In ordinary times – by which I mean every single geological
epoch minus the present one – species extinction takes place only very rarely. The “background
extinction rate” for mammals (the best-studied group) has historically been
approximately 0.25 per million species-years.
Translated that means that since there are about 5,500 mammal species
alive today, at the background extinction rate, you would expect one species to
disappear every seven hundred years.
That average is, however, distorted by the mass extinction
events that have certainly occurred throughout planet earth’s history. As you may have guessed from the title of
Elizabeth Kolbert’s Pulitzer Prize-winning book, we are confident that at least
five mass extinction events have previously occurred. Kolbert, an environmental journalist by
trade, defines a mass extinction event as “an event that eliminates a
significant proportion of the World’s biota in a geologically insignificant
amount of time.” Spoiler alert: Kolbert
has collected some evidence that we may be in the midst of the sixth.
Background and
Dinosaurs
Looking back on the scientific understanding of extinction
events, it is worth noting that Darwin’s theory evolution has been used to
explain both how species originated and how they vanished. Extinction and evolution were to each other like
the two strands of the double helix on which DNA is written. “The appearance of new forms and the
disappearance of old forms” were, Darwin wrote, “bound together.” Driving both was the “struggle for
existence”, which rewarded the fit and eliminated the less so.
The End-Cretaceous (or K-T) extinction event is the most
well-known of the five recognised mass extinction events because it is the most
recent and, more likely, because it allowed Steven Spielberg to come up with
Jurassic Park. Whilst not as well-known,
the other four mass extinction events have been equally if not more devastating.
In addition to the five major mass extinctions, there is
evidence to support the notion that there are also much more regular, but less
severe extinction events. A study
presented by Kolbert suggests that when mass and lesser extinction events are
viewed together, Earth’s history appears to follow a pattern in which an
extinction event of some magnitude takes place at regular intervals of roughly
26 million years.
Stepping back again, we can see from the graph below that earth’s
biodiversity shows a great expansion in the number and variety of life forms
that have come to call this planet home over the last 4.5 billion years.
(Paleozoic
(ancient life), Mesozoic (middle life) and Cenozoic (new life))
When the diversity of life expands, the reciprocal event
must be that more species are likely to become extinct, at least according to
the Darwinian theory of extinction. Yet,
there is a distinct lack of consensus amongst the scientific community when it
comes to the causes of extinction. One
of the most prominent themes in Kolbert’s book is that if 25 years ago it
seemed that all mass extinctions would ultimately be traced to the same cause,
now the reverse seems true. Whilst some
extinction events are thought to have been caused by glaciation or changes to
ocean chemistry, others are can be traced to extraneous causes like asteroid
impact, whilst others are a combination of the above.
Take, for example, the extinction of the dinosaurs that
occurred approximately 65 million years ago (the K-T). The most contemporary hypotheses propose that
the main cause was not the impact of an asteroid or even the immediate
aftermath, but the dust.
It is now thought that an asteroid hit the Yucatan Peninsula
at a speed approaching 45,000 miles per hour.
A vast cloud of searing vapour and debris then raced over the continent,
expanding as it moved and incinerating anything in its path. In the process, the asteroid blasted more
than 50 times its own mass of pulverised rock into the air. As the ejecta fell back through the
atmosphere, it lit the sky and generated enough heat to, in effect, broil the
surface of the planet. As catastrophic
as this event must have been, it would not have been enough to kill all the
dinosaurs. Instead, owing to the
composition of the Yucatan Peninsula, the dust thrown up was rich in sulphur
which then formed sulphate aerosols that blocked the sun and caused a multi-season
“impact winter” which slowly killed off the remaining dinosaurs and
precipitated a several million year period in which almost all marine and
forest ecosystems completely collapsed.
Anthropocene
If the causes of previous extinction events are still being
debated, Kolbert is much more confident regarding the current predicament.
What is sometimes labelled neo-catastrophism, but is mostly
nowadays just regarded as standard geology, holds that conditions on earth
change only very slowly, except when they don’t. In this sense the reigning paradigm of life
on earth can be summarised as “long periods of boredom interrupted occasionally
by panic”. Though rare, these moments of
global panic are disproportionately important.
They determine the pattern of extinction, which is to say, the pattern
of life.
The present moment of panic aligns in geology with the
Anthropocene epoch. As detailed in Paul
Crutzen’s essay, “Geology of Mankind”, the term “Anthropocene” means the
human-dominated geological age. Among
the most radical geological changes that have occurred in the Anthropocene are:
- Human activity has transformed between a third and a half of the land surface of the planet;
- Most of the world’s major rivers have been dammed or diverted;
- Fertilizer factories produce more nitrogen than is fixed naturally by all terrestrial ecosystems (the process by which nitrogen in the atmosphere is converted into ammonia for living organisms);
- Fisheries have removed more than a third of the primary production of the oceans’ coastal waters; and
- Humans use more than half of the world’s readily accessible fresh water runoff.
Currently, about 50 million square miles of land on the
planet is ice-free, and this is the baseline that’s generally used for
calculating human impact. According to a
recent study published by the Geological Society of America, people have
“directly transformed” more than half of this land – roughly 27 million square
miles – mostly by converting it to cropland and pasture, but also by building
cities, shopping malls and reservoirs, and by logging, mining and
quarrying. Of the remaining 23 million
square miles, about three-fifths is covered by forest and the rest is either
high mountains, tundra or desert.
Consequently, since humans first emerged approximately
200,000 years ago, ecosystems around the world have changed remarkably quickly
(at least in geological terms). But why
should such a change be issue? As Darwin
would posit, the strongest will surely adapt.
The primary issuing is one of extremely accelerated mixing
of species. For millions of years,
species moved only as fast as their legs or wings could carry them. The process of mixing the world’s flora and
fauna was thus a slow and gradual one.
However, as early human migration began to establish new routes and
methods, the game has changed completely.
In recent decades, the accelerated mixing of flora and fauna via human
intervention has reached the point where, in some parts of the world,
non-native plants now outnumber native ones.
In fact, during any given 24-hour period, it is estimated that 10,000
different species are being moved around the world just in ballast water. Thus a single supertanker (or, for that
matter, a jet passenger) can undo millions of years of geographic separation.
From the standpoint of the world’s biota, global travel and
commerce represent a radical new variable.
Around 200 million years ago, the entire planet’s biota lived on the
supercontinent of Pangea. Whilst there
was no water to physically prevent the mixing of species, they did so
slowly. Over millions of years the
continental plates drifted apart and segregated many species, allowing them to
develop their own niches and to thrive.
Now, in the geological blink of an eye, we are seeing all the
continents’ species being mixed back together at incredible speed.
Kolbert presents several lines of evidence that argue in
favour humans being the cause of accelerated patterns of species extinction
around the world. The most compelling of
these is timing. The megafauna
extinction that began at the end of the last ice age, and which caused the
world to lose its population of 14 foot mammoths and mastadons, and rodents
that weighed up to 700 kgs, did not take place all at once. Rather, it occurred in pulses. The first pulse, about 40,000 years ago, took
out Australia’s giants. A second pulse
hit North and South America around 25,000 years later. Madagascar’s giant lemurs, pygmy hippos and
elephant birds survived all way into the Middle Ages. New Zealand’s moas (12 foot tall flightless
birds) made it as far as the Renaissance.
It’s hard to see how such a sequence could be squared with a single
climate change event. Instead, the
sequence of the pulses and the sequence of human settlement line up almost
exactly.
The Anthropocene is usually said to have begun with the
industrial revolution, or perhaps even later, with the explosive growth in
population that followed WWII. By this
account, it’s with the introduction of modern technologies – turbines,
railroads, chainsaws – that humans became a world-altering force. But the megafauna extinction suggests that we
should view the Anthropocene as an age that began more like 40,000 years ago at
a much more primitive stage in human history.
Evidently, we humans have not needed to be technologically
advanced in order to cause large extinction events. Before humans emerged on the scene, being
large and slow to reproduce was a highly successful strategy, and outsized
creatures dominated the planet. Then, in
what amounts to a geological instant, this strategy became a loser’s game. And so it remains today, which is why
elephants, bears and big cats are in so much trouble. Though it might be nice to imagine there once
was a time in which man lived in harmony with nature, it’s not clear that he
ever really did.
Forests and Trees
In the popular imagination, climate change is mostly seen as
a threat to cold-loving species. But climate
change is going to have just as great an impact in the tropics. The reasons for this are complicated, but
they start with the fact that the tropics are where most species actually live.
This simple observation is more scientifically referred to
as the Latitudinal Diversity Gradient (the LDG, as discovered by Alexander von
Humboldt in the early 1800s): the
general rule that the variety of life is most impoverished at the poles and
richest at low latitudes.
At the equator, around Columbia, Venezuela, Peru and Brazil
there are approximately 1,035 tree species; roughly five times as many as in all
of Canada’s boreal forest. And what
holds for trees also holds for birds and butterflies and frogs and fungi.
Kolbert connects the disparate ideas of the Anthropocene,
climate change and the LDG when visiting a researcher by the name of Miles
Silman in South America. Silman has been
studying in the Peruvian Amazon for the past two decades where he has recorded
the different species of trees and their size within 17 two and a half acre
plots arranged along a ridge running from the Amazon basin to the top of the
Andes. Astoundingly, he has found that climate
change is driving the average tree genus up the mountain (to cooler
temperatures) at a rate of eight feet per year (of course, trees can’t actually
move, but they do the next best thing which is to disperse seeds uphill for the
next generation).
Silman’s research is like a time capsule that reveals there
is slow-motion race taking place between species of trees in the Amazon. Trees are moving faster than ever previously
recorded up the mountainside. However,
many more are learning they are not as fast or strong as their neighbours or
that they will be thwarted by human intervention.
That is because one of the defining features of the
Anthropocene is that we are segregating and chopping up the land in which
the world’s species live by creating barriers – roads, clear-cuts, cities –
that prevent species from rehousing themselves after this eviction. A species that needs to migrate to keep up
with rising temperatures has a hard enough time as it is, but the problem is
exacerbated when you realise that so many species are racing towards a cliff
edge.
The Sea
This problem is clearly not contained to land-faring species
either. Humans are both altering the
chemistry of the sea and slicing up the environments in which sea creatures can
live.
Sea chemistry is being altered by
the excess CO2 we emit. Roughly
one-third of the CO2 that humans have so far pumped into the air has
been absorbed by the oceans. This comes
to a stunning 150 billion tons in less than 100 years (to find CO2
levels higher than today’s requires going back to the mid-Miocene, 15 million
years ago). As with most aspects of the
Anthropocene, it is not only the scale of the transfer that is significant, but
the speed.
As a result of this absorption of CO2, the ocean
is becoming more acidic and, according to research, ocean acidification played
a major role in at least two of the Big Five extinctions (the End-Permian and
the End-Triassic) and quite possibly a third (the End-Cretaceous).
But why is ocean acidification so dangerous to species? Depending on how tightly organisms are able
to regulate their internal chemistry, acidification may affect such basic
processes as metabolism, enzyme activity and protein function. Because it will change the makeup of microbial
communities, ocean acidification will alter the availability of key nutrients,
like iron and nitrogen. For similar
reasons, it will change the amount of light that passes through the water and it
will alter the way sound propagates. It
will impact photosynthesis and alter the compounds formed by dissolved metals, often
in ways that will be poisonous.
The species most vulnerable to these changes is the coral
reef. A recent study by a team of
Australian researchers found that coral cover in the Great Barrier Reef has
declined by 50 per cent in just the last 30 years.
Add to this change in chemistry, human-created changes to
the physical landscape of the sea and it becomes obvious why sea
creatures are going extinct at such an alarming rate. The roster of perils includes: overfishing,
which promotes the growth of algae that compete with corals; agricultural
runoff, which also encourages algae growth; deforestation, which leads to
siltation and reduces water clarity; and dynamite fishing, whose destructive
potential would seem to be self-explanatory.
If you thought that the ocean would be safe from human intervention, the
data suggests otherwise.
Amphibians
Whilst Kolbert takes a fairly broad brush approach in
describing the plight of most species in the forests and in the seas, the point
is driven home by the specific example of amphibians.
Amphibians emerged at the time of the supercontinent,
Pangea. Since the breakup of Pangea,
they’ve adapted to conditions on every continent except Antarctica.
Yet, within the last two decades, a micro-organism (a fungus
from the chrytids family called Batrachochytrium Dendrobatidis or Bd for short)
has been found to be living on amphibians’ skin and killing them off at an incredible
rate. Bd interferes with amphibians’
ability to take up critical electrolytes through their skin. This causes them to suffer what is, in
effect, a heart attack.
Worringly, Bd is everywhere and appears to be unstoppable at
present. Its origin is presently
unknown. The plight of amphibians is
such that, as a result of the spread of Bd, there is a group called EVACC (El
Valle Amphibian Conservation Centre) operating in Costa Rica and Panama to
attempt to capture a male and a female of every species of amphibian (like an
Amphibian Noah’s Ark).
One theory as to how Bd is moving is that it has been moved
around the globe with shipments of African clawed frogs, which were used in the
nineteen fifties and sixties in pregnancy tests.
A second theory is that the fungus was spread on North
American bullfrogs which are often exported for human consumption.
Either way, the etiology is the same: human activity. Without being loaded by someone onto a boat
or a plane, it would have been impossible for a frog carrying Bd to get from
Africa to Australia or from North America to Europe.
Consequently, amphibians now enjoy the dubious distinction
of being the World’s most endangered class of animals; it has been calculated
that the group’s current extinction rate could be as much as 45,000 times
higher than the background rate.
Conclusion
Beyond amphibians, Kolbert presents data to suggest that the
extinction rates around many other groups are approaching similar levels. It is established that one-third of all
reef-building corals, a third of all freshwater mollusks, a third of sharks and
rays, a quarter of all mammals, a fifth of all reptiles and a sixth of all
birds are headed toward oblivion.
Ultimately though, Kolbert falls on her own sword. By Kolbert’s own admission, this is not the
sixth mass extinction event that the title of the book would suggest. Climate change alone is unlikely to generate
a mass extinction as large as one of the Big Five. However, there is a high likelihood that
climate change on its own could generate a level of extinction on par with or
exceeding the slightly “lesser” extinction events of the past and be the end of
so many precious species across the world.
Cause for alarm is heightened when you consider how human
behaviour could add to the rising extinction rates caused by climate change. The primary human sin in this regard is the
accelerated speed at which we are constantly transporting non-native species to
new environments.
Kolbert argues that, as soon as humans started using signs
and symbols to represent the natural world, they pushed beyond the limits of
this world. So if you want to think
about why humans are so dangerous to other species, you can picture a poacher
in Africa carrying an AK-47 or a logger in the Amazon gripping an axe, but
better still, you can picture yourself, holding a pen. For Kolbert’s conclusion is somewhat
paralysing: extinction is inevitable for every species, but humans are a quite
unique species in that our mastery of our minds and the world around us may ultimately
lead us to live on a very lonely planet.
