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 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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