March 2, 2004-4
Copyright © 2004 Earth Policy Institute
THE SIXTH GREAT EXTINCTION: A Status
Report
Janet Larsen
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
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FOR ADDITIONAL INFORMATION
From Earth Policy Institute
Lester R. Brown, Janet Larsen, and Bernie Fischlowitz-Roberts,
The Earth Policy Reader
(New York: W.W. Norton & Company, 2002).
Lester R. Brown, Eco-Economy:
Building an Economy for the Earth (New York: W.W. Norton
& Company, 2001).
Janet Larsen, "Other Fish in the Sea, But for How
Long?," Eco-Economy Update,
16 July 2003.
Janet Larsen, "Illegal Logging Threatens Ecological
and Economic Stability," Eco-Economy
Update, 21 May 2002.
Janet Larsen, "Our Closest Relatives are Disappearing,"
Eco-Economy Update, 5
March 2002.
From Other Sources
Gerardo Ceballos and Paul R. Ehrlich, "Mammal Population
Losses and the Extinction Crisis," Science, vol. 296 (3 May
2002), pp. 904-07.
F. Stuart Chapin III, et al., "Consequences of Changing
Biodiversity," Nature, vol. 405 (11 May 2000), pp. 234-42.
Convention on Biological Diversity, Executive Secretary,
"Provisional Global Indicators for Assessing Progress Towards
the 2010 Biodiversity Target," document for the Seventh Meeting
of the Conference of the Parties to the Convention on Biological
Diversity, 20 January 2004.
IUCN-World Conservation Union, 2003
IUCN Red List of Threatened Species, on-line database (Gland,
Switzerland, and Cambridge, UK: World Conservation Union-IUCN, 2003).
Chris D. Thomas, et al., "Extinction Risk from Climate
Change," Nature, vol. 427 (8 January 2004), pp. 145-48.
WWF, Living
Planet Report 2002 (Gland, Switzerland: 2002).
LINKS
Biodiversity Dossier of SciDev.net
http:/www.scidev.net
Conservation International
http:/www.conservation.org
Convention on Biological Diversity
http:/www.biodiv.org
Convention on International Trade in Endangered
Species of Wild Fauna and Flora (CITES)
http:/www.cites.org
IUCN Red List of Threatened Species (Database)
http:/www.redlist.org
The Species Survival Commission of the World Conservation
Union (IUCN)
http:/www.iucn.org/themes/ssc
World Conservation Monitoring Center
http:/www.unep-wcmc.org
World Resources Institute
http:/www.wri.org
WWF
http:/www.panda.org
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