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MAHATMA GANDHI ~ Truth never damages a cause that is just.
Thursday 20 July 2017
Past extinctions point to a current and future biodiversity crisis
Rapid climate change is a unifying feature of ancient mass extinctions – how bad might it be now?
Arabian Oryx are at risk of extinction becuase of limited habitats and
hunting. Evidence from past mass extinctions suggest many species and
not just endangered ones are under threat.
At one level extinction normal and natural. Most of the diversity of
life on Earth that has ever existed is now gone, and all species will
one day pass from being extant to being extinct. But although it is
normal for species to die out, the normal rate is considered to be quite
low. On average perhaps just one or two species go extinct in any given
year out of all of the bewildering diversity of beetles, mammals,
plants, microbes, worms, fungi and fish. In short, a tiny percentage of
the literally millions of species.
The current rate of loss though, while naturally hard to measure (not
least when we have perhaps only described around 20% of the species on
Earth) is considered to be considerably higher. There is a growing list
of species we know have gone extinct in the last century and plenty of
others are critically endangered or already doomed to extinction (some
species have no known breeding animals, or are known from populations of
only males). Even though many species are still apparently flourishing
they are at a fraction of their previous distribution and populations
– there may be tens of thousands left in one forest, but if they
formerly numbered in the millions and were much more widespread, then
they are now clearly very vulnerable to an outbreak of disease or a
single disaster like a flood or fire. Naturally then, there is huge concern about
what is happening to these species and what it might mean in the future
and scientists are keen to show how bad this may be and for people to
respond.
There have been suggestions for many years that the Earth is in the
process of a sixth mass extinction, which of course somewhat glosses
over the fact that palaeontologists recognise five previous mass
extinction events (as well as many more others which also accounted for
significant fractions of the Earth’s diversity at the time). These go by
the names of the geological ages that they ended, or the names of the
two ages that they separated.
This artist’s conception shows an asteroid crashing into
Earth in an event that scientists believe occurred in the Caribbean
region at the boundary of the Cretaceous and Tertiary periods.
Photograph: NASA/REUTERS
The oldest is the Ordovician-Silurian extinction which is linked to
mass glaciation leading to rapid cooling and then falling sea levels.
This accounted for the deaths of around 85% of marine species, though
life on land was not very diverse at this point and we have a limited
idea of its effect on terrestrial plants and animals.
Next
is the Late Devonian extinction which was responsible for around 75% of
species being lost. In geological terms, this was rather protracted and
may have had multiple causes. Major candidates for part or all of these
effects are sea level changes and ocean anoxia which would then leading
to rapid cooling.
The End Permian extinction is known as ‘the great dying’ and with
good reason. Volcanic eruptions and the mass release of carbon dioxide
and methane led to rapid warming. A staggering 96% of species are
estimated to have died out during this event.
Not too long after this, the end Triassic extinction took out around
70% of species (and incidentally paved the way for the takeover of the
dinosaurs). The origins of this event are not clear but suggestions
include vulcanism and climate change and the planet may have warmed or
cooled rapidly.
Finally, there is the famous Cretaceous-Tertiary extinction which took out most of the dinosaurs (the birds
limped over the line and diversified) which is known to have been the
result of a massive impact from space, perhaps coupled with huge
volcanic eruptions. This would have led to rapid cooling and a dramatic
shift in climate, and once more, around 75% of the species alive are
estimated to have been lost.
Although there is much uncertainty over exactly what triggered some
of these five great extinctions (and various other minor ones), they do
have something in common – sudden and dramatic changes in climate.
Species have a variety of responses to such changes, some can simply
migrate to new environments (birds especially), of if they are already
tolerant of wide ranges of conditions they may be able to stay and
simply get on with things. However, many lack the first option (not
least plants) and lack the adaptions to the latter, and so if such
species are to survive they need to evolve. This, though, takes time. It
can take multiple generations to see any kind of adaptation to a new
environment or different climate (fur can’t evolve overnight) and in the
meantime if conditions are harsh, most individuals are dying or
struggling. Not only that, but inevitably they will rely on other
species and other resources they need (light, water, pollinators for
plants, food from other organisms for animals) which may be in short
supply or themselves have moved or died. It’s no big surprise that any
kind of disturbance to the climate can have a massive and cascading
effect – each species that is massively reduced in population or killed
will take with it multiple other species.
A big tree may provide leaves and nuts for dozens of other species,
pollen for numerous insects, wood for beetles and act as a home for
various birds and bats, plus all the parasites and predators associated
with these. It can take decades for trees to reach maturity, and if in
that time the climate has changed multiple degrees and become much drier
(or wetter, or monsoonal etc.) then both the current population is
doomed and there’s no time for evolution to act on their offspring and
these later generations to adapt.
Coral reefs are suffering from rising temperatures
killing both the corals and the huge number of species that live in and
around the reefs. Photograph: Photograph by Greg Torda/ARC Centre of
Excellence for Coral Reef Studies (CoECRS)
Looking back, it is hard not to look at some of the major extinction
culprits (mass release of carbon dioxide and methane, rapid warming,
climate change, sea level changes) and wince. Add to that the ongoing
issues of pollution, fragmentation of landscapes and habitats,
introduction of invasive species and things like overfishing and it is
hard not to appreciate that the suggestions of an ongoing mass
extinction are no exaggeration. Certainly we have not wiped out 75% of
the living species around us, but mass extinctions take perhaps
thousands or tens of thousands of years to have an effect and reach the
point where so many species have been lost and entire ecosystems have
collapsed or gone. Areas like coral reefs
and rainforests are especially diverse and especially vulnerable – a
few percentage points of the global species total really can be found in
surprisingly small pockets of high diversity and we know that these are
in grave danger and could be gone in a single human lifetime, let alone
the year 7000.
It is hard not to worry thatwhile the numbers of losses might not
seem dramatic now, the problems are ongoing and only likely to worsen.
Species we do not even know exist are being lost, and it’s impossible to
know what cascades may follow. The lesson from the past though is
clear, such rapid changes will lead to dramatic losses and if we are to
preserve the world’s biodiversity in the wild, we desperately need to
protect and indeed restore lost environments, and restore lost
populations as well as trying to halt the man made effects of climate
change and our other destructive tendencies.
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