(gentle music)
- [Narrator] Biodiversity is in crisis today.
This much we know.
However, extinction, the human-caused basis of the crisis,
can also be a natural phenomenon.
Species go extinct for a variety of natural reasons,
like meteor strikes, volcanic eruptions,
natural climate shifts, movements of continents
over geological eons, things that occur all the time
without the involvement of humans.
Therefore, scientists know there's a natural background rate
of extinction that's estimated to be somewhere
in the neighborhood of one to five extinctions a year,
averaged over geologic time.
This background rate exists because organisms
are constantly being confronted by environmental changes.
Some can deal with it, others can't.
A major change has the potential result of an extinction.
Even without a direct, natural phenomenon
that could kill individuals directly,
a species could go extinct due to its inability
to keep up with the competitors, predators,
or parasites that are always influencing
its local environment or changing the way
that it interacts with its ecosystem.
It's now estimated that the present extinction rate
is somewhere between 1000
to 10,000 times the background rate,
and we also know the main reason for this.
The reason is us.
What I'd like to do here is frame this
within nothing less than the entire history
of life on earth.
Have extinctions of this magnitude happened before?
And what can we learn from that?
The answer is yes, there have been massive extinctions
in the past, and we're now experiencing
the highest rate of extinction since the last major
mass extinction that occurred at the end of the Cretaceous,
some 65 million years ago.
This is the one that a lot of people,
especially seven year olds,
are familiar with because it ended the age of dinosaurs.
This extinction was likely caused by effects
that occurred after the impact of a large asteroid,
which scientists like to call a bolide.
So, when you see the term bolide impact,
you can tell your friends that that means,
basically, kablooey.
Some suggest that the smoking gun of this bolide impact
is a huge crater 15 to 20 kilometers deep,
and over 170 kilometers across,
buried deeply in sediments along the Yucatan Peninsula.
This crater was made by a bolide about 10 kilometers across.
Events like this leave a physical,
detectable mark in Earth's crust.
Geologists use markers like this,
along with other changes in the rocks,
to give special names to specific periods,
intervals of geologic time
and history bounded by these markers,
names like Paleozoic, Cambrian,
Carboniferous,
Cenozoic.
These are names of epochs, eras, periods, et cetera,
in the history of the planet.
Scientists ask if we are, right now,
making an event that will be detectable
millions of years from now in the geologic record.
Some claim that we are, and that we should be calling
this present epoch the Anthropocene,
anthro for human, with cene being the suffix that we use
to designate these epochs.
So, these scientists suggest that we signify
a special epoch in the history of the Earth,
an epoch marked by a new type of fossil evidence,
such as plastics or some crusty carbon layer
that indicates that we have left an indelible mark
in the geology that will be preserved for eons.
Right now, what can we learn from the past
by looking at this big picture history
of extinction on Earth?
We know that biodiversity is governed by a balancing act
between speciation and extinction.
Does biodiversity increase steadily with time?
Has it reached an equilibrium limit?
Speciation is caused by one species splitting
to give rise to two new species,
each of those giving rise to two more, and so on.
You'd expect the number of species to increase over time.
However, that multiplication is offset by extinction.
If you look at a graph of biodiversity over the history
of our planet, you get something that looks like this.
You can see that there is indeed
an overall trend towards an increase in biodiversity,
but you can also mark certain spots on this curve,
here,
here, and here,
that indicate significant drops in biodiversity.
These are known as major mass extinction events,
times during which extraordinarily high numbers
of species disappeared, times when the extinction rate
was well above the background rate.
It's worth looking at these because they suggest
some interesting and, I think, relevant reasons
for why these extinctions happened,
so I want to go into these in a little bit of detail.
The first one, here,
occurs at the Ordovician-Silurian boundary, 440
to 450 million years ago.
What on earth can we learn from something
that happened so long ago?
Well, for one thing, during this time,
60% of marine invertebrate species died out.
These extinctions seemed to be the result of effects
caused by movement of the huge supercontinent
that existed at that time, Gondwana.
Sounds like a pretty interesting place,
except that it moved slowly into the south polar region,
with a consequent sea level fall
as significant amounts of the planet's precipitation
became tied up in snow and ice on Gondwana,
rather than flowing back into the sea,
and this change disrupted marine ecosystems
along the continental shelf, resulting in extinctions,
extinctions recorded as another marker in the fossil record.
The second major extinction, here,
occurred during the late Devonian period,
about 374 million years ago.
I'm amazed that scientists can get that precise,
but they can.
We now know that 50% of all the genera on Earth
went extinct at this time.
Think about that for a moment.
We're not talking about species.
We're talking about entire genera,
and that means a lot of species.
There's some thought that this was due to a bolide impact
that occurred long before the one
that wiped out the dinosaurs, but maybe not.
Others think maybe it was oceanic volcanism
or some global cooling mechanism
that we don't fully understand.
Some suggest that it was due to a drop in speciation rate,
rather than an increase in extinction rate,
and clearly, more research is needed here.
The third of the big five extinction events, here,
is something that occurred at the end of the Permian,
between the Permian and Triassic periods,
about 252 million years ago.
This is sometimes known as The Great Dying,
the biggest known extinction event,
during which 96% of all marine
and 70% of all terrestrial vertebrates died out.
It also appears that it's one of the few mass extinctions
in which insects took a big hit.
I guess it takes a lot to kill all those cockroaches.
In total, 83% of all the genera on earth disappeared.
The suggested reasons behind this mass extinction
range all over the map.
Some people think it might have been one
or more bolide impacts,
or volcanism on a huge scale,
possibly from a place called the Siberian Traps,
where there were huge outpourings of lava,
big fires going on, and all kinds of horrible things
happening to the environment because of that volcanism.
It could also have been a runaway greenhouse effect
triggered by the sudden release of methane
from the ocean floor.
Methane can be stored by the activities of bacteria
as something called methane clathrates
in sediments of the ocean floor.
Geologic disturbance might be able to release
these stores of clathrates, releasing huge amounts
of the potent greenhouse gas, methane.
Other people think that there might have been
major sea level changes at the time,
due to overall climate changes.
Increasing anoxic conditions
or lack of oxygen in the deep sea,
or perhaps shifts in oceanic circulation patterns
could be results of those climate changes,
or maybe a hellish convergence of all these things.
I can easily imagine the volcanism causing earthquakes
that would release methane clathrates,
and cause oceanic circulation changes,
and so on and so forth, a cascade of disasters.
The fourth major extinction occurred
at the Triassic-Jurassic boundary, 201 million years ago.
34% of all marine genera died out,
along with a whole bunch of terrestrial vertebrate groups,
clearing the stage for the rise of the dinosaurs.
Hooray.
Where would all of our five to 10 year olds be
if it weren't for the Triassic-Jurassic extinction?
Statistical analysis suggests some complications
in figuring out what the story is here,
but a drop in speciation,
rather than an increase in extinction,
might have caused this drop in biodiversity, too.
The fifth and possibly most famous of all
the big five extinctions is the Cretaceous-Paleogene event,
65 to 66 million years ago, and we know the story.
A kablooey, a bolide impact with dinosaurs dying out
in the aftermath, along with lots of other groups.
About 75% of all known species went extinct,
but this was followed by rapid recovery,
giving way to what many scientists refer to
as the modern biota,
including the rise of mammals,
and I think we can learn something from that, too.
Life is amazingly tenacious.
For all the delicacy of an individual or a species,
life recovered in impressive ways
at least five different times,
which leads to a fascinating question.
What happens after a mass extinction?
What are the contingencies, as evolutionists call them,
that happen when ecosystems are suddenly opened up
by the removal of many species.
Who comes next?
Who can take advantage of the changes and the openings,
and how fast can all that happen?
Those are difficult but very interesting
and very relevant scientific questions,
so there are lots of people working on them because,
as I like to say, the past is where you came from.
Without knowing where you came from,
sometimes you can't know where you're going,
and lots of the extinction causes I just described
should sound a bit familiar,
because they bring us back full circle
to the beginning of this video
in thinking about a sixth mass extinction
during the Anthropocene.
For the first time in the entire history of life on earth,
there's a mass extinction happening due to a single species:
us.
What's gonna happen while this is going on
and after it runs it present course?
I hate to imagine a world without tigers
or blue whales or Sequoias or salmon,
giant clams, coral reefs,
but without them and the ecosystems that depend on them,
our time is limited.
We'll not only be the cause of a mass extinction,
we'll be part of it.