Extinction is a natural event and, from a geological perspective, routine.
We now know that most species that have ever lived have gone extinct.
The average rate over the past 200 million years is 1-2 species per million
species present per year. The average duration of a species is 1-10 million
years (based on the last 200 million years). There have also been several
episodes of mass extinction, when many taxa representing a wide array
of life forms have gone extinct in the same blink of geological time.
In the modern era, due to human actions, species and ecosystems are threatened
with destruction to an extent rarely seen in Earth history. Probably only
during the handful of mass extinction events have so many species been
threatened, in so short a time.
What are these human actions that threaten biodiversity? There are many
ways to conceive of these; let's consider two.
First, we can attribute the loss of species and ecosystems to the accelerating
transformation of the Earth by a growing human population. As the human
population passes the 6 billion mark, we have transformed, degraded or
destroyed roughly half of the word's forests. We appropriate roughly half
of the world's net primary productivity for human use. We appropriate
most available fresh water, and we harvest virtually all of the available
productivity of the oceans. It is little wonder that species are disappearing
and ecosystems are being destroyed.
Second, we can examine six specific types of human actions that threaten species and ecosystems--the "sinister sextet."
Over-hunting
Over-hunting has been a significant cause of the extinction of hundreds
of species and the endangerment of many more, such as whales and many
African large mammals. Most extinctions over the past several hundred
years are mainly due to over-harvesting for food, fashion, and profit.
Commercial hunting, both legal and illegal (poaching), is the principal
threat. The snowy egret, passenger pigeon, and heath hen are US examples.
At US $16,000 per pound, and US $40,000 to US $100,000 per horn, it is
little wonder that some rhino species are down to only a few thousand
individuals, with only a slim hope of survival in the wild. The recent
expansion of road networks into previously remote tropical forests enables
the bushmeat trade, resulting in what some conservationsist describe as
"empty forests" as more and more wild animals are shot for food.
The pet and decorative plant trade falls within this commercial hunting
category, and includes a mix of legal and illegal activities. The annual
trade is estimated to be at least US $5 billion, with perhaps one-quarter
to one-third of it illegal. Sport or recreational hunting causes no endangerment
of species where it is well regulated, and may help to bring back a species
from the edge of extinction. Many wildlife managers view sport hunting
as the principal basis for protection of wildlife.
While over-hunting, particularly illegal poaching, remains a serious
threat to certain species, for the future, it is globally less important
than other factors mentioned next.
Habitat loss, degradation, fragmentation
Habitat loss, degradation, and fragmentation are important causes of
known extinctions. As deforestation proceeds in tropical forests, this
promises to become the main cause of mass extinctions caused by human activity.
All species have specific food and habitat needs. The more specific these
needs and localized the habitat, the greater the vulnerability of species
to loss of habitat to agricultural land, livestock, roads and cities.
In the future, the only species that survive are likely to be those whose
habitats are highly protected, or whose habitat corresponds to the degraded
state associated with human activity (human commensals).
Habitat damage, especially the conversion of forested land to agriculture
(and, often, subsequent abandonment as marginal land), has a long human
history. It began in China about 4,000 years ago, was largely completed
in Europe by about 400 years ago, and swept across the US over the past
200 years or so. Viewed in this historical context, we are now mopping
up the last forests of the Pacific Northwest.
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Thinking Point |
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The
destruction of tropical rainforests in economically developing countries
may well have serious worldwide ramifications (e.g., loss of species,
global warming). How might foreign policy in first world nations be
guided to support economic growth in rainforest states while fostering
a sustainable environment? |
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In the New World tropics, lowland, seasonal, deciduous forests began
to disappear after 1500 with Spanish and Portuguese colonization of the
New World. These were the forested regions most easily converted to agriculture,
and with a more welcoming climate. The more forbidding, tropical humid
forests came under attack mainly in twentieth century, under the combined
influences of population growth, inequitable land and income distribution,
and development policies that targeted rain forests as the new frontier
to colonize.
Tropical forests are so important because they harbor at least 50 percent,
and perhaps more, of the world's biodiversity. Direct observations, reinforced
by satellite data, document that these forests are declining. The original
extent of tropical rain forests was 15 million square km. Now there remains
about 7.5-8 million square km, so half is gone. The current rate of loss
is estimated at near 2 percent annually (100,000 square km destroyed,
another 100,000 square km degraded). While there is uncertainty regarding
the rate of loss, and what it will be in future, the likelihood is that
tropical forests will be reduced to 10-25 percent of their original extent
by late twenty-first century.
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Discussion
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{Dis: Deforestation in the Pacific Northwest is a volatile topic.}
Discuss arguments for forest harvesting (e.g., supporting the local economy, demand for wood
and paper, rotational harvest, tradition) and arguments against doing so (e.g., soil
erosion, habitat loss, threat to species including salmon). What types of conciliatory
approaches might be taken?
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Habitat fragmentation is a further aspect of habitat loss that often
goes unrecognized. The forest, meadow, or other habitat that remains generally
is in small, isolated bits rather than in large, intact units. Each is
a tiny island that can at best maintain a very small population. Environmental
fluctuations, disease, and other chance factors make such small isolates
highly vulnerable to extinction. Any species that requires a large home
range, such as a grizzly bear, will not survive if the area is too small.
Finally, we know that small land units are strongly affected by their
surroundings, in terms of climate, dispersing species, etc. As a consequence,
the ecology of a small isolate may differ from that of a similar ecosystem
on a larger scale.
For the future, habitat loss, degradation, and fragmentation combined
is the single most important factor in the projected extinction crisis.
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Thinking Point |
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In
Chicago, an infestation of Asian longhorned beetles in 1998--introduced
inadvertently through packing crates imported from China--forced the
destruction of hundreds of trees to control its spread. Create a list
of introduced non-native species you know of, and how they impacted
the environment. |
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Invasion of non-native species
Invasion of non-native species is an important and often overlooked cause of extinctions. The
African Great Lakes--Victoria, Malawi and Tanganyika--are famous for their great diversity of
endemic species, termed "species flocks," of cichlid fishes. In Lake Victoria, a single,
exotic species, the Nile Perch, has become established and may cause the extinction of most of the
native species, by simply eating them all. It was a purposeful introduction for subsistence and
sports fishing, and a great disaster.
Of all documented extinctions since 1600, introduced species appear to have played a role in
at least half. The clue is the disproportionate number of species lost from islands: some 93
percent of 30 documented extinctions of species and sub-species of amphibians and reptiles, 93
percent of 176 species and sub-species of land and freshwater birds, but only 27 percent of
114 species and subspecies of mammals. Why are island species so vulnerable, and why is this
evidence of the role of non-indigenous species? Islands are laboratories for evolution.
Domino effects
Domino effects occur when the removal of one species (an extinction event)
or the addition of one species (an invasion event) affects the entire
biological system. Domino effects are especially likely when two or more
species are highly interdependent, or when the affected species is a "keystone"
species, meaning that it has strong connections to many other species.
A keystone species is one whose influence on others is disproportionately great. A seminal study of
marine invertebrates in the rocky intertidal region of Washington State found that the top predator,
a starfish, facilitated the coexistence of many other invertebrates by selectively consuming
mussels, which otherwise would crowd out other organisms. Thus a keystone species is one whose
presence or absence both directly and indirectly influences other species through food web
connectivity. Contrary to what some may think, not all species are "keystones", and it requires
careful experimental studies to identify keystone species.
Pollution
Pollution from chemical contaminants certainly poses a further threat
to species and ecosystems. While not commonly a cause of extinction, it
likely can be for species whose range is extremely small, and threatened
by contamination. Several species of desert pupfish, occurring in small
isolated pools in the US Southwest, are examples.
Climate change
A changing global climate threatens species and ecosystems. The distribution
of species (biogeography) is largely determined by climate, as is the
distribution of ecosystems and plant vegetation zones (biomes). Climate
change may simply shift these distributions but, for a number of reasons,
plants and animals may not be able to adjust. The pace of climate change
almost certainly will be more rapid than most plants are able to migrate.
The presence of roads, cities, and other barriers associated with human
presence may provide no opportunity for distributional shifts. Parks and
nature reserves are fixed locations. The climate that characterizes present-day
Yellowstone Park will shift several hundred miles northward. The park
itself is a fixed location. For these reasons, some species and ecosystems
are likely to be eliminated by climate change. Mountaintop species are
especially vulnerable. The plants and animals found on high mountains
of the American West include many remnants of a Pleistocene fauna that
long ago was displaced toward the arctic, or upslope. With further warming,
many of these mountaintop species likely will be eliminated.
A changing climate will have many other effects. The southern extent
of the Everglades, today the site of the most ambitious and expensive
restoration project ever undertaken, may be underwater, along with significant
areas of human habitation. Agricultural production likely will show regional
variation in gains and losses, depending upon crops and climate. Some
coral reefs will expand, and others will contract or die off. Ecological
changes due to an altered climate are difficult to forecast, but expected
to be serious.
As a consequence of these multiple forces, many scientists fear that by
end of next century, perhaps 25 percent of existing species will be lost.
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