The role of extinction through long-term history
Possible causes for mass extinctions
Some consequences of mass extinctions
Today we fret or even panic about the prospect of extinction, and we strive to protect endangered species, usually from problems we have caused them rather than from any inherent faults they possess. This is a surprisingly recent phenomenon, however, and so it is useful to begin with a brief rundown of the history of the idea of extinction.
When people generally believed that all of the
universe was recently created by a higher power according to a
grandly-conceived and definitive plan, the very idea that a piece of that plan
might disappear – unless it was a predestined disappearance mandated by the
Creator to make a point – was essentially heresy. How could God have made an
error? Surely extinction would be an indicator of failure, and how could
this be possible? Extinctions did occur from time to time when people
thought this way, but it was a period when global biodiversity was scarcely
recorded, so the losses went unnoticed. It was also a time – pre-industrial –
during which human agency was almost always too weak to cause major damage to
any species or habitat (e.g. whaling involved rowboats and hand-thrown
harpoons, forest-clearing required men swinging axes, and so forth). Evidence
of what might be extinct organisms, such as huge fossil bones unearthed in rock
quarries or agricultural fields, was explained
away as either non-biological (perhaps subtle messages in the form of
God-given artefacts), or as the remains of animals currently locally absent,
but probably still
living elsewhere (insufficient data existed to discount this possibility, and
it built on the idea of Utopian survivals in distant untouched places, very
popular at the time).
Global exploration and colonization by Europeans
began to make the “species-which-still-live-elsewhere” idea untenable by the
beginning of the 20th century, and today we can be quite sure that
dinosaur fossils, for example, represent truly extinct animals. During the same
period, as mechanisms of evolutionary change were taking hold of the scientific
imagination, the possibility of extinction gradually became more respectable
just as the process of speciation did. In addition, over that same period, the
growing reach of humans and the products of their industrial “progress”, like
steamships, harpoon-cannons, chainsaws, and bulldozers, had made the likelihood
– indeed the certainty – of extinctions all too clear.
The simple answer to this question is “the end of a species” – literally the word is cognate with “extinguish” – that is to say the point at which the last member of a species dies or is unable to breed. In practice, however, we need to broaden and vary this definition somewhat to make it more useful, both as a description of events and as a mechanism to use for prediction.
Extinction definitions are fundamentally scale-dependent.
Most species of organisms consist of several separate breeding populations
occupying multiple habitat locations. If a small population of an organism in
one habitat is wiped out by a catastrophic event, we refer to the loss as a local extinction (or
extirpation). Although local extinction can have a dramatic local
effect, disrupting that particular ecosystem, the problem is manageable for two
reasons: (1) the area may receive colonizing individuals of the species from
other populations nearby, and thus the extirpated population may re-establish;
and (2) the species is not lost, since other populations are likely to persist
outside the range of the extinction-causing event. (Human activity can
sometimes deliberately reintroduce the organism, not waiting for natural
recolonization to occur.)
True extinction, sometimes called species extinction or
just “extinction”, involves either the loss of a species which existed in a
single population instead of several, or the loss of all populations of a
species to a larger causal event. Once an entire species is gone, there is no
recovery. Species loss can have a major destabilizing effect (if
a top predator species disappears, for example), or a more subtle effect. For
example, when the dodo (a large flightless species of pigeon) was driven
extinct by hunting on the Indian Ocean island of Mauritius, a steady decline in
the breeding success of some tree species (such as Calvaria) began to be
noticed. Today, some 350 years after the last dodos were killed, few stands of
these trees younger than 350 years are observed. Though many other forces (such
as deforestation) have also been at work in Mauritius, the
role of the dodo remains clear: dodos used to eat the fruits of these
trees, digesting the thin flesh on the surface and passing (only slightly
scarred-up) the large seeds. Probably giant tortoises also had this effect on
seeds. The trees are adapted to herbivory, and in fact “expect” it, and the
lack of herbivores has led to reduced natural rates of germination, and a
change in forest structure. You can demonstrate the “dodo effect” by scraping
seeds yourself, or force-feeding them to dodo-analogue turkeys (the experiment
actually was done!); germination rates are enhanced.
Extinction has been a steady and influential force
over evolutionary time, and few species escape it for more than 10 million
years or so – mammal species, for instance, tend to persist for only 1-3
million years. New species are arising all the time, and it’s possible for
older species to be outcompeted by new types, or eaten by enhanced new
predators, or wiped out by newly-evolved diseases. Probably most extinctions
are caused not by such biotic changes, but rather by abiotic ones:
subtle climate-changes, drying-up of lakes and ponds, forests transitioning
into grasslands or grasslands into deserts, all may leave specialized local
organisms with no options. New organisms flood into the spaces created by
extinction, and resources seldom go unused for very long – life is enterprising
and attentive to opportunity.
Several times since the
origin of life, the landscape of the biosphere has been modified almost
overnight by massive forces of several different kinds: the result in each case
has been a mass extinction
event. Most people, even if they know nothing else about the
history of biodiversity, are aware that a mass extinction 65 million years ago
spelled the end of the “Age of Dinosaurs” (the Mesozoic Era), and ushered in
the “Age of Mammals” (the Cenozoic), through the agency of an extraterrestrial
impact from an asteroid. Fewer people know that this event also wiped out many
other types of organisms, while leaving an unusual set of survivors standing…
or that it was far from being the worst such event known.
The “dinosaur-ending” event, marking the “K-T
Boundary” (the dividing line between the Cretaceous period (K) of the Mesozoic
Era and the Tertiary period (T) of the Cenozoic era) is now well-characterized.
We know from a convergence of data that an asteroid (or perhaps a more
comet-like body) of about 10km diameter, an object roughly the size of Mount
Everest, struck the surface of the Earth at some 40 times the speed of sound on
the north coast of the Yucutan Peninsula, near the modern town of Chicxulub,
Mexico, leaving behind as it vaporized a crater of about 150km diameter (which
is today partly under the sea, and partly buried by sediments, but detectable
using geologic methods). Billions of tonnes of material, from the object and
from the Earth, were hurled into the upper atmosphere by the force of the
impact, and circulated globally within a few days to blanket the planet in a
dense veil of dust, which circulation-models suggest remained thick for years
if not decades. We know this occurred because all over the world there is a
clay layer of that age containing extraordinarily high concentrations of the
element iridium, and element almost unknown in the Earth’s crust but relatively
abundant in asteroids and meteorites, deposited in dust fallout following the
impact. Huge tsunamis would have devastated the Earth’s coastlines for weeks,
and in fact extraterrestrial impacts striking the sea may have more devastating
effects than impacts on land for just this reason. No doubt global
photosynthetic production was dramatically curtailed by the dust, and many
species disappeared through starvation. Temperature also took some hits, first having
been pretty hot from global-scale fires triggered by the extreme radiant heat
of the impact, then chilled by the reduced light-inputs from the smoke added to
the atmospheric dust. Species-losses from these forces directly probably also
triggered further indirect losses, like a chain of dominoes falling when one is
struck. Overall, about 70% of species known at the time had disappeared in what
must be described as a geologic “blink of an eye”, and probably the abundance
of the remaining species was much reduced – the biosphere would have seemed
pretty empty by our standards. Although some recent data suggest that the
Chicxulub crater was a little too early to be the direct culprit for the K-T
mass extinction, the occurrence of a global-scale impact is nevertheless
clearly marked in the geologic record; possibly a string of impacting objects
reached the Earth over a few hundred millennia, much as the comet Shoemaker-Levy 9 sent a string
of impacting pieces at Jupiter recently.
The largest mass extinction event, however, preceded
the K-T event. At the boundary between the Permian period (the last part of the
Palaeozoic era) and the Triassic period (the first part of the Mesozoic), a
little over 250 million years ago, about 90% of
known species perished in another very rapid phase. Recently evidence has
come to light for a crater of about 200km diameter off the NW coast of
Australia, which appears to be just the right age for a suitable impactor –
representing an object of perhaps 10km or a little more in diameter, but there
is not widespread agreement about this case. Around the same time as the
Permo-Triassic event, there was a period of extensive volcanic activity in
modern Siberia – perhaps this was a partial driving force for atmospheric, and
thus biotic, change. Although extraterrestrial impacts play a part in mass extinctions,
they are not the only factor involved; the older the event, that harder it is
to pin down a specific cause, of course.
Asteroid
impacts. How:
by reducing sunlight penetration of the atmosphere, changing climate
and reducing production. Definitely occurs, but probably not for every mass
extinction.
Tectonic
rearrangement.
How: by bringing together isolated continents (or oceans, as the
case may be) and exposing organisms on each to the diseases, predators etc.
upon the others. We know this occurred when South America was invaded by North
American species a few million years ago, and most southern animal species went
extinct and were replaced by northern ones. We don’t know how general or rapid
a phenomenon it may be. (And some loss of biodiversity is likely whenever
formerly separate areas are joined up, simply because there are fewer
geographically isolated places to hold variant species.)
Loss
of shallow seas. How: partly an artefact; most well-fossilized organisms
like molluscs prefer shallow seas, so when shallow seas disappear, we measure
(notice) “species-loss”. Shallow seas disappear when continents coalesce, and
this loss may also be reflected in changes in global climate, since water
warmed in coastal areas is less available to be circulated to cooler regions.
Volcanic
activity. How:
by releasing large quantities of sulphur-containing or carbon-dioxide-rich
gases into the atmosphere, altering climate and often inducing severely acidic
precipitation. Requires a very large (thousands of square kilometres at least)
area of intense eruptive activity, persisting for many millennia. Such areas
have occurred (Siberia, Deccan Plateau of India), but their role in mass
extinctions is still debated.
Climate
change for other reasons. How: by changing the “life-zones” available to organisms
and driving some off the map, as it were. We don’t know in most cases why
climate changes, but it has done many times.
Ecological
“cascades”.
How: when some species go extinct, they destabilize life for
others, in a “chain reaction”. Not clear if this is a strong enough force on
its own to account for really major losses, but likely locally significant,
and/or in combination with other forces.
Human
activity. How:
by direct hunting in part, but mostly by environmental pollution, habitat
destruction, etc. Somewhat questionable if the last century has seen
enough damage to count as a true mass extinction, but we may have more dramatic
impacts yet to come.
A
combination of reasons. How: one or more of the others all occurring at about
the same time. They may easily interact to wipe out more species than any could
alone.
When a force wipes the majority of species off the planet,
and drastically reduces the abundance of surviving species, ecological forces
which formerly were instrumental in controlling populations simply lose their
strength. Competition is likely to disappear for many organisms, since it
relies on nutrient- and/or space-shortages, and these simply would not occur in
the post-event conditions. Predation could also dwindle in effect, since there
would be many hiding-places and much area in which potential prey could escape.
Diseases and parasites would find it difficult to spread, since their hosts
would be dispersed widely, and many would fail to persist in such host
situations. Thus overall the biosphere would have an odd, disjointed
appearance, and would easily be disrupted by any disturbances (no real “stable states” would prevail). In
such conditions, a much wider variety of organisms than previously would have
at least a chance to live out their lives, and unlikely but
in-the-right-place-at-the-right-time organisms could give rise to entirely new
biotas in the post-mass-extinction world – a nice example of the serendipitous
nature of evolutionary history.
Before the Permo-Triassic (“P-T”) extinction mentioned above, land communities were dominated by a particular subgroup of reptiles, the therapsids (meaning “mammal-like skull-structure animals”). There were many species, from cat-size to as large as a modern elephant, filling all ecological roles. They coexisted with many species you would recognize today, including ferns and turtles as well as other reptiles (not snakes, but lizard-like forms), and with species you probably would not recognize (a tough-leaved plant group called cycads, and large crocodile-like amphibians). The extinction of nearly all the therapsids at the P-T boundary opened the field for – ironically – dinosaurs, that other group so renowned for their association with a mass extinction. Dinosaurs dominated life on land for more than 150 million years (that’s 2.5 times as long as mammals have done it, so don’t feel too smug about your own “superiority”), and several distinct waves of dinosaur-types successively “ruled”, though nothing else was able to move in on their success even during periods when their diversity briefly declined.
At the end of the Mesozoic, the K-T boundary period, all the larger types of dinosaurs disappeared, leaving behind only that small dinosaur remnant which we refer to today as birds. The first birds emerged quite early on in the Mesozoic, and they existed in many forms alongside other dinosaurs for tens of millions of years; why they persisted when other dinosaurs died out is still a mystery. Also coexisting with dinosaurs were the remnant of the ancient therapsids, which had become the earliest true mammals. Until they experienced a dinosaur-free world, however, they were consigned to life in the shadows, and if the K-T mass extinction had not occurred, they – that is, we! – might never have become the dominant large animals on the planet today.
Thus we can safely state that mass extinctions – though obviously they do not do this deliberately! – tend to “clear the decks”, wiping out nearly everything present at a given time, whether successful dominant organisms or rare delicate ones. Such wholesale slaughter of species truly re-sets the stage, and no doubt the occurrence of mass extinctions has been responsible for the great variety of sets of species which have dominated over the entire history of life on the Earth: otherwise, evolution might easily have found one successful “type”, and it would have been the only one to dominate. Ironically, then, mass extinctions may be among the most important creative forces for the changes to biota which have characterized the evolution of life on the Earth.
