Extinction is the process by which a species, genus, or family, becomes extinct — no longer existing and living in the world. It is the abolition and annihilation of something that previously existed in the world. In the case of biology, it refers specifically to the end of an evolutionary line, or a branch on the tree of life.

The actual moment of extinction is considered to be when the last individual representative of a species or group is no longer living. But functional extinction can occur considerably earlier than that — as the result of loss of genetic diversity, range, and/or the ability for a population to breed and recover.

Most types of life, especially animals, are closely tied to their ecological niches and environments. With a loss of their living environment, and its accompanying species, extinction is almost inevitable for many types of life. Species diversification and emergence typically doesn’t occur in these circumstances, it usually happens within healthy ecosystems. The long-period of time that follows large extinction events when no new species emerge is referred to as a dead zone .

Mass extinctions are events in which a very significant percentage of the species on the planet go extinct. There are have been at least 5 in the history of the Earth so far, and it has been well argued that we are currently in the midst of a 6th mass extinction event, the Holocene extinction, or alternately, the Anthropocene extinction. This extinction event, which began towards the end of the last ice-age, has resulted in the extinction of nearly all forms of the large mammals known as megafauna, with the remaining species almost all threatened with extinction in the near-future. Given the close association between the extinction dates of various species, and first contact with humans, it is very likely that humans contributed to the extinction of many of these animals — likely in conjunction with a changing climate, ecosystem collapse, and possibly introduced diseases.

The International Union for the Conservation of Nature has developed a system to classify the estimated health of a species. The system factors in: the rate at which populations are declining, total population-size, habitat-loss, total species range and ecosystem health, and the degree of habitat and population-fragmentation. The classification spectrum includes: Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Near Threatened, Least Concern, Data Deficient, and Not Evaluated.

Effects on Ecosystems, The Physical Environment, and Climate

The extinction of a species can have significant consequences for an ecosystem and even for the physical environment and climate, often leading to further extinctions. This is especially true when it comes to what are known as keystone species. Which is a species that has a very pronounced effect on its environment in relation to its numbers and total biomass.

Examples would be the many species of sea stars that prey on sea urchins, mussels, and other shellfish. These animals typically have no predators other than sea stars, and without the sea stars their numbers explode, out competing and annihilating other species in the ecosystem. Another good example is the sea otter, which also preys on sea urchins, without them some sea urchin populations grow exponentially and can destroy whole kelp forests.

Less direct examples include animals such as the grizzly bear and jaguar, which effectively balance the whole ecosystem, and assist in the necessary transfer of nutrients across the whole ecosystem, as a result of their extremely varied diet. In the case of the grizzly bear, they transfer significant amounts of nutrients from the ocean onto the land as a result of their salmon fishing during spawning season. Animals such as this are also often called ecosystem engineers. Other examples of these engineers include prairie dogs, beavers, and elephants. When one of these engineers goes extinct, or their numbers decline significantly, it has a very large effect on their environment. All of the life forms that have evolved to benefit from the environment- changing actions that these animals do see a decline in their numbers because of the extinction. Many often go extinct themselves.

The extinction of some species also results in larger changes to the environment, and to the local, and occasionally global, climate. The extinction of important plants species, in particular, can lead to vast changes in the environment, sometimes turning previously biologically-productive areas into wastelands. This is often the case after large-scale deforestation, but also sometimes as a result of selective logging. Deforestation itself contributes very significantly to the extinction of many species, perhaps more than any other human activity.

Causes of Extinction, Before Modern Civilization, and Now

The causes of extinctions before modern human activities varied widely, and was generally very specific to the individual extinction, with the exception of larger extinction events, such as the 6 Great Mass Extinctions. The causes of extinctions often occur well before the actual extinction itself, this is referred to as extinction debt.

There is debate about whether more of the extinctions observed in the fossil record were as a result of competition between species or a result of catastrophic change. Generally though, the causes of extinction are as a result of the species losing its ability to survive and reproduce in its environment while being limited from expanding or spreading to other areas. The extreme fragmentation of the world’s current natural habitats, as a result of the roads and farmlands that cut through and divide nearly all of it, is often brought up in discussions about future extinctions for this reason. While in the past many species could simple slowly adjust, and change their range over time, in order to follow their preferred temperatures or environments, that simply isn’t the case for many animals and plants in the world today. Many animals and plants, especially the larger and slower-growing ones, are essentially imprisoned in their fragmented habitats, being unable to shift their range or make it to the other fragmented wild areas. It’s essentially like if they were stuck on an island, which largely wasn’t the case in the past.

In recent years, humans themselves have contributed very significantly to an incredible number of species extinctions. Deforestation, over-hunting, introduced diseases, introduced invasive-species, agriculture, and large-scale environmental degradation via pollution, have been the primary causes.

More than half of the world’s forests have been destroyed over just the past 10,000 years. This no doubt has contributed to a vast number of extinctions during those years. Many extinctions we may not even be aware of, those of animals that didn’t leave anything behind in the fossil record (that has been found yet). Over-hunting is thought to have contributed very significantly to the extinction of many of the previously ubiquitous megafauna animals, such as the animal that the unicorn legend is based on, Elasmotherium. Diseases introduced with the spread of domesticated animals, human pests, and agriculture, have been responsible for a large chunk of the extinctions that have occurred over the past few thousand years. The brown rat and the black rat, have followed humans nearly everywhere, living off their waste primarily, but also being particularly destructive to the native wildlife of many regions. The modern large-scale release of many chemical pollutants and heavy metals has also been implicated in the extinction of many species in recent years. And that is not including the effect that the large-scale release of greenhouse gases is having on the climate. A 2003 study published in the journal Nature estimates that 15-37% of all land species will be “committed to extinction” by 2050 as a result of climate change.

Mass Extinctions

Mass extinctions, also known as extinction events, are brief periods in time when there is a widespread and rapid decrease in the amount and diversity of life on the planet. Microbial life is difficult to study, especially in the fossil record, so mass extinctions refer only to macroscopic life, which is life that is large enough to be easily observed.

There are currently 5 mass extinction events that are known to have occurred in the past 540 million years, with a sixth ongoing as of right now. All of these have individually resulted in the extinction of over 50% of the animal species known to be alive at the time, and often times considerably more. The End-Permian mass extinction event resulted in the disappearance of 90-96% of all plant and animal species alive in the world at the time.

These mass extinction events do not typically occur as an abrupt event where all of the species involved suddenly become extinct, but are often the result of cascading effects and “boom and bust” cycles. Often the very qualities that may make an organism well-suited and adapted to its static environment become severe liabilities in a changed environment. This seems to be especially true with regards to environments featuring more organism-against-organism conflict, where the “arms race” between the different organisms may be highly specialized.

It seems to have often been the case that animals that did extremely well after the initial extinctions of other organisms then tended to over-expand and becomes too “successful”, over-consuming and destroying their environment and livelihood. This seems to have occurred continually after mass extinctions, over long periods of time, repeated “boom and bust” cycles, with different species periodically becoming the “booming” one.

After a mass extinction ends, and things have equalized somewhat, there is typically a long period when no new species emerge, referred to as a dead zone. These dead zones usually last for a few tens of thousands of years, but sometimes for much longer. No new species emerged after the End-Permian mass extinction for the next 5 million years. Biodiversity levels don’t return to their previous state for, on average, about 5-10 million years, but sometimes it takes as long as 30 million years.

Some details on The 5 Great Mass Extinctions are below:

1. Ordovician–Silurian extinction event (End Ordovician): It occurred about 450–440 million years ago. It actually consisted of two different events, in total they annihilated 27% of all families, 57% of all genera and 60% to 70% of all known species. This event is considered to be the second largest of the 5 major extinctions with regards to the percentage of genera that became extinct.

2. Late Devonian extinction: This event occurred about 375–360 million years ago. It was a somewhat prolonged series of different extinctions, going on for possibly as long as 20 million years, that wiped out around 19% of all families, 50% of all genera and 70% of all species.

3. Permian–Triassic extinction event (End Permian): This one occurred right about 251 million years ago. It is the largest extinction event that is currently known. It led to the extinction of “57% of all families, 83% of all genera and 90% to 96% of all species. 53% of marine families, 84% of marine genera, about 96% of all marine species and an estimated 70% of land species, including insects.” It is not clear exactly how many plant species went extinct, but it is clear a great number did. An entirely new taxa of plants became dominant after it was over. The event, also known as the “Great Dying”, “had enormous evolutionary significance: on land, it ended the primacy of mammal-like reptiles.” This allowed archosaurs to become ascendant.

4. Triassic–Jurassic extinction event (End Triassic): This event occurred about 200 million years ago. It led to the end of about “23% of all families, 48% of all genera (20% of marine families and 55% of marine genera) and 70% to 75% of all species went extinct.” Almost all non-dinosaurian archosaurs, including the majority of therapsids, and nearly all large amphibians were wiped out. This left dinosaurs with pretty much no terrestrial competition. “Non-dinosaurian archosaurs continued to dominate aquatic environments, while non-archosaurian diapsids continued to dominate marine environments. The Temnospondyl lineage of large amphibians also survived until the Cretaceous in Australia (e.g. Koolasuchus).”

5. Cretaceous–Paleogene extinction event (End Cretaceous): This event is well-known to the general public. It occurred 65.5 million years ago, and has been repeatedly associated with an asteroid or comet impact. Though such an impact wouldn’t have been the only cause of extinction, simply the domino that sets others into motion. “About 17% of all families, 50% of all genera and 75% of all species became extinct. In the seas it reduced the percentage of sessile animals to about 33%.” Almost all non-avian dinosaurs became extinct during this time. Mammals and birds became the dominant land vertebrates after this event.

We are currently in the midst of what is being considered the 6th great mass extinction, or the Holocene extinction, or sometimes the Anthropocene extinction. This extinction event started at the end of the last ice age, around 13,000 BP. And has resulted in the extinction of nearly all forms of megafauna mammals.

This includes animals such as the mammoth and mastodon, the American lion, saber-toothed cats such as Smilodon and Homotherium, giant sloths, giant birds such as the Moa, gorilla-sized lemurs such as Archaeoindris, an 8 foot long tortoise called Meiolania, the large and aggressive wild ancestor of cattle the aurochs, the 30 foot long Stellar’s Sea Cow, the giant short-faced bear, the unicorn Elasmotherium, the dragon Megalania, a species of giant 600 lbs kangaroo, car-sized giant armadillos, cave hyenas, cave lions, giant beavers, a species of horned crocodile, the 9 foot long saber-tooth salmon, a species of 500 lbs rodent, the largest owl to have ever existed Ornimegalonyx, the enormous 8 feet at the shoulder and 4500 lbs ice age bison Bison latifrons, several species of giant freshwater fish, a giraffe species that lived in India Sivatherium, the American cheetah Miracinonyx, the largest eagle ever known the Haast’s eagle, the strange giant hoofed-animal Toxodon, several species of small-plane-sized teratorn birds in North America, the stag-moose, the 6-meter-long constrictor snake Wonambi, the giant North American camel Camelops, the enormous relative of modern polar bears the Tyrant Sea Bear, the dire wolf, and the 7 foot tall 1500 lbs Irish Elk, among others.

Extinct Species

As the list of extinct megafauna-animals posted just above shows, when a large number of species become extinct, the world changes considerably. Whole ecosystems can be entirely restructured, greatly limited in their extent, or may completely collapse. This is especially true when there are many extinctions all at once, or the extinction is of a keystone or engineer species.

Below I’m going to highlight a couple of recent extinctions:

Thylacine also known as the Tasmanian Tiger: The thylacine was the largest carnivorous marsupial of modern times. It went extinct in the early 20th century, primarily as the result of a bounty program created by the Tasmanian government — but deforestation and habitat-loss no doubt contributed. They had previously gone extinct on the Australian mainland in the 19th century, and had been near-extinction there for at least the past few thousand years as a result of the earlier aboriginal human settlement, the introduced dingo, and possibly diseases that arrived with the dingo. The thylacine was primarily nocturnal though, and powerfully built, so it’s unclear how much the dingo may have contributed to their extinction. It is completely clear though that they were hunted by humans, as indigenous rock art attests to this.

There has been some speculation that there may be a very small population of the animals still alive in Tasmania. But no solid evidence has been produced, and there is a clear financial benefit to the claim, as it attracts valuable tourism to the areas where they have been “spotted”.

Sabertooth Salmon or Oncorhynchus rastrosus: The saber-tooth salmon was an enormous species of salmon, growing up to 9-feet in length — that likely went extinct sometime in the last few hundred thousand years. Though they may have lived until as recently as 11,000 years ago — or even more recently than that. Their fossils have been found along nearly all of the Pacific-coast of North America. In addition to their size, they are notable for the pair of large “fangs” that they possess near the tip of their mouth. The rest of their teeth were rather small though, which strongly suggests that they were primarily plankton-eaters, as many species of salmon are today. It is unclear exactly what led to their extinction, but a changing climate was likely the primary-cause.

Aiolornis incredibilis: Aiolornis incredibilis was the largest known North American flight-capable bird to have definitely had contact with humans. It was roughly the size of a small-plane, with a wingspan of over 16-feet –,not quite as large as its ancestor Argentavis magnificens, which had a wingspan of over 26-feet by some estimates. Its fossils have been found all throughout the west-coast, southwest, and central-regions of North America. Their range and description matches up very well with animals that feature in various stories from various American Indian tribes. It is thought that they went extinct about 11,000 years ago.

American Lion or Panthera leo atrox: The American lion was a species of lion closely related to the African lion, except much larger. They were very common throughout North American and northern South America until about 11,000 years ago — when much of the other North American megafauna also went extinct. They were about 25%-larger than the largest modern African lions, and apparently (from their depictions in cave art) didn’t possess a significant mane. They were about 8-feet-long and 4-feet-tall at the shoulders, and likely peaked around 800-lbs. Which makes them a good bit less-powerfully built than their competitors the saber-toothed cats, which also went extinct. There is evidence that they lined their dens with grasses and leaves, as modern Siberian tigers do. It is currently thought that they went extinct through the combined-actions of humans — via hunting and deforestation/fire — and the changing climate.

Macoun’s Shining Moss or Neomacounia nitida: Macoun’s Shining Moss was a species of moss that was only found in a small area in Ontario at the time of its discovery. The distinctive moss occurred at only three locations near Belleville, Ontario, growing on the trunks of several elm and cedar species. It was well known to many First Nations tribes as an effective remedy for colds and coughs. The very limited area where it lived was clear-cut for development between 1864 and 1892. Extensive searches done in the years since have found no trace of it. It’s presumed extinct.

If it wasn’t for the previous discovery, the moss could very easily have gone extinct without its existence ever being known to modern science. It’s an important lesson. When one considers how massive the forested and wild regions of the past several-million years were, and how rare fossils (especially plants) are compared to total population-sizes, it becomes very apparent that many species, including very-large animal species with limited-ranges or population-sizes, could have existed without leaving any trace that we have yet found. We have a very limited insight into the types of life that may have existed on the planet in the past. The ones that we are aware of are simply the ones that are/were the most common, the most easily preserved, and the most easily recognizable.

6th Great Mass Extinction: The Holocene Extinction, or The Anthropocene Extinction

The world is currently in the middle of a very pronounced and rapid mass extinction — sometimes referred to as the 6th Great Mass extinction, the Holocene extinction, or the Anthropocene extinction. Though not all recent extinctions have been caused entirely by humans, all have been at the very least partially so — mostly as a result of human expansion, deforestation, agriculture, over-hunting, and domesticated animals.

Humans have significantly modified the vast majority of the previously most-productive regions and ecosystems of the world. While the argument is often made that humans don’t take up that much of the world spatially, and that you could fit all of human development into a so-and-so sized region, this completely ignores the fact that humans have preferentially migrated to and “developed” the previously most biologically-productive regions. Rivers, coasts, forests, valleys, etc, have all been largely settled by humans — leaving primarily the marginal regions: deserts, dry-land ecosystems, mountains, etc. And these remaining regions are often now unconnected to each other — being separated by roads, cities, and farmland. This greatly limits the ability of many species to maintain a healthy population-range, and limits their ability to adapt and move in reaction to changes in the climate.

When this is combined with the directly human-related causes — over-hunting, deforestation, introduced invasive species, introduced disease, domesticated animals, pollution, over-fishing, greenhouse gas pollution, and water diversion — it is no surprise that the current rate of extinction is 10 to 100 times higher than during any of the previous mass extinction events on the Earth.

And of particular concern, is that this extinction event involves a significantly higher proportion of plants than during any of the previous mass extinction events. It is unknown what will happen as a result of the extinction of so many different species of plants at once.

These reasons are why this 6th Great Mass Extinction is often referred to as the Anthropocene extinction, meaning anthropogenic, or due to human-activity.

The rate of species extinction has been increasing in recent years — and with it the health of many ecosystems has been diminishing. Humans are entirely reliant upon these natural ecosystems for their lives. Without them there would be no food, clean water, livable climate, crop pollination, etc.

It has been estimated that if the current-rate of “human-disruption of the biosphere” continues, that one-half of all of the world’s multicellular life-forms will be extinct by 2100.

Source: Wikipedia

Image Credits: American Lion, Last Wild Thylacine, Bison Skull Pile and Extinction Tree via Wikimedia Commons; Globe via Flickr CC