Organization for Economic Cooperation and Development Category: Organizations, agencies, and programs Date: Establishing treaty signed December 14, 1960; took effect September 30, 1961 W
Trang 15 kilometers thick, and it records the products of
seafloor spreading The top kilometer is composed of
pillowed basalt (Basalt is a dense black lava
contain-ing about 50 percent by weight silicon dioxide formed
as a result of melting of the mantle.) A sequence of
these basalts looks like a huge pile of black to green
pillows Pillows form when basalt lava erupts and is
quenched underwater, perhaps 3 kilometers or more
beneath the surface These basalt flows are fed from a
system of vertical magma channels known as a sheeted
dyke complex (about 1 kilometer thick) The dykes are
supplied magma from an underlying magma
cham-ber, which upon cooling forms gabbro (about 3
kilo-meters thick) Gabbro is the base of the crustal section
and is underlain by mantle rocks called peridotite
Although there is wide agreement that ophiolites
form by seafloor spreading, scientists do not agree on
precisely where these form Early ideas, that
ophio-lites formed at true mid-ocean ridges such as the East
Pacific Rise or Mid-Atlantic Ridge, have been
super-seded by the idea that they form at convergent
mar-gins undergoing extension A more recent idea, that
ophiolites for the most part represent back-arc basin
crust, has been challenged by the hypothesis that
ophiolites form in forearc environments during the
initiation of subduction zones
Ophiolites prove that processes of seafloor
spread-ing and plate tectonics operated at the time the
ophio-lites formed Therefore, the record of ophioophio-lites can
be interpreted as a record of when plate tectonics
occurred on Earth Ophiolites are common in rock
sequences of the following time ages: 1,800 million
years ago, 800 to 400 million years ago, and younger
than 180 million years ago
Robert J Stern
See also: Earth’s crust; Lithosphere; Plate tectonics;
Seafloor spreading
Organization for Economic
Cooperation and Development
Category: Organizations, agencies, and programs
Date: Establishing treaty signed December 14,
1960; took effect September 30, 1961
With a 2009 membership of thirty of the world’s
indus-trialized countries, the Organization for Economic
Cooperation and Development (OECD) is a forum for intergovernmental policy discussions and policy coor-dination as well as a “think tank” that researches and publishes information on a variety of worldwide eco-nomic, financial, and environmental problems and resource issues.
Background The OECD was created as a successor to the Organiza-tion for European Economic CooperaOrganiza-tion (OEEC), which had been established in 1948 At that time, there were only two members, the United States and Canada, and the goal of the organization was to coor-dinate the use of Marshall Plan aid following World War II Today, the goals of the organization are to as-sist member countries in helping their citizens reach better standards of living and to develop sustainable economic growth in member countries, while con-tributing to the development of the economies of nonmember countries The OECD is also concerned with the free flow of international trade, human re-sources, scientific knowledge, and financial capital, while promoting development in industry and agri-culture and protecting the environment
Headquartered in Paris, France, the OECD con-ducts extensive research and publishes on a broad range of economic issues OECD has regional offices
in Germany, Japan, Mexico, and the United States It has worked with developing nations to help them en-ter the world free-market system
In 1994, Mexico became the first new member of the OECD in twenty years In 2007, the organization opened discussions with Chile, Estonia, Israel, Russia, and Slovenia for possible membership The OECD was also contemplating possible membership for five more countries: Brazil, China, India, Indonesia, and South Africa These countries are referred to as “en-hanced engagement countries.” To promote and ad-minister relations with nonmember countries, the OECD created the Center for Cooperation with Non-Members (CCNM) Through CCNM, the OECD has a working relationship with more than seventy non-member countries on all continents
The governance of the OECD is composed of three major bodies The first governing body is the Council
of Member Countries Each country is represented
by an ambassador who leads a delegation from the country The council has decision-making power and hence is an overriding body The second body is the Secretariat, managed by the secretary general, with a
888 • Organization for Economic Cooperation and Development Global Resources
Trang 2staff of some twenty-five hundred employees The
Sec-retariat is organized into fifteen departments and
di-rectorates The third body is formed of committees,
experts, and working groups The OECD committee
structure includes approximately two hundred
com-mittees and working groups The comcom-mittees and
working groups are composed of experts from
mem-bers and nonmember countries The goal of these
committees and working groups is to promote ideas,
review scientific progress, and gather information in
specific policy areas, such as environment,
econom-ics, agriculture, trade, science, technology,
employ-ment, education, taxation, and financial markets The
committees also develop guidelines and codes The
committee meetings are attended by approximately
forty thousand people from member and
nonmem-ber countries The OECD disseminates information
about member countries’ economic indicators and
the issues that their economies face The OECD
com-mittees and working groups focus on five areas: public
governance, trade and liberalization, policy reform
and development, managing new and evolving
tech-nologies, and social protection
Impact on Resource Use
A number of OECD committees work on natural
re-sources, energy, environment, and sustainable
devel-opment issues The Industry and Energy Committee
studies alternate sources of energy in order to lessen
reliance on oil and encourage the development of
multiple energy sources Among the Agricultural
Com-mittee’s concerns are fisheries issues, including
con-servation of stocks
During the Annual Meeting of Sustainable
De-velopment Experts (AMSDE), a group of experts
works on sustainability issues The AMSDE
encour-ages countries to mainstream sustainable
develop-ment approaches and policies into their developdevelop-ment
strategies It also oversees projects geared toward
de-veloping approaches for measuring sustainable
devel-opment and methodologies for conducting
sustain-ability assessments The AMSDE also addresses the
gender issue as it relates to sustainable development
Climate change is the most pressing
environmen-tal issue with which the OECD continues to work A
meeting held in May, 2009, among high-level
repre-sentatives from all member countries and the five
countries being considered for membership endorsed
a major report to provide guidance for the adaptation
to climate change into development cooperation
ac-tivities In addition, the meeting delegates laid out a road map on how to create better working partner-ships between the environment and the development communities One of the priorities of the OECD is to help member countries integrate their responses to climate change into their development strategies The OECD also conducts analysis of the links be-tween environmental, economic, and social issues in order to promote sustainable development in the members and nonmember countries The OECD has
a number of publications on this and related topics, including the impact of the environment on employ-ment, human health, and how government policies affect the environment
The OECD has extensive publications on the sub-ject of water resources and their sustainable manage-ment One of the issues that some countries face is the poor management of water resources that support human consumption, ecosystem needs, and agricul-ture production The OECD reports that to achieve the goal of cutting in half the number of people worldwide without access to safe water and sanitation services by 2015, countries need to develop better management of water supplies, put good governance structures in place, provide reliable financing, and involve the private sector
Since the inception of the organization, the work done by the OECD has been geared toward providing
a framework and a forum in which member countries can compare their experiences, get answers to their problems, and coordinate their policies in the five major areas of the OECD’s activities Over the years, the OECD has provided valuable information and data on social, environmental, agricultural, and finan-cial issues, as well as many other issues, about its mem-ber countries
Colleen M Driscoll, updated by Lakhdar Boukerrou
Web Site Organization for Economic Cooperation and Development
http://www.oecd.org/home/
0,2987,en_2649_201185_1_1_1_1_1,00.html See also: Developing countries; Energy economics; Energy politics; Fisheries; Greenhouse gases and global climate change; Lakes; United Nations Envi-ronment Programme; United Nations Framework Convention on Climate Change; Water pollution and water pollution control
Global Resources Organization for Economic Cooperation and Development • 889
Trang 3Organization of Arab Petroleum
Exporting Countries
Category: Organizations, agencies, and programs
Date: Established 1968
The Organization of Arab Petroleum Exporting
Coun-tries profoundly affected international economics and
politics in 1973 when it curtailed oil shipments to
countries supporting Israel.
Background
In 1968, Kuwait, Libya, and Saudi Arabia founded
the Organization of Arab Petroleum Exporting
Coun-tries (OAPEC) to advance the political and economic
interests of Arab oil-exporting states Unlike the
non-Arab members of the Organization of Petroleum
Ex-porting Countries (OPEC), OAPEC founders shared
a Mideast political agenda: the use of oil as a weapon
in the Arab-Israeli conflict Most of the Persian Gulf’s
oil-exporting states eventually joined OAPEC
On October 17, 1973, OAPEC ministers responded
to a U.S decision to resupply Israel during the
ongo-ing Yom Kippur War by agreeongo-ing to reduce oil
ship-ments to countries supporting Israel OAPEC’s
ac-tion set petroleum-importing states bidding against
one another for the oil upon which their
econo-mies depended The price of oil soared, and OPEC
was able to wrest control over the international
pe-troleum market from the cartel of private oil
compa-nies which had controlled it for much of the
twenti-eth century
Impact on Resource Use
It took the oil-importing countries nearly three years
to adjust their economies to the resultant fourfold
in-crease in the price of oil The political fallout from
OAPEC’s embargo was equally substantial Japan and
most European allies broke with the United States on
Mideast policy in order to avoid having their oil
ship-ments curtailed Energy became a major policy issue
everywhere in the developed industrial world
Joseph R Rudolph, Jr.
Web Site
Organization of Arab Petroleum Exporting
Countries
http://www.oapecorg.org/
See also: Energy economics; Energy politics; Oil em-bargo and energy crises of 1973 and 1979; Oil indus-try; Organization of Petroleum Exporting Countries; Resources as a source of international conflict; Saudi Arabia
Organization of Petroleum Exporting Countries
Category: Organizations, agencies, and programs Date: Established September 14, 1960
Largely ignored for twelve years, the Organization of Petroleum Exporting Countries achieved control over the international oil market in 1973 Since that time, its successes and failures in stabilizing the price of petroleum on the international market have had an impact on and have been affected by the health of the international economy, in general, and the pace of globalization, in particular.
Background Venezuela, Iran, Iraq, Kuwait, and Saudi Arabia estab-lished OPEC in September, 1960, in order to attain
as much control as possible over the international oil market Subsequently, the United Arab Emirates, Qatar, Libya, Algeria, Indonesia, Angola, Nigeria, Gabon, and Ecuador have joined the organization; two of these countries, Indonesia and Gabon, have dropped out
Prior to 1973, OPEC was essentially a bureaucratic entity providing its members with oil market informa-tion Between 1960 and 1973, however, the growing de-mand for imported oil from Western Europe and the United States gradually created a seller’s market In October, 1973, a crisis ensued when OPEC declared
an oil embargo against states friendly to Israel OPEC exploited the situation, taking over the pricing of oil
Impact on Resource Use Because of petroleum’s importance as an energy source, stable and affordable petroleum prices were associated with economic recovery and sustained eco-nomic growth in the Western world for most of the second half of the twentieth century Conversely, high energy prices during the 1970’s and early 1980’s led to
a global recession and an opposition to globalization
890 • Organization of Arab Petroleum Exporting Countries Global Resources
Trang 4in parts of the developed world, which feared job
losses because of outsourcing in economies already
experiencing unacceptably high levels of
unemploy-ment During the thirty-five-year span between 1973
and 2008, three principal factors made creating and
maintaining an affordable price for petroleum on the
world market difficult for OPEC: the organization’s
internal composition; the presence of outside
com-petitors in the oil-export market; and the intermittent
tightening of the oil market, often as a result of
un-foreseen political events
As an organization, OPEC is a confederate
alli-ance, not a governing body for its members Its
suc-cess in maintaining the market price that it establishes
generally requires its members to voluntarily abide
by the production quotas assigned to them If they
overproduce, a glut of oil can bring prices down If
they underproduce, the price of oil may increase
However, the price hike is likely to be followed by
lower demand, and hence lower profits over a
poten-tially long period Establishing compliance between
member states has often been difficult because of the
diverse interests and situations of OPEC member
states OPEC’s effective decision-making periods have
generally been those in which its members followed
the guidelines of its Arabian Peninsula producers:
principally Saudi Arabia, Kuwait, and the oil-exporting
members of the United Arab Emirates These are
gen-erally states with very deep reserves Most have small
populations, and hence, most have a generally low
ability to absorb all of the capital earned by their
ex-ports Moreover, those profits are often enormous
compared to the earnings of other OPEC states
be-cause the cost of producing their oil is often less than
one-tenth or one-fifteenth that of the more populous
oil producers (for example, Nigeria and Venezuela)
with expensive development needs Thus, states like
Saudi Arabia, where the cost of producing a barrel of
oil in 2008 was only two dollars, have a long-term
in-terest in maintaining the price of oil at a level unlikely
to induce their consumers to pursue alternative
en-ergy sources, because they are under no pressure to
make significant profits today and have the reserves to
remain exporters for decades to come They
consti-tute OPEC’s price moderates
Meanwhile, OPEC’s members with high import
bills and large populations, especially if they are
politi-cally ambitious and/or have thin reserves, normally
push for the highest prices possible As a practical
matter, OPEC has generally had to accommodate the
capital needs of these countries by allowing them to exceed their production quotas by approximately 5 percent even in times of abundant oil supply None-theless, even this system has sometimes failed to satisfy its producer states, as in the case of Ecuador, which suspended its membership in OPEC from 1992-2007 because it felt its quotas were set too low and its dues were too high for the benefits it was receiving from OPEC membership In tight energy markets like those that followed the disruption of Iranian and Iraqi exports in 1979 and the loss of Iraq’s export po-tential following the U.S invasion of that country in
2003, the price “extremists” inside OPEC have often gained the upper hand, if only temporarily The resul-tant high energy prices have often led to recessionary conditions in the oil-importing world, to a diminished demand for oil in the world market, and to falling profits for oil-exporting states
During such moments, OPEC has usually had diffi-culty reestablishing order in the petroleum market OPEC contains some of the major oil-exporting states, and its members account for the vast majority of the world’s known petroleum reserves It does not, how-ever, have a monopoly on the global oil-export mar-ket To the contrary, Mexico, Russia, and—in the past—Great Britain have constituted major export ri-vals, and they are joined by a variety of other, smaller non-OPEC exporting states like Norway and Oman Some, like Mexico, have frequently attended OPEC meetings and have functioned within OPEC price guidelines During times of declining demand, how-ever, others have often sold their oil below OPEC’s of-ficial price, resulting in a price-cutting war for shrink-ing markets among OPEC states, gluts of oil on the world market, and a downward plunge in the price of oil that OPEC has had difficulty arresting It was pre-cisely this situation that caused OPEC to fail the first real test of its ability to control the oil market During the second energy crisis in 1979, OPEC had bowed to the demands of its price extremists and allowed the price of a barrel of oil to rise to thirty-six dollars, trig-gering a global recession that drastically reduced the demand for oil at a moment when many producers needed revenue to pay their development bills Un-der pressure from England’s prime minister Margaret Thatcher, who repeatedly offered to sell British North Sea oil at a price below the prices set by OPEC minis-ters scrambling to stabilize the market, Nigeria and other OPEC members also began selling below OPEC’s posted price in the hope of expanding their
Trang 5individual shares of the market In the resultant price
war, the value of a barrel oil dropped dramatically,
eventually to less than seven dollars per barrel, before
order was restored
Finally, unforeseen political events, and sudden
and equally unexpected economic downturns, have
often made establishing and maintaining a stable
price structure difficult for OPEC, even when its
allo-cation process has enjoyed widespread support
among OPEC member states For example, in 1990,
Iraq’s invasion of Kuwait, and the threatened loss of
both Iraqi and Kuwaiti oil exports, created a
specula-tor’s market and sharp rise in the cost of oil on the
spot market to more than ten dollars per barrel above
OPEC’s target price of approximately twenty dollars
per barrel To reestablish order in the market, not
only did Saudi Arabia step up its production, but also
OPEC had to free all of its members from their quota
limits in order to fill the export gap and avoid
an-other, potentially recession-inducing, run-up in the
price of oil Then, at the end of the 1990’s, an
oppo-site challenge to OPEC’s ability to control the market
materialized At a time when many members were ex-ceeding their quotas by approximately 3 percent, an economic crisis in East Asia produced a sudden drop
in anticipated demand and threatened to produce a glut of oil on the world market and to collapse OPEC’s target price, which was then approximately twenty-five dollars per barrel Once again, Saudi action, not OPEC action, largely held the price line Saudi Arabia cut back its production sufficiently to offset the loss of demand from East Asia
In the early twenty-first century, the U.S invasion
of Iraq and the growing demand for oil from China and India provided a new test of OPEC’s ability to pre-vent a recession-inducing price explosion The test was particularly crucial because, at the time, the dif-ference between the world’s demand for imported oil and the amount of oil available for export was less than 1 million barrels of oil per day, and much of that available oil was located in politically volatile areas By
2008, the price of oil had increased fivefold, another global recession was building, and OPEC’s ability to control the market had once again been undone by its
OPEC delegates take an official photograph at the organization’s 2000 summit in Caracas, Venezuela Venezuelan president Hugo Chávez is front-row center (AP/Wide World Photos)
Trang 6price extremists exploiting economic and political
conditions for their individual, short-term advantage
Once again, OPEC had to trim production quotas (in
this case by 4 million barrels/day) in an effort to
re-turn from a late-2008 low of thirty-five dollars per
bar-rel to a target price of seventy-five dollars barbar-rel
What has sustained OPEC as a major influence in
the petroleum resources marketplace despite these
obstacles to its effective functioning has been, on one
hand, the combination of a repetitive economic cycle
that often turns to OPEC’s favor and, on the other, the
failure of oil-importing states to break their oil habits
Low oil prices not only rekindle consumer demand
but also usually bring a renewed complacency to the
Western world Corporations abandon alternative
en-ergy schemes because they are no longer
commer-cially profitable Governments back away from
poli-cies that would lower the use of oil (for example, by
invoking very high taxes on all types of low
kilometer-per-gallon vehicles) but would also be politically
un-popular, especially in eras of cheap oil Thus, the
collapse of oil prices in the 1980’s was quickly
fol-lowed by the abandonment of synthetic oil projects
and gas-from-coal projects throughout the Western
world The concern with developing alternative
en-ergy sources inspired by Iraq’s invasion of Kuwait in
1990 quickly vanished after the liberation of Kuwait
the following year and amid the decade of cheap
imported oil of the 1990’s Similarly, the drop in the
cost of imported oil in 2008 immediately called into
question the life expectancy of the energy-efficient
and alternative-energy projects born when imported
oil was $150 per barrel and gasoline was $1 per liter in
the United States In turn, the possible abandonment
of these and other import-reducing projects quickly
gave rise to projections that a new oil shock would
soon reappear, once the global demand for oil began
to grow significantly again
Joseph R Rudolph, Jr.
Further Reading
Amuzegar, Jahangir Managing the Oil Wealth: OPEC’s
Windfalls and Pitfalls New York: I B Tauris, 2001.
Kohl, Wilfrid L., ed After the Oil Price Collapse: OPEC,
the United States, and the World Oil Market Baltimore,
Md.: Johns Hopkins University Press, 1991
Learsey, Raymond J Over a Barrel: Breaking Oil’s Grip on
Our Future New York: Encounter Books, 2007.
Yergin, Daniel The Prize: The Epic Quest for Oil, Money,
and Power New ed New York: The Free Press, 2008.
Web Site Organization of Petroleum Exporting Countries
http://www.opec.org/home See also: Energy politics; Oil and natural gas distribu-tion; Oil embargo and energy crises of 1973 and 1979; Oil industry; Organization of Arab Petroleum Ex-porting Countries; Peak oil; Resources as a medium of economic exchange; Resources as a source of interna-tional conflict; Saudi Arabia
Orthosilicate minerals
Category: Mineral and other nonliving resources
Orthosilicates are silicate minerals (minerals contain-ing silicon and oxygen) in which the silicate groups are not linked to one another Found worldwide, they are often used as abrasives, and some are used as gem-stones.
Definition Orthosilicates, also known as nesosilicates, are distin-guished from other silicates by the fact that their sili-cate groups (one silicon atom bonded to four oxygen atoms) are not linked to one another The properties
of the different orthosilicates are determined by the type of metal atoms that are bound to the silicate groups Orthosilicates are usually hard minerals with
a glassy luster in a variety of colors Orthosilicates are found worldwide, usually within igneous and meta-morphic rocks The most common orthosilicates are the olivines, which make up about 3 percent of the Earth’s crust by weight
Overview Because of their hardness, orthosilicates are often used as abrasives Many orthosilicates are used as gem-stones because of their varied colors and glassy luster Some orthosilicates are used to line the interior of fur-naces A few orthosilicates are used as sources for cer-tain elements
Orthosilicates, unlike other silicates, have silicate groups that are not linked to neighboring silicate groups These silicate groups may be bound to atoms
of magnesium, iron, aluminum, oxygen, fluorine, manganese, calcium, chromium, zirconium, or
Trang 7nium They may also be bound to hydroxyl groups
(one oxygen atom bonded to one hydrogen atom)
The most common orthosilicates are the olivines,
which contain magnesium or iron Olivines are green,
glassy minerals Transparent olivines, known as
peridots, are used as gemstones Garnets are a group
of orthosilicates with various chemical compositions,
usually containing aluminum atoms in combination
with other metallic atoms In some garnets another
metal takes the place of aluminum Garnets are hard,
glassy minerals in a variety of colors including red,
yel-low, green, and brown Garnets are often used as
abra-sives and gemstones
Orthosilicates in which the silicate groups are
bound only to aluminum atoms exist in three forms:
andalusite (usually pink or red), sillimanite (usually
white), and kyanite (usually light blue) All three
vari-eties are often used to line the inside of furnaces
be-cause they resist high temperatures Kyanite’s unusual
color has led to its use as a gemstone
Topaz is an orthosilicate containing aluminum and
fluorine atoms along with hydroxyl groups It is a very
hard, glassy mineral which is used as a gemstone
To-paz may be colorless, resembling diamond, or may be
yellow, blue, brown, or green Red topaz is rare Yellow
topaz turns pink when subjected to heat, and some
colors of topaz fade when exposed to light
Zircon, which contains zirconium, is a hard, brown,
glassy mineral used as a gemstone or as a source of
zir-conium Sphene, which contains calcium and
tita-nium, is a moderately hard, brown, shiny mineral
used as a source of titanium Staurolite, which
con-tains iron, aluminum, oxygen, and hydroxyl groups, is
a hard, brown, glassy mineral which often occurs as
in-tersecting crystals that form natural crosses used as
decorations
Rose Secrest
See also: Abrasives; Garnet; Gems; Olivine; Silicates;
Silicon
Overgrazing
Category: Environment, conservation, and
resource management
Effects of overgrazing occur where there are more
graz-ing animals than the land and vegetation can
sup-port Overgrazing has negatively affected many regions globally, including large portions of sub-Saharan Af-rica, the southwestern United States, and the Indian subcontinent in Asia Areas that have been severely damaged by overgrazing may experience desertifica-tion, invasion by nonnative plant species, and an in-crease in endangered plant and animal species.
Background Herbivores are animals that feed on plant material, and grazers are herbivores that feed primarily on grasses Common examples are horses, cattle, ante-lope, and bison Overgrazing occurs when grazer pop-ulations exceed the carrying capacity of a specified area Carrying capacity is the number of individual or-ganisms the resources of a given area can support without the area being damaged; carrying capacity varies from region to region In overgrazing condi-tions, there are insufficient grasses and forbs to sup-port the animal population in question Depending
on the grazer’s strategy, emigration or starvation will follow Grasslands can handle normal grazing; only overgrazing adversely affects them
Overgrazing can result either from the presence of too many domestic grazers pastured in a specific area
or the presence of too many wild herbivores on the rangeland The latter can happen when natural events, such as fire, or human developments, such as farming
or suburbanization, crowd wildlife into a more re-stricted range than the organisms had occupied un-der undisturbed conditions In many regions of the world, species of wild herbivores that once experi-enced seasonal migrations over a wide geographic area have become confined to a much smaller range, resulting in more stress on forage resources As part of rangeland management, national parks and wildlife refuges must thin herds of grazers periodically to pre-serve the forage and ensure the long-term survival of the various plant and animal species in an ecosystem
Grasses’ Defenses Against Grazing Grasslands and grazers coevolved; thus, grasses can withstand grazing within the ecosystem’s carrying ca-pacity All plants have a site of new cell growth, called the meristem, where growth in height and girth oc-cur In most plants, the meristem is located at the top
of the plant (the apical meristem) On grasses the meristem is located at the junction of the shoot and root, close to the ground If a plant’s meristem is re-moved, the plant dies If grasses had an apical
Trang 8stem, grazers—and lawn mowers—would kill grasses.
Grasses survive mowing and grazing because they do
not have an apical meristem With a few exceptions,
such as sheep, grazers do not disturb the meristem,
and sheep do so only during overgrazing conditions
At proper levels of grazing, grazing actually stimulates
grass to grow in height in an attempt to produce a
flowering head for reproduction Grazing also
stimu-lates grass growth by removing older plant tissue at
the top that is functioning at a lower photosynthetic
rate
Grazers
Mammalian grazers have high, crowned teeth with a
great area for grinding to facilitate opening of plants’
cell walls in an attempt to release nutrients The cell
wall is composed of cellulose, which is very difficult for
grazers to digest Two major digestive-system grazing
strategies have evolved Ruminants, such as cows and
sheep, evolved stomachs with four chambers to allow
regurgitation in order to chew food twice to maximize
cellulose breakdown In addition, intestinal bacteria
digest the cellulose, releasing fatty acids that nourish
the ruminants Other grazers, such as rabbits and
horses, house bacteria in the cecum, a pouch at the
junction of the small and large intestines These
bac-teria ferment the ingested plant mabac-terial The
fer-mented products of the bacteria nourish these grazers
Global Causes of Overgrazing Overgrazing by livestock has been caused by a variety
of factors In pastoral societies—in which wealth is tra-ditionally measured in terms of how many cattle, sheep, or other grazer individuals are owned—the rangeland is held in common; thus, the commons may become overgrazed This happens because no single individual feels responsible for the land and in-stead worries only about increasing the size of his or her herd Prior to industrialization many pastoral so-cieties were nomadic and followed seasonal migration patterns, which helped prevent overgrazing How-ever, as world population has grown and the amount
of common land available for grazing has shrunk, few truly nomadic societies remain Estimates indicate that approximately 6 percent of the world’s population
is pastoralist, but many formerly nomadic groups— such as the Tuareg in North Africa, Maasai of Kenya, and Tsaatan (also known as Dukha) of Mongolia— now reside in fixed settlements year-round The in-ability to move their herds as they once did can result
in overgrazing in the areas around their villages Although overgrazing is often associated with pas-toral nomads, overgrazing can and does occur where raising livestock is a purely commercial enterprise The western United States, for example, experienced severe overgrazing on public lands in the nineteenth and early twentieth centuries when investors
over-stocked the range in hopes of making
a quick profit in the cattle industry Similarly, private landowners occa-sionally attempt to raise more graz-ers than their pasturage can support, but may not realize they have over-stocked until the land begins to show negative effects, such as erosion or desertification
Effects of Overgrazing Overgrazing can lead to a number
of basic ecological problems Over-grazed regions can experience de-sertification, for example Many of the grasslands of the world occur
in drier regions, such as the Ameri-can Southwest and northern Africa, where annual rainfall is not enough
to support forests Desertification is the intensification and expansion of deserts at the expense of
On overgrazed land in Jonesborough, Tennessee, these goats struggle for survival (AP/
Wide World Photos)
Trang 9ing grasslands When overgrazing occurs along desert
perimeters, the plant removal leads to decreased
shad-ing This decreased shading increases the local air
temperature When the temperature increases, the
air may no longer be able to cool down enough to
re-lease moisture in the form of dew Dew is the primary
source of precipitation in deserts, so without it, desert
conditions intensify Even a slight decrease in desert
precipitation is serious The result is hot and dry
con-ditions, which lead to further plant loss and,
poten-tially, to monocultures
The overgrazing of grasslands, combined with the
existence of nonnative species in an ecosystem, can
re-sult in the endangerment of species of native grasses
and the creation of monocultures in regions where
certain species have been removed A monoculture is
an ecosystem that has lost diversity; that is, one species
becomes dominant and crowds out everything else
Because of the globalization of agriculture and trade,
invasive nonnative species have become a problem in
grasslands worldwide For example, at one time, cattle
in the American Southwest fed exclusively on native
grasses Then nonnative grass species arrived in the
New World via the guts of cows shipped from Europe
The nonnative grasses began to compete with the
na-tive grasses European grass species have seeds with
prickles and burs; southwestern native grasses do not,
making them softer and more desirable to the cattle
Hence, European grasses experienced little, if any,
grazing, while the much more palatable southwestern
native grasses experienced cattle grazing to the point
of overgrazing The result was drastic decline or loss
of native grassland species Animals dependent on
na-tive grassland species must then emigrate or risk
ex-tinction For example, many ecologists conjecture
that the Coachella Valley kit fox in California is
threat-ened because of the loss of grassland habitat upon
which it is dependent
Solutions
Desertification may be irreversible, but the
elimina-tion of grazing along desert perimeters can at least
help to prevent further desertification One method
to reestablish native grass species involves
controlled-burn programs Nonnative grassland species do not
appear to be as fire-resistant as native grass species are
Therefore, controlled-burn programs have been used
in some overgrazed grassland areas in an attempt to
reestablish native grass species and eliminate
nonna-tives If successful, such programs will increase the
biodiversity of the area and will improve the health of the ecosystem
Jessica O Ellison, updated by Nancy Farm Männikkö
Further Reading
Barnes, Robert F., et al., eds Forages, Volume One: An In-troduction to Grassland Agriculture New York:
Wiley-Blackwell, 2003
Brown, Lester R “Eroding Soils and Shrinking
Crop-land.” In Plan B: Rescuing a Planet Under Stress and a Civilization in Trouble New York: Norton, 2003 Collins, Scott L., and Linda L Wallace, eds Fire in North American Tallgrass Prairies Norman:
Univer-sity of Oklahoma Press, 1990
DeBano, Leonard F., Daniel G Neary, and Peter F
Ffolliott Fire’s Effects on Ecosystems New York:
J Wiley, 1998
Gerrish, Jim Management-Intensive Grazing: The Grass-roots of Grass Farming Purvis, Miss.: Green Park
Press, 2004
Gilles, Jere Lee, and Keith Jamtgaard “Overgrazing
in Pastoral Areas: The Commons Reconsidered.”
Sociologica Ruralis 21 (2008): 129-141.
Gordon, I J., and H H T Prins, eds The Ecology of Browsing and Grazing Berlin: Springer, 2008.
Holechek, Jerry L., Rex D Pieper, and Carlton H
Herbel Range Management: Principles and Practices.
5th ed New York: Prentice-Hall, 2004
Humphrey, L R The Evolving Science of Grassland Im-provement New York: Cambridge University Press,
1997
Mysterud, Atle “The Concept of Overgrazing and Its
Role in Management of Large Herbivores.” Wildlife Biology 12, no 2 (2006): 129-141.
Nation, Allan Paddock Shift: Changing Views on Grass-land Farming Jackson, Miss.: Green Park Press, 1997.
Pollan, Michael “Grass: Thirteen Ways of Looking at
a Pasture.” In The Omnivore’s Dilemma: A Natural History of Four Meals New York: Penguin Press,
2006
Stanley, Jim “Overgrazing.” In Hill Country Landowner’s Guide College Station: Texas A&M University Press,
2009
Vallentine, John F Grazing Management 2d ed San
Diego, Calif.: Academic Press, 2001
See also: Biodiversity; Bureau of Land Management, U.S.; Deep ecology; Ecozones and biogeographic realms; Fires; Land ethic; Sagebrush Rebellion; Spe-cies loss; Taylor Grazing Act
Trang 10Category: Mineral and other nonliving resources
Oxygen is an abundant element that is also reactive;
that is, it easily forms compounds—oxides—with other
elements.
Definition
Oxides are combinations of oxygen and metals
Ox-ides are ores of a number of minerals and may
them-selves be used as abrasives
Overview
A simple oxide contains only one metal, whereas a
multiple oxide contains two metals Examples of
sim-ple oxides are tin oxides, manganese oxides, and iron
oxides When one tin atom is combined with two
oxy-gen atoms, tin dioxide (SnO2) is formed Tin dioxide
is also called cassiterite, which is derived from the
Greek word for tin, kassiteros Cassiterite can be found
in high-temperature hydrothermal veins When one manganese atom is combined with two oxygen atoms, manganese dioxide (MnO2, or pyrolusite) is formed Pyrolusite can be found in bog, lake, or shallow ma-rine deposits
If two iron atoms are combined with three oxygen atoms, hematite (Fe2O3) is formed If three iron at-oms are combined with four oxygen atat-oms, magne-tite (Fe3O4) is formed Examples of multiple oxides are magnesium-aluminum-oxide (spinel, MgAl2O4), beryllium-aluminum-oxide (chrysoberyl, BeAl2O4), and iron-titanium-oxide (ilmenite, FeTiO3) Spinel can be found in basic igneous rocks, the original rocks
of the Earth, formed directly from molten mineral matter billions of years ago Chrysoberyl can be found
in granite pegmatites, which are coarse-grained rocks formed after the bulk of the igneous rocks had been produced Ilmenite can be found in both igneous rocks and pegmatite When hydrogen is provided, hy-droxides such as MnO(OH) (manganite), Al(OH)3
(gibbsite), and Mg(OH)2(brucite) may be formed
Percentages of Oxides in Crustal Rocks
Oxide
Unmelted Peridotite
in the Mantle
Basalt Formed at Oceanic Ridges or Rises
Andesite Formed at Subduction Zones
Granitic Rock Along Continental Subduction Zones
Continental Rift Basalt Shale
Sandstone Near the Source
Sandstone Far from the Source Limestone
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
Note: Compositions are given as weight percentages of the oxide in the entire rock.