The
Sixth Great Extinction
By Janet Larsen
07 March, 2004
Earth Policy Institute
Almost
440 million years ago, some 85 percent of marine animal species were
wiped out in the earth's first known mass extinction. Roughly 367 million
years ago, once again many species of fish and 70 percent of marine
invertebrates perished in a major extinction event. Then about 245 million
years ago, up to 95 percent of all animalsnearly the entire animal
kingdomwere lost in what is thought to be the worst extinction
in history.
Some 208 million
years ago, another mass extinction took a toll primarily on sea creatures,
but also some land animals. And 65 million years ago, three quarters
of all speciesincluding the dinosaurswere eliminated.
Among the possible
causes of these mass extinctions are volcanic eruptions, meteorites
colliding with the earth, and a changing climate. After each extinction,
it took upwards of 10 million years for biological richness to recover.
Yet once a species is gone, it is gone forever.
The consensus among
biologists is that we now are moving toward another mass extinction
that could rival the past big five. This potential sixth great extinction
is unique in that it is caused largely by the activities of a single
species. It is the first mass extinction that humans will witness firsthandand
not just as innocent bystanders.
While scientists
are not sure how many species inhabit the planet today, their estimates
top 10 million. Yet each year thousands of species, ranging from the
smallest microorganisms to larger mammals, are lost for good. Some disappear
even before we know of their existence.
The average extinction
rate is now some 1,000 to 10,000 times faster than the rate that prevailed
over the past 60 million years. Throughout most of geological history,
new species evolved faster than existing species disappeared, thus continuously
increasing the planet's biological diversity. Now evolution is falling
behind.
Only a small fraction
of the world's plant species has been studied in detail, but as many
as half are threatened with extinction. South and Central America, Central
and West Africa, and Southeast Asiaall home to diverse tropical
forestsare losing plants most rapidly.
Today nearly 5,500
animal species are known to be threatened with extinction. The IUCNWorld
Conservation Union's 2003 Red List survey of the world's flora and fauna
shows that almost one in every four mammal species and one in eight
bird species are threatened with extinction within the next several
decades. (For access to IUCN's Red List of Threatened Species database,
see www.redlist.org).
Of 1,130 threatened
mammal species, 16 percent are critically endangeredthe highest
threat level. This means that 184 mammal species have suffered extreme
and rapid reduction in population or habitat and may not survive this
decade. Their remaining numbers range from under a few hundred to, at
most, a few thousand individuals. For birds, 182 of the 1,194 threatened
species are critically endangered.
Although the status
of most of the world's mammals and birds is fairly well documented,
we know relatively little about the rest of the world's fauna. Only
5 percent of fish, 6 percent of reptiles, and 7 percent of amphibians
have been evaluated. Of those studied, at least 750 fish species, 290
reptiles, and 150 amphibians are at risk. Worrisome signslike
the mysterious disappearance of entire amphibian populations and fishers'
nets that come up empty more frequentlyreveal that there may be
more species in trouble. Of invertebrates, including insects, mollusks,
and crustaceans, we know the least. But what is known is far from reassuring.
At the advent of
agriculture some 11,000 years ago, the world was home to 6 million people.
Since then our ranks have grown a thousandfold. Yet the increase in
our numbers has come at the expense of many other species.
The greatest threat
to the world's living creatures is the degradation and destruction of
habitat, affecting 9 out of 10 threatened species. Humans have transformed
nearly half of the planet's ice-free land areas, with serious effects
on the rest of nature. We have made agricultural fields out of prairies
and forests. We have dammed rivers and drained wetlands. We have paved
over soil to build cities and roads.
Each year the earth's
forest cover shrinks by 16 million hectares (40 million acres), with
most of the loss occurring in tropical forests, where levels of biodiversity
are high. Ecologically rich wetlands have been cut in half over the
past century. Other freshwater and terrestrial ecosystems have been
degraded by pollution. Deserts have expanded to overtake previously
vegetated areas, accelerated in some cases by overgrazing of domesticated
animals.
A recent study of
173 species of mammals from around the world showed that their collective
geographical ranges have been halved over the past several decades,
signifying a loss of breeding and foraging area. Overall, between 2
and 10 percent of mammal populations (groups of a single species in
a specific geographical location) are thought to have disappeared along
with their habitat.
Direct human exploitation
of organisms, such as through hunting and harvesting, threatens more
than a third of the listed birds and mammals. Other threats to biodiversity
include exotic species, often transported by humans, which can outcompete
and displace native species.
A recent survey
of some 1,100 animal and plant species found that climate change could
wipe out between 15 and 37 percent of them by 2050. Yet the actual losses
may be greater because of the complexity of natural systems. The extinction
of key species could have cascading effects throughout the food web.
As John Donne wrote, "no man is an island." The same is true
for the other species we share this planet with: the loss of any single
species from the web of life can affect many others.
Healthy ecosystems
support us with many servicesmost fundamentally by supplying the
air we breathe and filtering the water we drink. They provide us with
food, medicine, and shelter. When ecosystems lose biological richness,
they also lose resilience, becoming more susceptible to the effects
of climate change, invasions of alien species, and other disturbances.
The 1992 Convention
on Biological Diversity provides a framework for countries to conserve
biological diversity and promote sustainable development. It has been
signed by 168 countries, notably excluding the United States. The parties,
which recently held their seventh conference in February 2004 in Kuala
Lumpur, have set a target of substantially reducing biodiversity loss
by 2010. Yet the convention lacks mechanisms for action and enforcement,
which may make it difficult to achieve the target.
Consciously avoiding
habitat destruction and mitigating the effects of land use change, reducing
the direct exploitation of plants and wildlife, and slowing climate
change can help us stop weakening the very life-support systems we depend
on. While this may be the first time in history that a single species
can precipitate a mass extinction event, it is also the first time in
history that a single species can act to prevent it.
Copyright © 2004 Earth Policy Institute