Tuesday, July 2, 2013

The Sixth Extinction

Thomas H. Wilson

Extinction is a way of life.  It is always occurring.  Something like 98% of all species that have ever lived are extinct.  Scientists estimate that species last for about 1-10 million years before they become extinct, the background or normal extinction rate.  Five times in the Earth’s history, there have been major spikes in extinction rates, or mass extinctions.  There is considerable evidence that we are currently in the midst of a sixth mass extinction, this one mostly caused by us.  What can the previous mass extinctions tell us about the causes of mass extinctions, and what may we learn about where we are headed in the current crisis?

The causes of mass extinctions are difficult to gather and interpret because of the vast time scales involved, the difficulties of precise dating so many years ago, finding the geological deposits at the times of the extinctions, counting the fossils involved, and determining causes.  For brevity, complex issues of the mass extinctions are necessarily simplified.

Ordovician Extinction (447-443 million years ago)

Much of the Ordovician Period (485-443 Ma) saw high sea levels and warm temperatures.  The continent Gondwana started the period in equatorial waters, but migrated towards the South Pole towards the end of the period.  The world at this time had no terrestrial floras or faunas.  Multicellular life existed exclusively in the seas.  Marine faunas included mollusks such as cephalopods (nautiloids, ammonites, now octopuses and squid), bivalves and gastropods (snails and slugs); corals, bryozoans (aquatic invertebrate filter feeders, mostly colonial), crinoids (sea lilies and feather stars), and graptolites (fossil colonial animals).

Major volcanism in the Late Ordovician increased CO2 levels and heated Earth, but CO2 levels fell to lower levels before extinction time.  Towards the end of the Ordovician, Gondwana’s migration to the South Pole caused widespread glaciation, which cooled the Earth and caused lowered sea levels, reducing marine ecological niches, especially along continental shelves.   Hard hit were planktonic forms (animals in the water column that flow with ocean currents), bryozoans, brachiopods, some trilobites and cephalopods.  This was the second most devastating extinction in Earth’s history, when 49% of genera, and more than 60% of marine invertebrates died.  In general, fluctuation of greenhouse gases, changes in sea levels, and climate change were the main causes of the Ordovician extinction.

Devonian Extinction (375-359 million years ago)
The Devonian Period (419-359 my) saw the rise of vascular land plants on the two super continents of Gondwana and Euramerica, the first forests, amphibians and insects on land, the rise of sharks and bony fishes in the seas, reef-building with corals and stromatoporpoids (reef-builders of laminated calcareous skeletons, related to sponges).  The causes of the Devonian extinction were possibly similar to the earlier Ordovician events:  glaciation and cooling temperatures, and lowered sea levels.  In addition, there was depletion of oceanic oxygen levels.  The extinctions involved mostly marine faunas:  reef-builders, brachiopods, trilobites, conodonts (eel-like creatures), jawless fish and armored fish.  There seems to have been little impact on terrestrial floras.  There were probably a number of extinction events, with two extinction spikes, one about 375 million years ago and the other at the end of the Devonian.  The events caused the extinction of 22% of families, 57% of genera and 75% of species of marine animals, mostly invertebrates.

Permian Extinction (252 million years ago)
 The extinction that closed the Permian Period (299-252 million years ago) is known as the Great Dying.  Perhaps 96% of all marine species and 70% of all terrestrial vertebrate species became extinct.  The end Permian not only marked the change to the Triassic Period, but also the boundary between the Paleozoic and Mesozoic Eras.

The super continent Pangea had great conifer forests, well stocked with large and complex terrestrial faunas, including amphibians, reptiles and therapsids (mammal like reptiles).  The end of the Permian saw the extinction of about 87% of genera of marine invertebrates, like foraminifera (amoeboid protozoans with calcium carbonate tests, or shells, living in seafloor sediment, or as floaters), corals, reefs, sea anemones, brachiopods, bivalves, gastropods and ammonites, among others.

On land, among terrestrial invertebrates, the end Permian featured the greatest insect die-off ever, the only known insect mass extinction.  Continental deposits in South Africa (Karoo) and Russia reveal tremendous diversity of terrestrial vertebrates at the end of the Permian.  36 of 48 families of tetrapods died out (75%), animals representing great diversity of size and life style, from small insect eaters to large herbivores and carnivores.

What caused the extinction of maybe 90% of life on Earth, over the course of perhaps 500,000 years?  It might have been climate change caused by massive volcanism of the Siberian Traps, which emitted up to three million cubic kilometres of lava, releasing CO2 and other emissions, leading to global warming, perhaps exacerbated by release of methane.  Oceans were warmer and shallow coastal waters saw a reduction of oxygen.

Life on Earth was slow to recover after the Great Dying.  The tetrapod survivor Lystrosaurus accounted for the great majority of the post-extinction faunas of the early Triassic, in contrast to the biological and ecological diversity preceding the event.  It required perhaps 4-6 million years for ecosystem recovery, and ultimately the extinction of the mammal-like reptiles made way for the rise of dinosaurs.

Triassic Extinction (201 million years ago)
The Triassic Period lasted for about 50 million years, from 252 to 201 million years.  Faunas and floras of the Triassic diversified from those plants and animals that made it through the Permian extinction.  In the seas were many kinds of marine reptiles, including ichthyosaurs and plesiosaurs.  Archosaurs, the crown group of the clade that ultimately included dinosaurs, birds, pterosaurs (flying reptiles) and crocodiles, were the most common land vertebrates.  Other land animals included scorpions, spiders, snakes and millipedes.

Most of the Triassic was hot and dry.  Towards the end of the period the supercontinent Pangea broke apart.  As a result, over a period of perhaps 40,000 years, from Nova Scotia to Brazil and West Africa there were volcanic eruptions, perhaps extruding two million cubic kilometres of lava, spewing CO2, sulfur, and methane, leading to greenhouse gas effects and acid rain.  This caused the extinction of non-dinosaurian archosaurs, large amphibians, 20% of marine families, half of marine genera, including conodonts and ammonoids; brachiopods, gastropods and marine reptiles.  Ultimately, this clearing of terrestrial faunas opened ecological niches for the spread of dinosaurs, and the early lineages of mammals.

Cretaceous Extinction (66 million years ago)
The end of the Cretaceous Period (145-66 million years ago) is well known for the extinction of the non-avian dinosaurs.  By the end of the Cretaceous, the continents and oceans were taking their positions similar to today.  It was also a time of high sea levels, with a seaway separating eastern and western North America.  On land, the Cretaceous is significant for the appearance and spread of flowering plants.  Dinosaurs were the dominant land animals, and mammals were still relatively small.

An asteroid struck the Earth about 65.5 million years ago, the evidence for which is the Chicxlub impact crater in Yucatan, a worldwide iridium layer at the top of the Cretaceous stratigraphic column, melted basalt droplets and shocked quartz from the impact, and a fern spike.  The climate may already have been changing from volcanic activity from the Deccan Traps in India. 

Diverse plant and animal species suffered declines and extinction.  Life forms affected included foraminifera, colonial corals in warm shallow seas, cephalopods (including ammonoids), echinoderms and bivalves, plesiosaurs (marine reptiles with long necks, thick bodies and flippers) and mosasaurs (apex predator large marine reptiles), insects, mass extinction of plants, 50% of crocodilian families, the last of the flying reptile pterosaurs, and all the non-avian dinosaurs, including giants such as Tyrannosaurus rex.  Mammalian lineages suffered but came across the extinction event.  Birds are the only dinosaurs to survive extinction.

Any time marine or terrestrial plants or animals become extinct, particularly in large numbers, opportunities arise for the evolution and diversification of new forms.  After the end of the Cretaceous, which is also the boundary between the Mesozoic and Cenozoic Eras, the opportunity arose for the adaptive radiation of mammals.

The Sixth Extinction

There is evidence that we have already entered Earth’s sixth mass extinction.  Absent a true cataclysmic event, like the asteroid that struck Earth and ended the Cretaceous, it is somewhat difficult to date the beginning and ending of mass extinction events.  A convenient place to set the beginning of the sixth extinction might be the peopling of the Americas, which occurred about the time of the extinction of the great native megafaunas.  As this happened about the time of the end of the last Ice Age, there is some debate about whether humans or climate change caused the extinctions.  More broadly, there is fairly strong correlation between the arrival of humans and extinctions in Australia, the Pacific Islands and North America.  In the Americas, the lost faunas include:  mammoth and mastodon, various kinds of sloths, American lion, dire wolf, camel, tapir, horse and other species.

Other large mammalian faunas are currently threatened.  These include polar bears, pandas, lions, leopards, tigers, rhinos and others.  It is hard to foresee the survival of our closest primate relatives, the great apes.  Many amphibians are now dying off, with a third of all amphibians at risk.  21% of reptiles may be endangered.  Bees, which evolved along with the flowering plants in the early Cretaceous, are facing hard times.  The Zoological Society of London recently estimated that 20% of the world’s invertebrates may be headed for extinction.  12% of birds worldwide are at risk.  Coral reefs are greatly threatened, and 21% of all fish species studied globally are at risk for extinction.  The rates of extinction now may be higher than any time in the past.

Causes of the previous five mass extinctions include global warming from greenhouse gas effects from CO2, SO2, methane and other emissions, from volcanism and rise of sea temperature.  Global cooling from continental drift, glaciation and falling sea levels caused ecosystem loss in shallow seas.  Anoxia, oxygen deprivation in shallow seas also contributed to catastrophic effects.  Asteroid strikes can cause cataclysmic devastation to ecosystems and marine and terrestrial floras and faunas.  Some of these issues face us today.

The reasons for the sixth extinction are reasonably well known.  Loss of habitat is a major factor.  Often, increasing human population stresses habitats in various ways, such as deforestation or agriculture.  Invasive exotics choke off native species.  Pollutants, such as chemical fertilizers or poisons, ruin land and water environments.  Air is polluted.  Naturally occurring greenhouse gases contributed to extinction events in Earth’s history.  Now, human activities, such as burning fossil fuels, are the main contributors to increasing greenhouse gases.  Climate change and changes in sea levels and temperatures were major factors in previous extinctions, and we ignore this knowledge at our peril.

We do not know when or how the sixth extinction will end.  We do know that this time, we are the primary cause.  The speed of extinction is accelerating, and so far there is little motivation to do anything about it.  It is a colossal mistake to believe that humans are immune from extinction processes.  Every day that we do nothing about it, we bring that result closer.

The next time you swat that little scorpion, enjoy your victory.  Scorpions were among the first animals to inhabit the land, from the Silurian Period, about 430 million years ago.  As species, scorpions have a much better likelihood than humans of surviving the sixth extinction.

Thomas H. Wilson is Chair of the Arizona Humanities Council and Director of the Arizona Museum of Natural History.

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