Environmental Encyclopedia

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Environmental Encyclopedia 3

Defoliation

Defenders of Wildlife

Defenders of Wildlife was founded in 1947 in Washington,

D.C. Superseding older groups such as Defenders of Furbear-

ers and the Anti-Steel-Trap League, the organization was es-

tablished to protect wild animals and the habitats that support

them. Today their goals include the preservation of

biodiver-

sity

and the defense of

species

as diverse as gray

wolves

(Canis lupus),

Florida panthers

(Felis concolor coryi), and griz-

zly bears (Ursus arctos), as well as the western yellow-billed

cuckoo (Coccyzus americanus), the

desert tortoise

(Gopherus

agassizii), and Kemp’s Ridley sea turtle (Lepidochelys kempii).

Defenders of Wildlife employs a wide variety of meth-

ods to accomplish their goals, from research and education

to lobbying and litigation. They have achieved a ban on

livestock grazing on 10,000 acres (4,050 ha) of tortoise

habi-

tat

in Nevada and lobbied for restrictions on the interna-

tional wildlife trade to protect

endangered species

in other

countries. In 1988 they successfully lobbied Congress for

funding to expand wildlife refuges throughout the country.

Ten million dollars was appropriated for the Lower Rio

Grande

National Wildlife Refuge

in Texas, two million

dollars to purchase land for a new preserve along the Sacra-

mento River in California, and $1 million for additions to

the Rachel Carson National

Wildlife Refuge

in Maine.

They are currently seeking passage of the National Biological

Diversity Conservation and Research Act and the American

Heritage Trust Fund, which would support the continued

acquisition of natural habitats.

The organization has been at the forefront of placing

preservation on an economic foundation. In Oregon, they

have overseen the establishment of a number of areas from

which to view wildlife on public and private land, thus im-

proving access to natural habitats and linking the

environ-

ment

with the economic benefits of the state’s tourism in-

dustry. Defenders maintains a speaker’s bureau, and they

support a number of educational programs for children de-

signed to nourish and expand their interest in wildlife. But

the group also participates in more direct action on behalf

of the environment. They coordinate grassroots campaigns

through their Defenders Activist Network, which has a

membership of 9,000. They work with the

Environmental

Protection Agency

on a hotline called the “Poison Patrol,”

which receives calls on the use of pesticides that damages

wildlife, and they belong to the Entanglement Network Coali-

tion, which works to prevent the loss of animal life through

entanglement in nets and plastic refuse.

Restoring wolves to their natural habitats has long

been one of the top priorities of Defenders of Wildlife. In

1985, they sponsored an exhibit in

Yellowstone National

Park

and at Boise, Idaho, called “Wolves and Humans,”

which received over 250,000 visitors and won the

Natural

357

Resources

Council of America Award of Achievement for

Education. They have helped reintroduce red and gray

wolves back into the northern Rockies. In order to assist

farmers and the owners of livestock herds that graze in these

areas, Defenders has raised funds to compensate them for the

loss of land. They are also working to reduce and eventually

eliminate the

poisoning

of predators, both by lobbying for

stricter legislation and by encouraging Western farmers to

participate in their guard dog program for livestock.

In order to conserve land, the Defenders have also

launched their own coffee line called “Java Forest.” The

coffee beans are grown under the forest canopy or on farms

which recreate a natural habitat. This reduces the large

amount of land that is used for hybrid coffee beans. Twenty-

five percent of each purchase is being returned to the De-

fenders to be used in other programs.

Defenders of Wildlife has 425,000 members and an

annual budget of $5.2 million. In addition to wildlife viewing

guides for different states, their publications include a bi-

monthly magazine for members called Defenders and In De-

fense of Wildlife: Preserving Communities and Corridors..

[Douglas Smith]

ESOURCES

RGANIZATIONS

Defenders of Wildlife, 1101 14th Street, NW #1400, Washington, D.C.

USA 20005 (202) 682-9400, Email: info@defenders.org, <http://

www.defenders.org>

Defoliation

Several factors can cause a plant to lose its leaves and become

defoliated. Defoliation is a natural and regular occurrence

in the case of deciduous trees and shrubs that drop their

leaves each year with the approach of winter. This process

is aided by an abscission layer that develops at the base of

the leaf petiole, weakens the attachment to the plant, and

eventually causes the leaf to drop. Severe

drought

may also

cause leaves to wilt, dry and drop from a plant. The result

of severe dehydration is usually lethal for herbaceous plants,

although some woody

species

may survive an episode of

drying. Heavy infestation by leaf-eating insects can lead to

partial, or complete defoliation. The

gypsy moth

(Porthetria

dispar) is an important defoliator that attacks many trees,

defoliating, and weakening, or killing them. Parasitic wasps

that feed on the larvae can help to control gypsy moth

outbreaks. Insecticide sprays have also be used to kill the

larvae. Spider mites, any of the plant-feeding mites of the

family Tetranychidae (subclass Acari), feed on house plants

and the foliage and fruit of orchard trees. Heavy infestation

can lead to serious or complete defoliation. Spider mites are

Environmental Encyclopedia 3

Deforestation

controlled with pesticides, although growing

resistance

chemical control agents has made this more difficult, and

alternative control measures are under investigation.

Along with natural causes of defoliation,

chemicals

can cause plants to drop their leaves. The best known and

most widely used chemical defoliators are 2,4,5 trichlorophe-

noxyacetic

acid

(

2,4,5-T

) and 2,4 dichlorophenoxyacetic

acid (

2,4-D

). Both chemicals are especially toxic to broad-

leaf plants. The

herbicide

2,4-D is widely used in lawn care

products to rid lawns of dandelions, clover, and other broad-

leaf plants that interfere with robust turf development. At

appropriate application rates, it selectively kills broad-leaf

herbaceous plants, and has little effect on narrow-leaf grasses.

The uses of 2,4,5-T are similar, although it has been more

widely used against woody species.

A mixture of 2,4-D and 2,4,5-T, in a product called

Agent Orange

has been extensively used to control the

growth and spread of woody trees and shrubs in sites ear-

marked for industrial or commercial development. Agent

Orange saw extensive use by American forces in Southeast

Asia during the Vietnam war, where it was used initially to

clear for power lines, roads, railroads, and other lines of

communication. Eventually, as the war continued, it was

used to spray enemy hiding places, and U.S. military base

perimeters to prevent surprise attack. Food crops, especially

rice, were also targets for Agent Orange to deprive enemy

forces of food. Although Agent Orange and other formula-

tions containing the chlorinated phenoxy acetic acid deriva-

tives are generally lethal to herbaceous plants, woody decidu-

ous plants may survive one or more treatments, depending

on the species treated, concentrations used, spacing of appli-

cations, and weather. In Vietnam it was found that mangrove

forests in the Mekong delta were especially sensitive, and

often killed by a single treatment. A member of the

dioxin

family of chemicals, 2,3,7,8-tetrachlorodibenzo-p-dioxin

(TCDD) has been found to be an accidental but common

contaminant of 2,4,5-T and Agent Orange. Dioxins are very

resistant to attack by

microbes

in the

environment

, and

are apt to persist in soils for a very long time. Although

few disorders have been definitively proven to be caused by

dioxins, their effects on laboratory animals have caused some

scientists to rank them among the most poisonous substances

known. The U.S. government banned some 2,4,5-T con-

taining products in 1979 because of uncertainties regarding

its safety, but its use continues in other products.

[Douglas C. Pratt Ph.D.]

ESOURCES

OOKS

Addicott, F. T. Abscission. Berkeley: University of California Press, 1952.

Gansner, D. A. Defoliation potential of gypsy moth. Radnor, Pa.: U.S. Dept.

of Agriculture, Northeastern Forest Experiment Station, 1993.

358

Teas, H. J. Herbicide toxicity in mangroves. U.S. Environmental Protection

Agency, Office of Research and Development, Environmental Research

Laboratory, Springfield Va.: for sale by the National Technical Information

Service, 1976.

Whiteside, T. The withering rain; Americas herbicidal folly. New York:

Dutton, 1971.

Deforestation

Deforestation is the complete removal of a forest

ecosystem

and conversion of the land to another type of landscape. It

differs from

clear-cutting

, which entails complete removal

of all standing trees but leaves the

soil

in a condition to

regrow a new forest if seeds are available. Humans destroy

forests for many reasons. American Indians burned forests

to convert them to

grasslands

that supported big game

animals. Early settlers cut and burned forest to convert them

to croplands. Between 1600 to 1909, European settlement

decreased forest cover in the United States by 30%. Since that

time, total forest acreage in the United States has actually

increased. In Germany about two-thirds of the forest was

lost through settlement. Food and Agriculture Organization

(FAO) estimated that from 1980 to 1990, 0.9% of remaining

tropical forests were deforested annually (65,251 mi

[169,000 km

] per year), an area equivalent to the state of

Washington. FAO defines forest as land with more than

10% tree cover, natural understory vegetation,

nature

ani-

mals, natural soils, and no agriculture. Analysis of deforesta-

tion is difficult because data is unreliable and the definitions

for “forest” and “deforestation” keep changing; for example,

clear-cuttings which reforest within five years have been

considered deforested in some studies but not in others.

The major direct causes of topical deforestation are

the expansion of shifting agriculture, livestock production,

and fuelwood harvest in drier regions. Forest conversion to

permanent cropland, infrastructure, urban areas, and com-

mercial fisheries also occurs. Although not necessarily re-

sulting in deforestation, timber harvest, grazing, and fires

can severely degrade the forest. The environmental costs

of deforestation can include

species extinction

erosion

flooding

, reduced land productivity,

desertification

, and

climate

change and increased atmospheric

carbon dioxide

As more

habitat

is destroyed, more species are facing extinc-

tions. Deforestation of watersheds causes erosion, flooding,

and

siltation

. Upstream land loses fertile

topsoil

and down-

stream crops are flooded, hydroelectric reservoirs are filled

with

silt

and fisheries are destroyed. In drier areas, deforesta-

tion contributes to desertification.

Deforestation can alter local and regional climates be-

cause evaporation of water from leaves makes up as much

as two-thirds of the rain that falls in some forest. Without

trees to hold back surface

runoff

and block wind, available

moisture is quickly drained away and winds dry the soil,

Environmental Encyclopedia 3

Delaney Clause

sometimes resulting in desert-like conditions. Another po-

tential effect on climate is the large scale release into the

atmosphere

carbon

dioxide stored as organic carbon in

forests and forest soils. In 1980, tropical deforestation re-

leased between 0.4 and 1.6 billion tons of carbon into the

atmosphere, an amount equal to 10–40% of that from

fos-

sil fuels

As a result of misguided deforestation in the moist

and dry tropics, the rural poor are deprived of construction

materials, fuel, food, and cash crops harvested from the

forest. Species extinctions, siltation, and flooding expand

these problems to national and international levels. Despite

these human and environmental costs, wasteful deforestation

continues. Current actions to halt and reverse deforestation

focus on creating economic and social incentives to reduce

wasteful land conversion by providing for wiser ways to

satisfy human needs. Other efforts are the reforestation of

deforested areas and the establishment and maintenance of

biodiversity

preserves.

[Edward Sucoff]

ESOURCES

OOKS

Rowe, R., N. P. Sharma, and J. Browder. “Deforestation: Problems, Causes

and Concerns.” In Managing the World’s Forests, edited by N. P. Sharma.

Dubuque, IA: Kendall Hunt, 1992.

ERIODICALS

Monastersky, R. “The Deforestation Debate.” Science News 144 (July 10,

1993): 26-27.

Delaney Clause

The Delaney Clause is a part of the Federal Food, Drug,

and Cosmetic Act of 1958, Section 409, and it prohibits

the addition to food of any substance that will cause

cancer

in animals or humans. The clause states “no additive will

be deemed to be safe if it is found to induce cancer when

ingested by man or animal, or if it is found, after tests which

are appropriate for the evaluation of the safety of

food

additives

, to induce cancer in man or animals...” The clause

addresses the safety of food intended for human consump-

tion and few, if any, reasonable individuals would argue with

its intent.

There is however, an emerging scientific controversy

over its application, and many now question the merits of the

clause as it is written. For example, safrole occurs naturally as

a constituent in sassafras tea and spices and thus permissibly

under the Delaney Clause, but it is illegal and banned as an

additive to natural root beer because it has been proven a

359

carcinogen

in animal tests. Coffee is regularly consumed

by many individuals, yet more than 70% of the tested

chemi-

cals

that occur naturally in coffee have been shown to be

carcinogenic in one or more tests. Naturally occurring carcin-

ogens are found in other foods including lettuce, apples,

pears, orange juice, and peanut butter. It is important to

note here that the National Cancer Institute recommends

the consumption of fruits and vegetables as part of a regimen

to reduce cancer risk. This is because it is widely believed

that the positive effects of fruits and vegetables far outweigh

the potential hazard of trace quantities of naturally occurring

carcinogens.

It has been estimated that about 10,000 natural pesti-

cides of plants are consumed in the human diet. These

natural pesticides protect the plants from disease and preda-

tion by other organisms. Only a few of these natural plant

pesticides (less than 60) have been adequately tested for

carcinogenic potential and of these about half of them tested

positive. Bruce N. Ames and his associates at the University

of California estimate that 99.99% of the pesticides ingested

by humans are not residues of chemicals applied by humans

but are chemicals that occur naturally and therefore legally.

It has been argued that such naturally occurring chemicals

are less hazardous and thus differ in their cancer-causing

potential from synthetic chemicals. But this does not appear

to be the case; although the mechanisms for chemical carci-

nogenesis are poorly understood, there seems to be no funda-

mental difference in how natural and synthetic carcinogens

are metabolized in the body.

The Delaney Clause addresses only the issue of addi-

tives to the food supply. It is noteworthy that salt, sugar,

corn syrup, citric

acid

and baking soda comprise 98% of the

additives listed, while chemical additives, which many fear,

constitute only a small fraction. It should also be noted that

there are other significant safety issues pertaining to the food

supply, including pathogens which cause botulism, hepatitis,

and salmonella food

poisoning

. Of similar concern to health

are traces of environmental pollutants, such as

mercury

fish, and cooking-induced production of carcinogens, such

benzopyrene

in beef cooked over an open flame. Excess

fat in the diet is also thought to be a significant health

hazard.

Scientists who are rethinking the significance of the

“zero risk” requirement of the Delaney Clause do not believe

society should be unconcerned about chemicals added to

food. They simply believe the clause is no longer consistent

with current scientific knowledge, and they argue that chem-

icals added in trace quantities, for worthwhile reasons, should

be considered from a different perspective. See also Agricul-

tural chemicals; Agricultural pollution; Drinking-water sup-

ply; Food and Drug Administration

[Robert G. McKinnell]

Environmental Encyclopedia 3

Demographic transition

ESOURCES

ERIODICALS

Corliss, J. “The Delaney Clause: Too Much of a Good Thing?” Journal of

the National Cancer Institute 85 (1993): 600-603.

Gold, L. S., et al. “Rodent Carcinogens: Setting Priorities.” Science 258 (9

October 1992): 261-265.

Demographic transition

Developed by demographer Frank Notestein in 1945, this

concept describes the typical pattern of falling death and

birth rates in response to better living conditions associated

with economic development. This idea is important, for it

offers the hope that developing countries will follow the

same pathway to population

stability

as have industrialized

countries. In response to the Industrial Revolution, for exam-

ple, Europe experienced a population explosion during the

nineteenth century. Emigration helped alleviate overpopula-

tion, but European couples clearly decided on their own to

limit family size.

Notestein identified three phases of demographic tran-

sition: preindustrial, developing, and modern industrialized

societies. Many authors add a fourth phase, postindustrial.

In phase one, birth rates and death rates are both high with

stable populations. As development provides a better food

supply and

sanitation

, death rates begin to plummet, mark-

ing the onset of phase two. However, birth rates remain

high, as families follow the pattern of preceding generations.

The gap between high birth rates and falling death rates

produces a population explosion, sometimes doubling in less

than 25 years.

After one or two generations of large, surviving fami-

lies, birth rates begin to taper off, and as the population

ages, death rates rise. Finally a new balance is established,

phase three, with low birth and death rates. The population

is now much larger yet stable. The experience of some Euro-

pean countries, especially in Central Europe and Russia,

suggests a fourth phase where populations actually decline.

This may be a response to past hardships and oppressive

political systems there, however.

Historically, birth rates have always been high. With

few exceptions population explosions are linked to declining

death rates, not rising birth rates. Infants and young children

are especially vulnerable; sanitation and proper food are vital.

Infant survival is seen by some as a threat because of the built-

in momentum for

population growth

. However, history

reveals that there has been no decline in birth rates which

has not been preceded by a drop in infant

mortality

.Ina

burgeoning world this makes infant survival a matter of top

priority. To this end, in 1986 the United Nations adopted

a program with the acronym GOBI: Growth monitoring,

Oral rehydration therapy (to combat killer diarrhea), Breast

360

feeding, and Immunization against major

communicable

diseases

. See also Child survival revolution; Population

Council

[Nathan H. Meleen]

ESOURCES

OOKS

Cunningham, W. P., and B. W. Saigo. Environmental Science: A Global

Concern. 2nd ed. Dubuque, IA: William C. Brown, 1992.

Maddox, J. The Doomsday Syndrome. New York: McGraw-Hill, 1972.

ERIODICALS

Keyfitz, N. “The Growing Human Population.” Scientific American 261

(September 1989): 7-16.

Dendroica kirtlandii

see

Kirtland’s warbler

Denitrification

A stage in the

nitrogen cycle

in which

nitrates

in the

soil

or in dead organic matter are converted into nitrite,

nitrous

oxide

, ammonia, or (primarily) elemental

nitrogen

. The

process is made possible by certain types of bacteria, known

as denitrifying bacteria. Denitrification is a reduction reac-

tion and occurs, therefore, in the absence of oxygen. For

example, flooded soil is likely to experience significant deni-

trification since it is cut off from atmospheric oxygen. Al-

though denitrification is an important process for the decay

of dead organisms, it can also be responsible for the loss of

natural and synthetic fertilizers from the soil.

Deoxyribose nucleic acid

Deoxyribose

nucleic acid

(DNA) molecules contain genetic

information that is the blueprint for life. DNA is made

up of long chains of subunits called nucleotides, which are

nitrogenous bases attached to ribose sugar molecules. Two of

these chains intertwine in the famous double helix structure

discovered in 1953 by James Watson and Francis Crick.

The genetic information contained in DNA molecules

is in a code spelled out by the linear sequence of nucleotides

in each chain. Each group of three nucleotides makes up a

codon, a unit resembling a letter in the alphabet. A string

of codons effectively spells a word of the genetic message.

This message is expressed when enzymes (cellular pro-

teins) synthesize new proteins using a copy of a short segment

of DNA as a template. Each nucleotide codon specifies

which amino

acid

subunit is inserted as the protein is

formed, thus determining the structure and function of the

Environmental Encyclopedia 3

Desalinization

proteins. Because the chains are very long, a single DNA

strand can contain enough information to direct the synthesis

of hundreds of different proteins. Since these proteins make

up the cell structure and the machinery (enzymes) by which

cells carry out the processes of life, such as synthesizing more

molecules including more copies of the DNA itself, DNA

can be said to be self-replicating. When cells divide, each

of the new cells receives a duplicate set of DNA molecules

giving them the necessary information to live and reproduce.

See also Ribosenucleic acid

Department of Agriculture

see

U.S. Department of Agriculture

Department of Energy

see

U.S. Department of Energy

Department of Health and Human

Services

see

U.S. Department of Health and

Human Services

Department of the Interior

see

U.S. Department of the Interior

Desalinization

Desalinization, also known as “desalination,” is the process

of separating sea water or

brackish

water from their dis-

solved salts. The average salt content of the ocean water is

about 3.4% (normally expressed as 34 parts per thousand).

The range of salt content varies from 18 parts per thousand

in the North Sea and near the mouths of large rivers to a

high of 44 parts per thousand in locked bodies of water

such as the Red Sea, where evaporation is very high. The

desalination process is accomplished commercially by either

distillation or reverse osmosis (RO).

Distillation of sea water is accomplished by boiling

water and condensing the vapor. The components of the

distillation system consist of a boiler and a condenser with

a source of cooling water. Reverse osmosis is accomplished

by forcing filtered sea water or brackish water through a

reverse osmosis membrane. In a reverse osmosis process,

approximately 45% of the pressurized sea water goes through

membranes and becomes fresh water. The remaining brine

(concentrated salt water) is returned to the sea.

361

In 1980, the United Nations declared 1981–1990 as

the “International Drinking Water Supply and

Sanitation

Decade.” The objective was to provide safe drinking water

and sanitation to developing nations. Despite some progress

in India, Indonesia, and a few other countries, the percentage

of the world population with access to safe drinking water

has not changed much since that declaration. In the period

between 1990 and 2000, the amount of people with access

has only increased by 5%.

The World Health Organization (WHO) estimates

that only two in five people in the

less developed countries

(LDCs) have access to safe drinking water. The WHO also

estimates that at least 25 million people of the LDCs die

each year because of polluted water and from water-born

diseases such as

cholera

, polio, dysentery, and typhoid.

Whether by distillation or by reverse osmosis, desalination

of water can transform water that is unusable because of its

salinity

into valuable fresh water. This could be an important

water source in many drought-prone areas.

Desalination plants, distribution, and functions

There are approximately 7,500 desalination plants

worldwide. Collectively they produce less than 0.1% of the

world’s fresh water supply. This supply is equal to about 3.5

billion gal per day (13 million l). The cost and the feasibility

of producing desalinated water depends upon the cost of

energy, labor, and relative costs of desalinated water to that

of imported fresh water. It is estimated that in the United

States, commercial desalinated water produced from sea

water by reverse osmosis costs about $3 per 1,000 gal (3,785

l). This price is four to five times the average price currently

paid by urban consumers for drinking water and over 100

times the price paid by farmers for

irrigation

water. The

current energy requirement is approximately three kilowatt

hours of electricity per one gallon of fresh water extracted

from sea water. Currently, using desalinated water for agri-

culture is cost prohibitive.

About two-thirds of the desalination water is produced

in Saudi Arabia, Kuwait, and North Africa. Several small-

scale reverse osmosis plants are now operating in the United

States, including California (Santa Barbara, Catalina Island,

and soon in Ventura and other coastal communities). Gener-

ally, desalination plants are used to supplement the existing

fresh water supply in areas adjacent to oceans and seas such

as southern California, the Persian Gulf region, and other

dry coastal areas. Among the advantages of desalinized water

are a dependable water supply regardless of rainfall patterns,

elimination of

water rights

disputes, and the preservation

of the fresh water supply, all of which are essential for existing

natural ecosystems.

Reverse osmosis

Reverse osmosis involves forcing water under pressure

through a

filtration

membrane that has pores small enough

Environmental Encyclopedia 3

Desert

to allow water molecules to pass through but exclude slightly

larger dissolved salt molecules. The basic parts of a reverse

osmosis system include onshore and offshore components.

The onshore components consist of a water pump, an electri-

cal power source, pre-treatment filtration (to remove sea-

weed and debris), reverse osmosis units connected in series,

solid waste

disposal equipment, and fresh water pumps.

The offshore components consist of an underwater intake

pipeline, approximately 1,093 yd (1 km) from shore, and a

second pipeline for brine

discharge

Small reverse osmosis units for home use with a few

gallons-per-day capacity are available. These units use a

disposable reverse osmosis membrane. Their main drawback

is that they waste four to five times the volume of water

they purify.

Producing potable water from sea water is an energy

intensive, costly process. The high cost of producing desali-

nized water limits its use to domestic consumption. In areas

such as the Persian Gulf and Saudi Arabia where energy is

plentiful at a low cost, desalinized water is a viable option

for drinking water and very limited greenhouse agriculture.

The notion of using desalinized water for wider agricultural

purposes is neither practical nor economical at today’s energy

prices and available technology.

[Muthena Naseri]

ESOURCES

OOKS

Kaufman, D. G., and C. M. Franz. Biosphere 2000: Protecting Our Global

Environment. New York: Harper-Collins, 1993.

Nebel, B. J., and R. T. Wright. Environmental Science: The Way the World

Works. 4th Edition. Englewood Cliffs, NJ: Prentice Hall, 1993.

THER

Lizarraga, S., and D. Brown. “Fresh Water from Santa Barbara Seas.”

Reprinted from Desalination and Water Reuse, 1992. Santa Barbara: Depart-

ment of Water Resources.

Desert

Six percent of the world’s land surface is desert, a

biome

which less than 10 in (25 cm) of precipitation occurs per year

or any place where evaporation greatly exceeds precipitation,

resulting in a lack of available moisture. Sometimes any area

lacking the necessary conditions to support life is called a

desert. Deserts occur around latitudes 30 degrees north and

south where masses of dry circulating air descend to the

earth’s surface. There are three kinds of deserts—hot (such

as Sahara), temperate (such as the Mojave), and cold (such

as the Gobi). The area of global desert is increasing yearly,

as marginal lands become degraded by human misuse re-

sulting in

desertification

. Deserts are potential sites for the

362

production of electricity using banks of solar cells or parabolic

solar collectors. The lack of water and remoteness of some

deserts may make them attractive places to store nuclear and

other hazardous wastes.

Desert tortoise

The

desert

tortoise (Gopherus agassizii) is a large, herbivo-

rous, terrestrial turtle of the family Testudinidae. It is found

in both the southwestern United States and in northwestern

Mexico. It is the official reptile in the states of California

and Nevada. No other turtle in North America shares the

extreme conditions of the habitats occupied by the desert

tortoise. It inhabits desert oases, washes, rocky hillsides, and

canyon bottoms with sandy or gravelly

soil

under hot,

arid

conditions.

Desert tortoises dig into dry, gravelly soil under bushes

in arroyo banks or at the base of cliffs to construct a burrow,

which is their home. Climatic conditions dictate daily activ-

ity patterns of these tortoises, and they can relieve the prob-

lems of high body temperature and evaporative water loss

by retreating into their burrows. Since many desert tortoises

live in areas devoid of water, except for infrequent rains,

they must rely on their food for their water.

The active period for the desert tortoise is from March

through September, after which they enter a hibernation

period. Nesting and egg laying activities extend from May

through July. Desert tortoises lay an average of five moisture-

proof eggs, an

adaptation

that helps retain water in its

harsh

environment

. These tortoises reach sexual maturity

at 15–20 years, and they have a life span of up to 80 years.

The desert tortoise is very sensitive to human distur-

bances, and this has led to the decimation of many of its

populations throughout the desert southwest. The Beaver

Dam Slope population of southwestern Utah has been stud-

ied over several decades and shows some of the general

tendencies of the overall population. In the 1930s and 1940s

the desert tortoise population in this area exhibited densities

of about 160 adults per square mile. By the 1970s this density

had fallen to less than 130 adults per square mile, and more

recent studies indicate the level is now about 60 adults per

square mile. In southeastern California at least one popula-

tion reaches densities of 200 adults per square mile, but

overall tendencies show that populations are drastically de-

clining. Recent estimates indicate that there are about

100,000 individual desert tortoises existing in the Mojave

and Sonoran deserts.

Desert tortoise populations are listed as threatened in

Arizona, California, Nevada, and Utah. Numerous factors

are contributing to its decline and vulnerability.

Habitat

loss

through human encroachment and development, overcol-

Environmental Encyclopedia 3

Desertification

A desert tortoise in the Desert Tortoise Natural

Area, Mojave Desert, California. (Photograph by

Dan Suzio. Reproduced by permission.)

lecting for the

pet trade

, and vandalism—including shoot-

ing tortoises for target practice and flipping them over onto

their backs, causing them to die from exposure—have deci-

mated populations. Other factors contributing to their de-

cline are grazing livestock, which trample them or their

burrows, and mining operations, which also causes respira-

tory infections among the desert tortoises. Numerous desert

tortoises have been killed or maimed by

off-road vehicles

which also collapse the tortoises’ burrows. Concern is

mounting as

conservation

efforts seem to be having little

effect throughout much of the desert tortoise’s range.

[Eugene C. Beckham]

ESOURCES

OOKS

Campbell, F. “The Desert Tortoise.” Audubon Wildlife Report. San Diego:

Academic Press, 1988–89.

Ernst, C., and R. Barbour. Turtles of the United States. Lexington: University

Press of Kentucky, 1972.

———. Turtles of the World. Washington, DC: Smithsonian Institute

Press, 1989.

Desertification

About one billion people live in

arid

or semiarid

desert

lands that occupy about one third of the world’s land surface.

In these drier parts of the world, deserts are increasing rapidly

from a combination of natural processes and human activi-

ties, a process known as desertification or land degradation.

An annual rainfall of less than 10 in (25 cm) will produce

a desert anywhere in the world. In the semiarid areas along

the desert margins, where the annual rainfall is around 16

363

in (40 cm), the

ecosystem

is inherently fragile with seasonal

rains supporting the temporary growth of plants. Recent

changes in the

climate

of these regions have meant that the

rains are now unreliable and the lands that were once semi-

arid are now becoming desert. The process of desertification

is precipitated by prolonged droughts, causing the top layers

of the

soil

to dry out and blow away. The eroded soils

become unstable and compacted and do not readily allow

for seeding. This means that desertified areas do not regener-

ate by themselves but remain bare and continue to erode.

Desertification of grazing lands or croplands is accompanied,

therefore, by a sharp drop in the productivity of the land.

Natural desertification is greatly accelerated by human

activities that leave soils vulnerable to

erosion

by wind and

water. The drier

grasslands

with too little rain to support

cultivated crops have traditionally been used for grazing

livestock. When semiarid land is overgrazed (by keeping too

many animals on too little land), plants that could survive

moderate grazing are uprooted and destroyed altogether.

Since plant roots no longer bind the soil together, the ex-

posed soil dries out and is blown away as dust. The destruc-

tion and removal of the

topsoil

means that soil productivity

drops drastically. The obvious solution to desertification

caused by

overgrazing

is to limit grazing to what the land

can sustain, a concept that is easy to espouse but difficult

to practice.

In the

Sahel

zone along the southern edge of the

Sahara desert, settled agriculture and overgrazing livestock

on the fragile scrublands have led to widespread soil erosion.

Nomadic pastoralists, who have traditionally followed their

herds and flocks in search of new pastures, are now prevented

by national borders from reaching their chosen grazing

grounds. Instead of migrating, the nomads have been en-

couraged to settle permanently and this has led to their herds

overgrazing available pastures.

Other human factors leading to desertification include

over-cultivation,

deforestation

, salting of the soil through

irrigation

, and the plowing of marginal land. These destruc-

tive practices are intensified in developing countries by rapid

population growth

, high population density, poverty, and

poor land management. The consequences of desertification

in some countries mean intensified

drought

and

famine

and lowered standards of living. It is estimated that desertifi-

cation worldwide has claimed an area the size of Brazil (2

billion acres or 810 million ha) in the past 50 years. Each

year new deserts consume an area the size of Belgium (15

million acres or 6 million ha), most of which is in the African

Sahel.

In marginal areas throughout the world, traditional

farming practices can lead to desertification. Plowing turns

the top layer of the soil upside down, burying and killing

weeds but exposing bare soil to erosion. In arid areas the

Environmental Encyclopedia 3

Desertification

World deserts and areas at risk of desertification. (Beacon Press. Reproduced by permission.)

exposed soil dries out rapidly and is easily lost through wind

erosion.

The processes of erosion and soil formation vary with

climate and with the composition of the parent material. In

balanced ecosystems, soil lost to erosion is replaced by new

soil created by natural processes. On average, new soil is

formed at a rate of about 5 tons an acre (12.5 tons per ha)

per year, which is equivalent to a layer of soil about 0.2 in

(0.4 cm) thick. This means that soils can sustain an erosion

rate of up to 5 tons per acre per year and still remain in

balance. However, much of the world’s crop and forest land

is not within this balance (with erosion running at two to

10 times the tolerable rate). In the United States about 6

tons of topsoil are lost for every ton of grains produced.

Forests are cut down for many different reasons. Some

are cleared for agriculture, some for construction, some for

paper products, and some to meet cooking and heating

needs. Unfortunately, deforestation results in more than just

the loss of trees, for soil is eroded, nutrients are lost to the

ecosystem, and the water cycle is disrupted. The roots of

the trees serve to bind the soil together and to hold water

in the ecosystem, while the leaves of the trees break the

364

force of the rain and allow it to soak into the topsoil. The

result is that surface

runoff

from a forested hillside is half

as much as from a grass-covered slope. Additionally, water

soaking into the ground (rather than running off) leads

to the natural recharge of

groundwater

and other

water

resources

. Water and soil runoff from deforested hillsides

cause

flooding

and

siltation

of agricultural and aquatic

ecosystems in adjacent lowlands. Forests are also more effi-

cient at reabsorbing and

recycling

the nutrients released

from decaying

detritus

than are grasslands. Clearing forests

therefore exposes the soil to both erosion and

nutrient

loss

and alters the recharge of water reserves in the ecosystem.

Industrialized countries experienced a period of in-

tense deforestation during the Industrial Revolution and

even today much of the land has low productivity. Fortu-

nately, most of these countries are now reforesting faster

than they are deforesting. This is possible because their

population growth is low, their agricultural production per

acre is high, and their need for fuelwood for cooking and

heating is optional, since

fossil fuels

and electricity are

widely available. All of these factors release the pressure to

further deforest the land.

Environmental Encyclopedia 3

Detoxification

In developing countries, on the other hand, high popu-

lation growth rates and widespread poverty put pressure on

the forests. Trees are needed for firewood, charcoal, and

export, and the land is needed for farmland. In some devel-

oping countries deforestation exceeds replanting by five

times. In Ethiopia, population and economic pressures

pushed people to deforest and cultivate hillsides and margin-

ally dry lands, and more than 1 billion tons of topsoil per

year are now lost, resulting in recurrent famines. In parts of

India and Africa there is so little wood available that dried

animal dung is used to fuel cooking fires, an act that further

robs the soil of potential nutrients. In Brazil, soil erosion and

desertification have resulted from the conversion of forests to

cattle ranches. In China, about one-third of its agricultural

land has been lost to erosion, and the story is similar for

many other countries.

The answer to erosion from deforestation is reforesta-

tion, better forest

conservation

, and better

forest manage-

ment

to increase productivity. Planting trees on hillsides

is particularly effective. Reforesting desertified areas first

requires mulching the soil to hold moisture and the protec-

tion of the seedlings for several years until natural processes

can regenerate the soil. Using these methods, Israel has

achieved spectacular success in bringing desertlands (a prod-

uct of past desertification) back to agriculture.

Desertification and its agents, deforestation and ero-

sion, have been powerful shapers of human history. Agricul-

ture had its roots in the once fertile crescent of the Middle

East and in the Mediterranean lands. However, deforesta-

tion, overgrazing, and poor agricultural practices have turned

once-productive pastureland and farmland into the near de-

serts of today. It is thought by some that deforestation and

desertification may have even contributed to the collapse of

the Greek and Roman Empires. Similar fates may have

befallen the Harappan civilization in India’s Indus Valley,

the Mayan civilization in Central America, and the Anasazi

civilization of Chaco Canyon, New Mexico, in what is today

desertland.

[Neil Cumberlidge Ph.D.]

ESOURCES

OOKS

Dejene, A. Environment, Famine, and Politics in Ethiopia: A View from the

Village. Boulder, CO: L. Rienner Publishers, 1990.

Grainger, A. Desertification: How People Make Deserts, How People Can

Stop, and Why They Don’t. London: International Institute for Environment

and Development, 1982.

McLeish, E. The Spread of Deserts. Austin, TX: Steck-Vaughn Library,

1991.

365

Design for disassembly

For

recycling

to be broadly applicable, many environmental-

ists believe that products should be cheaper and easier to

take apart so that they can be put together again and reused

in other products.

Many large companies like Whirlpool, Digital Equip-

ment, 3M, and General Electric have begun to design prod-

ucts in this regard. The BMW car, Z1, is considered to be

the first to incorporate the concepts of disassembly. The

plastic exterior of the car can be removed from the metal

chassis in 20 minutes. The doors, bumpers, and front, rear,

and side panels have been manufactured with recyclable

plastic, and pop-in/pop-out fasteners replace screws and

glues, whenever possible.

Composite materials, on the other hand, which are

being used on a more widespread basis are difficult to recycle.

Small juice boxes, for instance, which have become very

popular are made of many layers of plastic, paper, and

alumi-

num

, which cannot be separated. See also Recyclables; Reuse;

Waste management; Waste reduction

Detergents

A group of organic compounds that cause foaming and serve

as cleansing agents based on their surface-active properties.

A detergent molecule is termed surface-active because a

portion of the molecule is hydrophilic and a portion is hy-

drophobic. It will therefore collect at the interface of water

and another medium such as a gas bubble. Bubbles are

stabilized by the surface-active molecules so that when the

bubbles rise to the top of the bulk liquid, they maintain their

integrity and form a foam. Surface-active agents can also

agglomerate by virtue of their hydrophilic/hydrophobic

na-

ture

to form micelles that can dissolve, trap, and/or envelop

soil

particles, oil, and grease. Surface-active agents are some-

times called surfactants. Synthetic detergents are sometimes

referred to as syndets.

Detoxification

When many toxic substances are introduced into the

envi-

ronment

, they do not remain in their original form, but are

transformed to other products by a variety of biological and

non-biological processes. The

chemicals

and their transfor-

mation products are degraded, converted progressively to

smaller molecules, and eventually utilized in various natural

cycles, such as the

carbon cycle

. Toxic metals are not de-

graded but are interconverted between available and non-

available forms. Some organic compounds, such as polychlo-

rinated biphenyl (PCBs), are degraded over a period of many

Environmental Encyclopedia 3

Detoxification

years to less toxic compounds, while compounds such as the

organophosphate

insecticides may break down in only a

few hours.

The chemical transformations that occur may either

increase (referred to as intoxication, or activation if the parent

compound was nontoxic) or decrease (referred to as detoxifi-

cation) the toxicity of the original compound. For example,

elemental

mercury

, which has low toxicity, can be converted

to methylmercury, a very hazardous chemical, through

methylation

. Parathion is a fairly nontoxic insecticide until

it is converted in a living system or by photochemical reac-

tions to paraoxon, an extremely toxic chemical. However,

parathion can also be degraded to less toxic products by the

process of hydrolysis.

In microbial degradation, the ultimate fate of the toxic

chemicals may be mineralization (that is, the conversion of

an organic compound to inorganic products), which results

characteristically in detoxification; however, intermediates

in the degradation sequence, which may be toxic or have

unknown toxicity, may persist for a period of time, or even

indefinitely. Likewise, since degradation pathways may con-

tain many steps, detoxification may occur early in the degra-

dation pathway, before mineralization has occurred. Detoxi-

fication may also be accomplished biologically through

cometabolism

, the

metabolism

microorganisms

of a

compound that cannot be used as a

nutrient

; cometabolism

does not result in mineralization, and organic transformation

products remain. Studies have also shown that the structure

and toxicity of many organic compounds can be altered by

plants.

In modifying a toxic chemical, detoxifying processes

destroy its actual or potential harmful influence on one or

more susceptible animal, plant, or microbial

species

. Detox-

ification may be measured with the use of bioassays. A

bioassay

involves the determination of the relative toxicity

of a substance by comparing its effect on a test organism

with the conditions of a control. The scale or degree of

response may include the rate of growth or decrease of a

population, colony, or individual; a behavioral, physiological,

or reproductive response; or a response measuring

mortality

Bioassays can be used for environmental samples through

time to determine detoxification of chemicals.

Both acute and chronic bioassays are used to assess

detoxification. In an acute bioassay, a severe and rapid re-

sponse to the toxic chemical is observed within a short period

of time (for example, within four days for fish and other

aquatic organisms and within 24 hours to two weeks for

mammalian species). Detoxification of a chemical may be

detected if there is a decrease in the observed toxicity of a test

solution over the time of the acute test, indicating removal

of the toxic chemical by degradation or other processes.

366

Similarly, an increase in toxicity could indicate the formation

of a more toxic transformation product.

Chronic bioassays are more likely to provide informa-

tion on the rates of degradation, transformation, and detoxi-

fication of toxic compounds. Partial or complete life cycle

bioassays may be used, with measurements of growth, repro-

duction, maturation, spawning, hatching, survival, behavior,

and

bioaccumulation

Detoxification of chemicals should also be measured

by toxicity testing that involves changes in different organ-

isms and interactions among organisms, especially if the

chemicals are persistent and stable and may accumulate and

magnify in the

food chain/web

. Model ecosystems can be

used to simulate processes and assess detoxification in a

terrestrial-aquatic

ecosystem

. A typical ecosystem could

include

soil

organisms, lake-bottom

fauna

, a plant, an in-

sect, a snail, an alga, a crustacean, and a fish species main-

tained under controlled conditions for a period of time.

Most major types of reactions that result in transfor-

mation and detoxification of toxic chemicals can be accom-

plished either by biological (enzymatic) or by non-biological

(nonenzymatic) mechanisms. Although significant changes

in structure and properties of organic compounds may result

from non-biological processes, the biological mechanism is

the major and often the only mechanism by which organic

compounds are converted to inorganic products. Microor-

ganisms are capable of degrading and detoxifying a wide

variety of organic compounds; presumably, every organic

molecule can be destroyed by one or more types of microor-

ganisms (referred to as the “principle of microbial infallibil-

ity.") However, since some organic compounds do accumu-

late in the environment, there must be factors such as

unfavorable environmental conditions that prevent the com-

plete degradation and detoxification of these persistent com-

pounds. There are many examples where certain microor-

ganisms have been identified as capable of detoxifying

specific organic compounds. In some cases, these microor-

ganisms can be isolated, cultured, and inoculated into con-

taminated environments in order to detoxify the compounds

of concern.

The major types of transformation reactions include:

oxidation, ring scission, photodecomposition,

combustion

reduction, dehydrohalogenation, hydrolysis, hydration, con-

jugation, and chelation. Conjugation is the only reaction

mediated by enzymes alone, while chelation is strictly nonen-

zymatic. Primary changes in organic compounds are usually

accomplished by oxidative, hydrolytic, or reductive reactions.

Oxidation reactions are reactions in which energy is

used in the incorporation of molecular oxygen into the toxic

molecule. In most mammalian systems, a monooxygenase

system is involved. One atom of molecular oxygen is added

to the toxic chemical, which usually results in a decrease in