Extinction can occur at the population level, called local extinction, or at the species level, called true extinction. True extinction involves the loss of all populations of a species globally. Mass extinctions have occurred throughout history due to various causes like asteroid impacts, volcanic activity, climate change, and more recently, human activity. Currently most extinctions are caused by human interference with habitats and ecosystems. International organizations monitor extinction rates and endangered species to try and control further biodiversity loss.

1. Extinction.
By Awais Bakshy.

2.  What Does Extinction Mean: 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 some what 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.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 decolonization to occur.)

3.  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.

4.  Pseudo extinction (or phyletic extinction): of a species occurs when all
members of the species are extinct, but members of a daughter species remain alive.
The term pseudo extinction refers to the evolution of a species into a new form, with
the resultant disappearance of the ancestral form. Pseudo extinction results in the
relationship between ancestor and descendant still existing even though the ancestor
species no longer exists.
 The classic example is that of the non-avian dinosaurs. While the non-avian dinosaurs
of the Mesozoic died out, their descendants, birds, live on today. As of now it is
accepted that modern birds evolved from Archaeopteryx, a small dinosaur with flight
feathers from the Mesozoic period. Many other families of bird-like dinosaurs also died
out as the heirs of the dinosaurs continued to evolve, but because their birds continue
to thrive in the world today their ancestors are only pseudo extinct.
 Example: Horse evolution while the earliest ancestor became extinct its descendant
survived to produce the modern ‘Equus’.

5.  The Role Of Extinction Through Long Term History:
 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 a biotic 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.

6.  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 tones of material,

7.  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.

8.  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.

9.  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.

10.  Cause Of Mass Extinction:
 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
mollusks 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.

11.  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
kilometers 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.

12.  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.
 At Present:
 Mostly due to human interference.
 The realization of such a cause lead to starting of several international voluntary
agencies to monitor and control extinctions.
 “The red-data book brought out regularly by W. W. F (World Wide Fund for nature,
Formerly IUCN - international union for the conservation of nature and natural
resources) provides a list of animals and plants that are endangered or have become
extinct.