1, 2, 3, 4, 5, and 6? Extinctions

“Extinction is the rule. Survival is the exception.”
Carl Sagan

An article in Scientific America asked an interesting question, Why Don’t We Hear about More Species Going Extinct? There have been a lot of stories about the planet being in the middle of the 6th mass extinction.  Reports are saying that the rate of extinction is as much a 1000 times normal.  If these articles are correct shouldn’t we see articles in the news about species going extinct?  However, I wonder if people even understand the context of mass extinctions?  If asked, what is a mass extinction, could you answer? 

To understand what a mass extinction is, we need to understand life on earth and the fossil record.  All five existing mass extinctions are in the fossil record.  The first life that appeared were microbes around 3.7 billion years ago.  They lived in a world that was quite different from present-day earth.  The atmosphere was almost devoid of O2 (molecular oxygen) and high in things like methane.  Molecular oxygen is highly reactive and will spontaneously react with any oxidizable compounds present.  The early earth was full of oxidizable compounds, any molecular oxygen that did appear was almost instantly removed by chemical reaction.

About 1.3 billion years later the first cyanobacteria evolved, these were the first photo-synthesizers. Over possibly hundreds of millions of years molecular oxygen produced by the cyanobacteria reacted with compounds in the environment until all the oxidizable compounds were used up.  A great example of this is banded iron deposits.  Only when molecular oxygen reacted with all the oxidizable compounds could molecular oxygen begin to accumulate in the environment. 

After another 1.7 billion years the first multicellular organisms, sponges appeared in the fossil record.  Around 65 million years later a group of multicellular organisms called the Ediacaran Biota joined the sponges on the seafloor.  Most of these organisms disappeared around 541 million years ago.  However, the loss of the Ediacaran Biota is not one of the five mass extinction events.  How much of an evolutionary impact the Ediacaran Biota had on modern multicellular organisms is still an open question.  Most of the Ediacaran Biota had body planes quite different from modern organisms.

The next period is especially important; it started about 541 million years ago and lasted for about 56 million years.  The period is known as the Cambrian.  This period is referred to as the Cambrian explosion because all existing types (phyla) of organisms we see in modern life emerged during this period. The Cambrian explosion is also essential because the diverse number and types of organisms that evolved during the Cambrian explosion form the backdrop for mass extinctions.

The first mass extinction occurred 444 million years ago at the end of the Ordovician period.  During this extinction event, 86% of all species disappeared from the fossil record over about 4.4 million years. Global recovery after the extinction event took about 20 Million years

The Second mass extinction occurred at the end of the late Devonian Period.  The Devonian extinction is the extinction that eliminated the Trilobites.  During this extinction event, 75% of all species disappeared from the fossil record over as much as 25 million years

The third and largest mass extinction occurred at the end of the Permian period 251 million years ago.  During this mass extinction, 96% of all species disappeared from the fossil record over 15 million years.  Research suggests that the Permian mass extinction took 30 million years for full global recovery.

The fourth mass extinction occurred 200 million years ago at the end of the Triassic period.  During this mass extinction, 80% of all species disappeared from the fossil record. The Triassic mass extinction appears to have occurred over an incredibly short period, less than 5000 years.

The fifth mass extinction occurred at the end of the Cretaceous period 66 million years ago.  This extinction is by far the most famous of the mass extinctions because it is the meteor strike that killed the dinosaurs.  During this extinction, 76% of all the spices disappeared from the fossil record.  Research suggests this mass extinction only took 32,000 years.

Now that we have looked at mass extinctions what about regular extinctions.  Normal or background extinction rate is the number of extinctions per million species per year (E/MSY).  Current estimates put the background extinction rate at 0.1 E/MSY.  If the the current extinction rate is 1000 times the background extinction rate, then currently the extinction rate is 100 E/MSY.

The current estimate for the total number of species is 8.9 million.  That means that 890 species are going extinct every year or 2.5 species a day.  So why don’t we hear more about spices going extinct if the extinction rate is that high?  First, the current catalog of identified species is 1.9 million, which means there are currently 7 million species (79%) that are undescribed.  That means 700 of the 890 extinctions a year would be in species that scientists haven’t identified.

The second problem is that even with identified species, it is often difficult to know if a species has gone extinct.  The International Union for Conservation of Nature (IUCN) maintains the Red List of critically endangered species.  One of the categories is Possibly Extinct (PE) based on the last time anyone saw an organism.  For example, no one has seen the San Quintin Kangaroo Rat in 33 years, no one has seen the Yangtze River Dolphin in 17 years, and no one has seen the Dwarf Hutia in 82 years.  It is likely that these three spices, along with several others, are extinct.

However, not being seen is not good enough to classify a species extinct.  After all, the Coelacanth was thought to be extinct for 65 million years until a fisherman caught one in 1938.  For a species to be declared extinct, a thorough and focused search must be made for the organism to declare it extinct.  These types of searches require time, personnel, and money.  Therefore, searches don’t often happen.  So with the exceptions of particular cases, like Martha the last Carrier pigeon who died on September 1, 1914, most species go extinct with a whimper, not a bang.

We don’t hear more about species going extinct because even knowing extinctions are occurring, in many cases, we don’t know about them.   Returning to the question of what is a mass extinction, and could there be a 6th happening?

Use the five existing mass extinctions as examples a simple definition of a mass extinction is an event where 75% or more of the existing species become extinct within a short (less than 30 million years) time.  Using the current estimated number of spices, and the current estimated rate of extinction we can calculate how long it will take to reach the 75% mark the answer is 7500 years. Since 7500 years is less than 30 million years, we could be on course for a 6th mass extinction.  However, as Doug Erwin says we are not in the middle of the 6th mass extinction.  If we were in the middle of a mass extinction like Dr. Erwin said cascade failures would already have started in the ecosystem and there would be anything we could do.  However, that is good news we still have time to do something.

What we need is an accurate count of extinct species.  So, do you have a class that could do fieldwork?  There is probably a critically endangered species near you.  Maybe you will even be lucky, and you will find the species then you can help with a plan to save it.

Thanks for Listening to My Musings
The Teaching Cyborg

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