous damage to underwater ecosystems in the Gulf of Mexico, which has yet to recover. Similarly, the Great Pacific Garbage Patch harbors plastics and other trash. These plastics are eaten by albatrosses and other animals, and the animals subsequently die.
An industrial port on Virginia's James River. Ports are rife with pollution.
Of all threats to the natural world, human overpopulation overshadows them all. Even if everyone on Earth drove vehicles with zero emissions and all power was clean and renewable, we'd still need to address population concerns. Since the year 1804, not long after the start of Industrial Revolution, our population has swelled from 1 billion to 7 billion. Over 2 billion people reside in China and India alone. The United States is a distant third with just over 321 million people.
By the year 2040, we are expected to have a population of 9 billion. With less space for arable land and declining fish stocks, one could argue that we are already overpopulated. And, with another 2 million mouths to feed over the next couple decades, the situation will only deteriorate.
Downtown San Diego, California, USA. City populations worldwide have been climbing.
These issues are just the tip of the melting iceberg. As global climates change, populations rise, and unsustainable hunting and fishing practices continue, the Sixth Mass Extinction will claim even more species. Even if we stopped everything we're doing today and switched to more environmentally conscious practices, we could not halt all the negative effects. The damage has been done, and some of the effects are irreversible.
Yet, it's not all fire and brimstone. Thanks to our current conservation efforts, many species have been saved, and many more are being protected. Species that have become rare or extinct in the wild are generally housed in zoos and aquariums, where they can be bred and reintroduced into the natural habitat.
Even for these deceased taxons, there is hope. With modern technology, we have the power to perform de-extinction, where we use cloning, selective breeding, and other techniques to resurrect extinct species. As Ian Malcom said in Jurassic Park "Life, uh, finds a way".
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