Population &
the Land
Introduction
Population Growth Rates
A Sustainable Society
Modification of Natural Systems
The Land Ethic
The Biosphere
Ecosystems & Biomes
Biodiversity & Species Preservation
Summary
Who cannot wonder at this harmony of things, at this symphony of nature
which seems to will the well-being of the world?
Cicero
Population, when unchecked, increases in a geometrical ratio.
Subsistence increases only in an arithmetical ratio. A slight acquaintance
with numbers will show the immensity of the first power in comparison
with the second.
Thomas Malthus
Introduction
•
•
•
Increasing human populations place increasing stresses on
Earth’s finite resources.
It currently takes about 11 years to add a billion people to
Earth.
Current global population is just over 6 billion.
English curate, Thomas Malthus, published a famous essay on
population growth in 1798 in which he predicted that
population size would grow faster than agricultural production
resulting in a lack of future food supplies. Although Malthus
correctly identified the upward trend of population growth, he
did not anticipate concomitant advances in food production and
resource extraction that would meet the physical and material
needs of the burgeoning global population.
However, Malthus's basic observation remains true,
population-related trends expand exponentially by a constant
rate, for example, 2% per year, whereas agricultural trends
typically increase arithmetically by a constant value, such as
2,000 tons per year (Fig. 1). For example, compare the rate of
increase in irrigated farmland worldwide (three million
hectares per year) with the growth in number of telephone lines
(5% per year). The second section of this chapter considers
how population growth rates have varied with time and the
implications for global population distributions in the next
century.
Population Distribution
A third of the world’s population lives in just two countries,
China and India, out of 227 nations, and just over half of the
world's people live in six nations (Fig. 2; China, India, U.S.,
Indonesia, Brazil, and Russia).
2
Figure 1. Arithmetic
vs. exponential
increases. The area
of irrigated farmland
has increased by a
near-constant value
each year (~3 million
hectares per year, an
arithmetic increase),
whereas the number
of telephone lines
increased at a
constant rate (~5%
per year, an
exponential increase)
since 1960. An
exponential increase
creates the concaveupward graph shown
on the right,
sometimes called a Jcurve.
Figure 2. Top twenty
nations ranked by
population. Note that
most populous
nations are located in
or near the tropics.
Concern over increasing population size has continued to the
present. Ecologist, Paul Ehrlich, has written, “No geological
event in a billion years has posed a threat to terrestrial life
comparable to that of human overpopulation.” Ehrlich sees a
finite world with a limited volume of resources that will be
diminished more rapidly as populations grow. In contrast,
economist Julian Simon suggests, “There is no meaningful
limit to our capacity to keep growing forever.” Simon views
human beings as the world’s greatest resource and anticipates
that we will create new technologies to deal with problems
associated with increasing population size. The fate of the
world rests on how the growing population chooses how to use
the planet's finite resources. In an ideal world, we would
develop into a sustainable society, a society that satisfies its
need for resources without jeopardizing the needs of future
generations.
Current world population is over 6 billion and is updated daily
at the U.S. Census Bureau website. The expansion of
population has resulted in the growth of cities and the
conversion of natural environments to agricultural lands. The
fourth section of the chapter reviews some of the changes that
we have made to the planet in the modification of natural
systems resulting from population growth.
Conservationist Aldo Leopold viewed each person as a
member of both the social community of people and the
ecological community of plants, animals, and the land. He
3
termed this concept the land ethic, and we use his idea as a
springboard to discuss humans impact on the other members of
the biosphere in the final sections of the chapter. Leopold
suggested that just as in society where we have certain
obligations and privileges, we have similar constraints on our
behavior as members of an ecological community.
Modern biology was born in the mind of Charles Darwin on
the Galapagos Islands (Fig. 3) in 1835. It was here that
Darwin’s observation “that the different islands to a
considerable extent are inhabited by a different set of beings”
identified the concept of the ecosystem that links organisms to
specific physical environments. The penultimate section of the
chapter reviews the characteristics of the biosphere including
the flow of energy and nutrients between organisms and the
physical environment. The Galapagos ecosystems of Darwin’s
time still exist but, like many elements of the biosphere, are
increasingly endangered by human activities. Native species
have been threatened by invaders such as goats, pigs, dogs, and
rats, introduced to the islands following the arrival of
Europeans in the sixteenth century. Recent programs have
successfully removed invasive pests from some islands and the
native vegetation has rebounded as a result.
The presence of increasing numbers of people, all attempting to
improve their standard of living, places greater stress on the
environment, not only from the perspective of resource use, but
also from pollution of air and water, and from the need to
dispose of larger volumes of waste. To evaluate the impact of
human activity on nature we must first identify the parameters
that influence the distribution of specific associations of plants
and animals at a regional scale. These associations are termed
biomes and are composed of multiple interrelated ecosystems.
The final sections of the chapter, ecosystems & biomes and
biodiversity & species preservation review the major biomes
of the world, the human activities that impact the natural order,
and efforts to preserve species that are threatened by extinction.
4
Figure 3. View of
Galapagos Islands
from space. Image
courtesy of NASA.
Population Growth Rates
•
•
•
•
Global population is expected to stabilizeto between 10 to
11 billion because of declining population growth rates.
Population growth rates have been in decline since the
early 1960s.
Population growth is dependent upon current population
size and growth rate.
Current population growth rate is approximately 1.3%.
Seventy-eight million people are added to the planet annually,
approximately the population of Vietnam (the world's 14th
most populous nation). An additional billion people are added
to the world's population every 12 to 13 years at current growth
rates. At that rate, world population would be over 14 billion
by the end of the next century. However, the good news is that
population growth rates have declined by approximately a
third in the last few decades. The continued decline in growth
rates is expected to result in a global population of around 10
billion by the year 2100 (Fig. 4).
Figure 4. Graph of
world population in
the past and
projected into the
future. Note: the time
scale increases in
irregular intervals.
The Basics of Population Growth
Population growth rates are determined by the balance between
the number of people added to a nation's population by birth
and immigration and the number who are lost through death or
emigration.
Population = birth rate - death rate + immigration - emigration
growth rate
Population growth in the U.S. is determined by:
• Birth rate of 14 per 1,000 people.
• Immigration of ~4 per 1,000 people.
• Death rate of 9 per 1,000 people.
• Emigration is negligible.
• The U.S. population growth rate is 14 + 4 – 9 = 9 per 1,000
people, or 0.9%.
5
Without ongoing immigration, U.S. natural population growth
rates would approach 0.5%. Population changes in individual
nations may result from an influx or an outflow of refugees
fleeing persecution. Over 22 million people were recognized as
refugees by the United Nations (UN) in 1998. Approximately
2.6 million citizens of Afghanistan were displaced by internal
conflicts. Many of these people fled to Iran which saw its
population increase by 2 million. Pakistan, Germany, and the
U.S. all saw their populations increase by over a million people
the UN considered refugees.
Figure 5. Global
population growth
(red line) vs. selected
growth rates. Note
that population
increased at low rates
(0.5%) for the first
quarter century and
accelerated to faster
average growth rates
(1.5%) in recent
years.
Life was relatively short and brutish for much of human history
and death rates were close to birth rates so population increased
relatively slowly until the 1900s. In some cases death rates
exceeded birthrates during outbreaks of rapidly spreading fatal
diseases such as the Black Death (plague) in medieval Europe
which caused global populations to decline. Global population
reached 1 billion in the early 1800s. It took over a hundred
years for the population to double to 2 billion in the late
1920’s. Death rates declined during the second half of the 20th
century with the advent of modern medicine, better sanitation,
and improved nutrition. Rapid population growth followed as
the gap between birth rates and death rates widened. Global
population passed the three billion people mark in 1960,
accelerated to 4 billion in 1974, 5 billion in 1987, and reached
six billion in October 1999 (Figs. 4, 5).
Demographers (people who study changing population trends)
recognize four stages of population growth (Fig. 6):
1. Both birth rate and death rate are high in relatively
primitive societies (e.g., pre-1800s); population growth
rates are low.
6
2. Birth rates remain high and death rates decline as
technology and health facilities improve; growth rates
increase.
3. Birth rates decline and death rates remain low as
populations become more affluent; growth rates decline.
4. Birth rate equals death rate and population growth rates are
zero.
Figure 6. The four
stages in population
growth. Population
growth rate is
reflected in the height
of the gap between
the curves. In this
example growth rate
is ~30/1,000 people
(3%).
Figure 7. World
population growth
rates from 1950
projected to 2050.
During the "Great
Leap Forward" China
tried to restructure its
economy, sending
millions of farm
workers into factories
to increase
manufacturing output.
The result was that
agricultural
production fell and
millions died of
starvation. Image from
Recently, cultural forces (urbanization, access to
contraception, increased female literacy, higher earnings) have
lead to a reduction in birth rates in many developed and
developing nations. Thirty-two countries (e.g., Japan, Italy,
Russia), representing 40% of the global population, have
natural population growth rates that are at or below
zero. Fertility rates (number of children born per woman) in
these nations have declined to 2.1 or less and are now at or
below replacement reproduction levels where the number of
deaths balances the number of births. Birth rates have also
fallen dramatically in many developing nations over the last
the U.S. Census
Bureau.
7
few decades. Fertility rates declined in Bangladesh, one of the
world's most densely populated nations, from 7 in 1975 to 3.3
today.
Population Growth vs. Population Growth Rates
The maximum world population growth rate (increase in
population relative to current population) of 2.2% was recorded
in 1963 when world population increased by 71 million people
(Fig. 7). The maximum population growth (actual number of
people added to the world) occurred in 1989 when population
increased by 87 million people. This represented a 1.7%
increase over the previous year. Both population growth rate
and population size are important in determining how
population increases.
A similar increase in population size can be obtained from a
small population with a high population growth rate or a larger
population with a smaller population growth rate (Fig. 8). For
example, the population of Nigeria is 114 million and it
increases at a rate of 3% per year, generating an annual
population increase of 3.4 million people. In contrast Indonesia
has nearly twice as many people (214 million) people and a
growth rate of 1.5%. Consequently, Indonesia’s population
increases by 3.2 million people per year. The population
increase in both nations is almost the same but Nigeria’s
growth rate is among the highest in the world, whereas
Indonesia’s is more moderate.
The average global growth rate has since fallen to 1.3% but
the range of growth rates varies from less than zero (population
decreases) to over 3%. The difference between population
Figure 8. Global
population growth
from 1950 to 2050.
Similar growth rates
can yield different
population increases
depending upon the
current population;
likewise, similar
population increases
may result from
different population
growth rates. Image
modified from a U.S.
Census Bureau graph.
8
growth rates will result in some nations changing rank in the
future. Nations with high growth rates (Nigeria) will rise,
whereas nations with low or negative growth rates (Japan,
Russia) will inevitably fall in rank.
Population
Rank 1998
1
2
3
4
5
6
7
8
9
10
Nation
Growth Rate
China
India
United States
Indonesia
Brazil
Russia
Pakistan
Bangladesh
Japan
Nigeria
0.9
1.7
0.9
1.5
1.2
-0.3
2.2
1.8
0.2
3.0
Predicted
Rank 2020
1
2
3
4
7
9
6
8
11
5
Population growth rates are in decline but population size will
continue to increase. Current population growth rates are
relatively low but the population base is so large that it ensures
a population increase of approximately 2 billion people in the
next quarter century. Approximately 80% of the world’s
population and 95% of the world’s population growth occurs in
developing nations in Africa, Central America, and Asia.
A Sustainable Society
•
•
More people will result in greater stress on the environment
because of an increasing demand for limited resources,
increased risk for natural hazards, changes in natural
systems, and greater potential for human-related hazards.
The impact of people on the environment may be measured
at a range of scales from the local to global.
It is theoretically possible that Earth could support many times
its current population but such speculation takes no account of
the quality of lives people would be required to lead to ensure
sufficient food (and other resources) for all. Increasing
population will result in an increase in the demand for
resources, the modification of natural systems, and greater
numbers of people living in hostile environments (Fig. 9).
9
Living standards are likely to impose the greatest constraints
on future population growth.
Increasing the population of people on planet Earth will have
impacts at a range of scales:
• Global concerns (greenhouse effect, ozone depletion).
• National problems (soil erosion, oil supply).
• Regional issues (groundwater sources, air quality).
• Local issues (pollution, waste disposal, urban sprawl).
When faced with a diminishing pool of resources, should it be
our objective to grab as much as we can as quickly as possible
or should we deliberately use less on the premise that future
generations may one day need these resources? Citizens and
governments of the developed nations in North America and
Europe have protested loudly over the depletion of the tropical
rain forests. Yet they rarely point out that little of the original
forests of Europe and the United States remain. If a sustainable
global future requires persuading individual nations like Brazil
and Zaire not to mow down their own trees, how will these
nations be compensated? Some developing nations may be able
to use otherwise scarce resources as a stepping stone toward a
more prosperous future. However, nations with less to offer
can do little more than protest their imperiled status.
Current estimates suggest a maximum global population of
over 10 billion people. In a finite world, how many people can
the planet support? If we are to reach the carrying capacity of
Earth (the maximum population with a given technology and
social organization that Earth can support indefinitely) we will
have to create a sustainable society.
10
Figure 9. Contrasting
impact of human
activity between the
sparsely populated
central Wyoming
community of Hiland
(that's it in the
background) and the
more densely
populated city of
Toronto, Canada. The
human impact on
Hiland is almost
negligible in
comparison to the
changes in natural
systems that occur in
association with
larger cities.
Sustainable Development
When humans first glimpsed the Earth from space it was from
the Apollo 8 spacecraft orbiting the Moon in December 1968
(Fig. 10). These early views of the planet from the inky
darkness of space helped illustrate for many the unique
wonders of the fragile environment we share on spaceship
Earth. Our isolation in space emphasizes our reliance on our
home planet's resources.
Figure 10. View of
Earth from space
taken by Apollo 8
astronauts. Image
courtesy of NASA.
Because of their restricted land areas and limited resources,
small island nations may feel the effects of environmental
problems earlier than larger continental nations. Island
residents must rely on the careful management of their finite
resource base to ensure a reasonable future for generations to
follow. The signs of environmental stress are apparent on many
Pacific island nations that suffer from a lack of freshwater
sources, pollution of coastal waters, lack of waste disposal
sites, destruction of fish stocks, loss of biodiversity, and a
shortage of suitable agricultural land (Fig. 11). Furthermore,
not all potential problems can be solved by the island nations
themselves.
Figure 11. Plantation
on Kauai, Hawaii.
Image courtesy of
NOAA Photo Collection.
•
The Maldive Islands in the eastern Indian Ocean lie a few
meters above sea level and may submerge beneath the
waves if global warming becomes reality. Unfortunately for
their citizens, the Maldives and similar island nations, have
little clout in the international political arena to force the
developed nations to reduce their carbon dioxide emissions,
the ultimate cause of global warming. The Maldive
islanders have little to offer except beachfront views and
these are readily available elsewhere.
•
Both Easter Island (Fig. 12), in the southern Pacific
Ocean, and Hawaii were settled 1,500 years ago by
Polynesian islanders. A rich culture flourished on Easter
Island before declining to the point of impoverishment by
the time the first Europeans encountered the island in 1722.
Visitors to the island marveled at the hundreds of massive
11
Figure 12. Left:
Statue on Easter
Island. Image courtesy
of NOAA Photo
Collection. Right:
Relative locations of
Hawaii, Easter Island,
and Nauru in the
Pacific Ocean
carved statues, relics of a once sophisticated culture that
peaked at 7,000 inhabitants around 1400 A.D. The society
was based around a culture of ancestor worship tied to the
statues. The growing population had developed a nonsustainable lifestyle that stripped the island of timber, its
principal resource, used for offshore fishing (canoes),
transportation (of 20-ton statues), fuel, and housing. When
the timber was gone the culture collapsed, reverting to
cannibalism and tribal warfare. The survivors lived in caves
and scratched out a meager living by farming.
•
Native inhabitants of the tiny (22 km2) island nation of
Nauru (Fig. 12) in the western Pacific Ocean became
enriched by the mining of phosphate used to fertilize
Australian farms. Unfortunately, the mining process
destroyed much of the island interior (80%) making it unfit
for any other purpose. Native residents, supported by
royalties from mining, pay others to do the work and grow
little of their own food. Islanders have low life expectancy
(54 years) and high rates of diabetes linked to a poor diet
rich in junk foods. Poor (non-sustainable) resource
management has ensured that there will be few economic
opportunities for future generations of islanders when the
phosphate is exhausted.
The question for us on Earth is, Will we show the same poor
stewardship for our planet as the residents of Easter Island and
Nauru or will we have a more enlightened approach that
considers the long-term consequences of our interactions with
the environment?
12
Modification of Natural Systems
•
•
•
•
38% of Earth’s land surface has been domesticated.
Both the area of grain producing lands and grain production
per person have declined in recent decades.
Wetlands provide many benefits for the environment.
Over half of U.S. wetlands have been drained for
agriculture or development.
The physical surface on which we live can represent an
aesthetic, economic, and/or recreational resource. Land has
long held a special place in American culture. Belting out a few
verses of “America the Beautiful” should be enough to
convince anyone of the significance of the connection between
Americans and the land. The appeal of the New World to
colonists was the availability of land for the common man. The
United States grew by adding lands by purchase, by treaty, or
by taking them in war. The nation’s early economic vitality
was largely based upon resources from the land. Majestic
unspoiled natural landscapes of the continent contrasted with
overdeveloped and overpopulated European nations. The land,
and how it was used, have been a source of inspiration and
consternation for centuries.
Friction between economic forces that seek to use the land and
cultural forces endeavoring to protect it continues to generate
some of the most heated debates on environmental policy in
America and elsewhere. Pressure to convert natural lands for
agriculture and urbanization will only increase as population
increases. Such increases are most acute in poor, densely
populated nations that can’t afford the high cost of imports and
must rely on their land base to generate food to sustain their
people.
Domestication of Natural Lands
Approximately 38% of Earth's surface has been
"domesticated," converted to cropland, pasture, forest
plantations, urban use, etc. The degree of domestication (Fig.
13) depends upon the land area, population of the country, and
character of the landscape. Nations with relatively high
population densities typically have more domestication of land.
For example, the small Asian nation of Bangladesh has 81%
domestication and 9.2 people per hectare. In contrast, the U.S.
has a 45% domestication rate and only 0.3 people per hectare.
13
Figure 13.
Domesticated land
area as a proportion
of land available for
continents and
selected nations. Bold
values represent land
use for selected
continents.
(Note: a hectare = 2.5 acres.) However, some small nations
with large populations have relatively low levels of
domestication because of the rugged nature of their landscape
(e.g., Japan).
Nation
United States
Brazil
Australia
India
Bangladesh
Land Area
(million
hectares)
957
846
764
297
13
Population
(density per
hectare)
0.28
0.19
0.02
3.25
9.60
%
Converted
Area
45
28
60
61
81
One of the most significant effects of increasing urbanization
and development is the loss of agricultural lands, especially
lands that were originally situated close to cities and that were
used to grow grain crops (Fig. 14). Approximately 1% of
world’s land area is dedicated to urban uses and represents
home to nearly half (45%) of the world's population. Twentysix million hectares (approximately the area of Wyoming) are
dedicated to urban uses in the U.S. Nearly half a million
hectares (467,000) of arable land in developing nations is lost
annually to urbanization. More land is lost to the growth of
cities in nations with rapidly expanding urban populations.
Population growth rates vary for the world’s largest cities, but
are typically higher in developing nations and in nations with
rapidly growing economies (e.g., China).
Rank
1
2
3
4
5
14
City
Tokyo, Japan
Sao Paulo, Brazil
New York, U.S.
Mexico City, Mexico
Bombay, India
Population
(millions)
26.8
16.4
16.3
15.6
15.1
Growth Rate
(1990-95)
1.41%
2.01%
0.34%
0.73%
4.22%
Figure 14. The city of
Cleveland has
expanded to the east
and west along the
southern shore of
Lake Erie.
The total land area available for grain production has decreased
in recent years but production has continued to increase
because of the increased use of fertilizers and improved
farming techniques (Fig. 15). Unfortunately, population growth
has increased more rapidly than grain production and grain
production per person has declined for over a decade.
Improved living standards have resulted in an increasing
demand for grain to support "Western-style" diets that include
grain-fed beef.
Figure 15. Changes
in grain-producing
area and grain
production from 1965
to 1995.
Valuable agricultural lands are lost to expanding cities. The
total area of land dedicated to growing grain crops has
remained essentially static since the early 1970’s but grain
production has risen because of increased use of fertilizers and
improvements in agricultural methods.
Year
1955
1975
1995
World Grain
Area (million
hectares)
639
708
679
World Grain
Production
(million tons)
759
1237
1703
World Grain
Production per
Person
(kilograms)
273
303
299
The China Syndrome
Improved living standards in developing nations have created a
greater demand for meat and dairy products that are supplied
by domestic animals fed a diet of grains. Grain production per
person has steadily declined from a high of 342 kg in 1984 to
less than 300 kg today. Continued declines in cropland area
and increases in population will make it increasingly difficult
for technological advances to meet the future demand for grain.
Nowhere is this more evident than in China. China is
undergoing a period of rapid industrialization as its economy is
expanding rapidly. This economic boom is fueling a rise in
living standards. Increasing affluence has meant more beef,
15
pork, poultry, and eggs are consumed, resulting in an
increasing demand for grain to feed domestic livestock.
Industrial expansion is gobbling up cropland, and contributing
to a decline in grain production. In 1994, China was a net
exporter of 8 million tons of grain, but in 1995 it became a net
grain importer of 16 million tons. Some estimates suggest that
Chinese grain imports will rise to at least 200 million tons by
2030. If China’s grain consumption per person increases to the
level of Taiwan today, consuming even more livestock
products, then grain imports may approach 400 million tons.
However, the important question is: Who can supply grain on
that scale? World grain exports are about 200 million tons per
year, of which the U.S. supplies about half. If China becomes
the 300-pound gorilla of world grain markets, other nations
will either go hungry or go broke.
The Land Ethic
•
•
•
Cultural forces are causing changes in Western views of
land use.
Approximately a third of the U.S. is made up of federal
lands.
The land ethic views each person as a member of an
ecological community.
With the passage of the 1785 Land Ordinance, government
lands were opened for public sale in tidy squares of 640 acres
each. Unfortunately, land ownership didn't always ensure good
stewardship. Since the nation’s earliest days, poor farming
methods have often depleted the soil and degraded the land.
While the nation was young and the western lands beckoned,
few concerned themselves with soil conservation. The land
seemed to stretch unlimited to the west in comparison to their
European homelands.
Even as the nation expanded westward, there were large
sections of the public domain that were never sold. Nearly a
third of the nation is made up of federal lands administered by
a variety of government agencies (National Park Service, NPS;
16
National Forest Service, NFS; Fish and Wildlife Service, FWS;
Bureau of Land Management, BLM). Much of this land may be
leased for logging, grazing, or mining, or used for recreation.
Of these public lands, the most heavily used are the national
park system, which is made up of over 50 parks plus national
historic sites, battlefields, parkways, recreation areas, trails,
seashores, and monuments. Although the National Park Service
was not created until 1916, the first lands were set aside for
public use by congress in 1832, when four square miles of
thermal springs were reserved near Hot Springs, Arkansas. It
was in this same year that the artist, George Catlin, first wrote
about “a nation's park” that would preserve the buffalo and the
native cultures of the western plains. However, it was not until
40 years later, in 1872, that Congress authorized Yellowstone
as the world's first national park.
Figure 16. National
forest lands in the
Black Hills, South
Dakota
The relative remoteness of sites like Yellowstone meant that
they were enjoyed by an affluent clientele and they earned a
reputation as elitist vacation spots, until the advent of widely
available motorized transportation in early 1900s. Today, there
are more visitors to our national park system than there are
people living in the U.S., and the parks suffer from the
problems attendant with such overcrowding. In Yellowstone's
earliest days, its principal environmental concern was
poaching. Today, park officials must monitor many other
problems, including (1) the development of surrounding lands
which fragments ecosystems essential to park animals, (2)
water pollution from abandoned (and potential future) mines on
adjoining federal lands, (3) increasing numbers of
confrontations between visitors and park animals, and (4) the
use of thermal waters by others outside the park boundaries.
Historian, Frederick Jackson Turner, speaking at the 1893
Columbian Exposition in Chicago, suggested that the frontier
had played a decisive role in creating the hallmarks of the
American character. Turner believed that life on the frontier led
to the development of personal traits such as pragmatism,
individualism, materialism, nationalism, optimism, and
democracy. Turner's view implied a type of social Darwinism
17
in which a niche was developed on the frontier for
Bunyanesque characters forging an American identity.
This theme of individualism and the frontier spirit it generated
has remained a constant theme in American art, literature, and
politics. In recent years, some westerners have taken symbolic
steps to demonstrate their opposition to the federal
management of public lands. Most of the initiative (sometimes
called wise use) comes from those using public lands for
grazing, logging, mining, or oil exploration. The public appears
sympathetic to those whose land values have been diminished
by the effects of environmental legislation. Yet, they are more
likely to choose protection of the environment over
development or property rights issues, and a majority favor
charging fees for grazing, drilling, mining, and logging on
federal lands.
Aldo Leopold was born at the end of the Nineteenth century,
into a time when people’s impact on natural systems was
becoming clear. Leopold, a trained forester, published the
conservation classic A Sand County Almanac and Sketches
Here and There, in 1949. The book included his seminal essay,
The Land Ethic, wherein Leopold presented an argument in
favor of the preservation of all elements of nature, which he
collectively considered as the land. The land ethic defined each
person as a member of both the social community of people
and the ecological community of plants, animals, and the land.
He suggested that just as in society, where we have certain
obligations and privileges, we have similar constraints on our
behavior as members of an ecological community.
The Biosphere
•
•
•
18
The biosphere represents all animal and plant life on Earth
and the environments in which they live.
Ecology is the study of the interaction of organisms with
each other and their environment.
The stability of ecosystems are influenced by a
combination of abiotic and biotic factors.
•
•
•
All organisms rely on the intake of energy from the Sun
(plants) or through the consumption of plants (herbivores)
and/or other animals (omnivores, carnivores).
Some elements serve as essential nutrients for life in plants
and animals and move through the Earth system in
biogeochemical cycles.
Most of Earth's biomass is concentrated in forests with
tropical rain forests accounting for the most biomass per
square kilometer.
The biosphere represents life on Earth and is confined to a
relatively narrow strip of the planet's surface that extends from
the deepest portion of the ocean floor to the Himalayas highest
mountains. The diversity of life on Earth ranges among
millions of species, from the simplest single-cell organisms to
giant marine mammals such as the blue whale. Ecology is the
study of the interaction of organisms with each other and their
environment. Communities of organisms that inhabit specific
physical environments, defined primarily by their climate and
landforms, make up ecosystems. Best estimates suggest that
there are from 5 to 30 million species worldwide. Biodiversity,
the number of species in an ecosystem, varies depending upon
the characteristics of the individual ecosystems but is typically
greatest in environments with warm temperatures, plentiful
rainfall, and rich soils.
The stability of ecosystems are influenced by a combination of
biotic (organic) and abiotic (non-organic, e.g., sunlight,
precipitation) factors (Fig. 17). Where we choose to live is
largely determined by the abiotic characteristics of our physical
environment (e.g., climate, landscape) and the availability of
basic resources such as food. Other organisms are little
different. Individual abiotic factors may be limiting factors
Figure 17. Abiotic
environmental factors
that influence the
distribution of
ecosystems. The
majority of species
live within the upper
200 meters (660 feet)
of the ocean
(euphotic zone) and
below 6200 m
(20,500 feet) on land.
19
that can control the growth of an organism’s population in a
given area. For example, water quality controls fish
populations in acidified lakes in the Adirondack Mountains of
New York. Reproduction in lake trout ceased as lakes became
increasingly contaminated by acid precipitation because of
upwind air pollution.
Plants are producers that manufacture the food they need from
inorganic compounds in the physical environment. Through
photosynthesis, most plants use light energy (from sunlight) to
convert atmospheric gases (e.g., carbon dioxide), water, and
soil nutrients into leaves, branches, and roots. Approximately
1% of solar radiation is used by plants for photosynthesis. The
actions of each plant mimic the earth system in miniature. The
atmosphere supplies key gases, the hydrosphere provides
water, the solid earth is the source of the soil material and the
plant itself represents the biosphere.
carbon dioxide + water + energy Æ glucose + oxygen
from air
from soil sunlight
photosynthesis
Animals are consumers that can't produce their own food from
inorganic materials, such as air or water, but derive their
energy from consuming plants or other organisms and use the
energy to grow and maintain tissue mass. Most of the energy
represented by the plant material is lost by consumers as heat
or is excreted from animals as waste. Primary consumers
(herbivores, e.g., cows, deer) devour plants and secondary
consumers such as wolves and humans eat primary consumers.
The consumers and producers are part of a food chain that
transfers energy between organisms within an ecosystem.
Approximately 5 to 20% of energy is transferred with each step
up the food chain. As energy is lost with each step, the
numbers of organisms decrease with each step up the chain.
glucose + oxygen Æ carbon dioxide + water + energy
from consumption
from respiration
of producers
Dead organisms are broken down and returned to their
constituent inorganic materials (that will become nutrients for
plants) through the actions of decomposers, for example, fungi
and bacteria.
Certain key elements serve as essential nutrients for life in
plants and animals. Six elements (nitrogen, carbon,
hydrogen, oxygen, phosphorus, and sulfur) make up
20
approximately 95% of materials in plants and animals and
combine to form compounds that are essential nutrients for life
on Earth. These and other elements pass through the Earth
system in biogeochemical cycles (Fig. 18) that link together
processes in the biosphere, atmosphere, hydrosphere, and solid
earth.
The amount of organic material in an ecosystem is the biomass
and can be divided into phytomass (plants) and zoomass
(animals). The vast majority (99%) of biomass is phytomass.
Phytomass represents a reservoir for key elements of chemical
cycles on Earth. Current debate over the role of tropical rain
forests in sequestering carbon from the atmosphere and thus
moderating the potential impact of the global warming
illustrates how important it is to understand the distribution of
biomass within the Earth system. Tropical rain forests
represent a third of Earth's biomass but cover only 7% of the
land surface. All of the planet's forests account for
approximately three-quarters of all biomass on 20% of the
land. In contrast, deserts cover about the same land area but
account for less than 2% of biomass
Figure 18.
Biogeochemical
cycles link some of
the major elements of
the Earth system
including carbon (C),
nitrogen (N), oxygen
(O), phosphorus (P),
and sulfur (S).
21
Element
Carbon (C)
Hydrogen (H)
Nitrogen (N)
Oxygen (O)
Phosphorus (P)
Sulfur (S)
Source
Air, water
Water
Air, water, soils
Air
Water, soils
Water, soils
Nutrients/compounds
Carbon dioxide (CO2)
Water (H2O)
Gas (N2), nitrate (NO3), ammonium (NH4)
Carbon dioxide (CO2)
Phosphate (PO4)
Sulfate (SO4)
The distribution of biomass is a proxy measure of biodiversity.
The next section compares the characteristics of ecosystems
and explores the role of human activity in reducing
biodiversity.
Ecosystems & Biomes
•
•
•
•
•
•
Ecosystems are communities of organisms that inhabit
specific physical environments.
Biomes are composed of several ecosystems and represent
a regional community of organisms named after the
dominant vegetation.
The three major types of biomes are grasslands, forests, and
desert.
Grasslands can be subdivided into savanna, temperate
grasslands (prairie), and tundra with decreasing average
temperature.
Forests receive more precipitation than other biomes and
vary from boreal, to temperate, to rain forest with
increasing temperature.
Deserts have the fewest species and the most extreme
climate.
Effective management and care of the biosphere requires that
we understand how organisms interact with the physical
environment to create their habitats. Management involves
understanding the scale at which such associations function and
the processes that control the distribution of species within
such systems. Ecosystems are communities of organisms that
inhabit specific physical environments, defined primarily by
their climate and landforms. A number of similar ecosystems
can be grouped together in a biome, a regional community of
plants and animals named after the dominant type of
22
Figure 19. Nine
global-scale biomes.
Many ecologists
would further divide
grasslands or tropical
forests into additional
biomes or
ecogregions. Map
adapted from Miller,
1995, Environmental
Science, ITP.
vegetation. Biomes are characterized by similar association of
species, comparable climates, and consistent soil types (Fig.
19).
Ecologists don't agree on an exact number of ecosystems and
biomes because the number will vary depending upon how
they are defined (Which species are used? What climate
characteristics are considered?). Much of the eastern U.S. is
considered a temperate forest biome (under natural conditions,
prior to domestication) but it is composed of several separate
eco-regions that include the eastern broadleaf forests of West
Virginia, the southeastern mixed forest of the western
Carolinas, and the coastal plain mixed forest of northern
Florida. Generally we recognize three major climate-related
biome groups, grasslands, forests, and deserts, each of which
can be further subdivided into individual biomes (Fig. 20).
Figure 20. The
distribution of biomes
is largely a function of
regional climate
patterns. The
transition between
deserts, grasslands,
and forest is
governed by rainfall
and different types of
each biome are
differentiated by
temperature.
23
Grasslands
Grasslands range from the high-temperature, low-moderate
precipitation of the African savanna characterized by scattered
trees, to the treeless, frigid plains of the tundra straddling the
Arctic Circle in northern Asia and North America (Fig. 21).
The Serengeti Plain of Kenya and Tanzania is the largest
example of savanna grasslands and is home to the largest land
animals such as the elephant, rhino, and giraffe. The tundra is
snow-covered for much of the year and its shallow soils and
rocky surfaces support little more than grasses, sedges, and
lichen that are adapted to the cold, dry climate. The extreme
cold of the tundra environment results in a short growing
season that barely stretches beyond two months. The
environment is characterized by a few large species like the
caribou that can migrate within the ecosystem and an
abundance of short-life-cycle insects.
The temperate grasslands are known by a variety of names,
pampas in South America, prairie in North America, steppe in
Russia, and veldt in South Africa, yet all share common
characteristics. All occur in continental interiors with cold
winters and hot summers, they may have tall or short grasses
depending upon precipitation (or the lack of it), and trees are
only found along waterways. Much of these original temperate
grasslands were converted to croplands, only to be devastated
by wind erosion when droughts wiped out crops poorly adapted
to dry climate cycles. In the U.S. alone, over 100 million
hectares of prairie is gone with only a few hundred acres left in
isolated remnants in states like Iowa and Kansas (Fig. 21).
Bison, pronghorn antelope, and prairie dogs, were abundant
throughout the prairies until the prairie became domesticated
and some species (bison, black-footed ferret) came close to the
brink of extinction.
24
Figure 21. North
American grassland
environments. Left:
Caribou feeding on
tundra vegetation,
Alaska. Image courtesy
of USFWS. Right:
Tallgrass Prairie
National Preserve,
Flint Hills, northeast
Kansas. Less than
1% of original
tallgrass prairie
remains, most of it in
the Flint Hills. Image
courtesy of the U.S.
NPS.
Forests
Three principal forest biomes can be separated on the basis of
the types of vegetation in each. Boreal forests (taiga) of
northern latitudes are composed of coniferous evergreens
(spruce, fir, pine) growing on acidic soils. Such environments
are characterized by cold temperatures and low plant diversity.
Moose, wolf, bear, and lynx are characteristic of boreal forest
environments. Much of the eastern U.S. and northern Europe
are covered by temperate forests dominated by broadleaf
deciduous tree species (oak, sycamore, maple, poplar) that lose
their leaves prior to their dormant winter season. The loss of
leaves during fall provides nutrients for the underlying soils.
Areas of especially high rainfall may be home to temperate
rain forests (e.g., Pacific Northwest) with both evergreen and
deciduous species, including the giant redwoods. The
temperate forests are home to the tallest species on Earth, the
coastal redwoods as well as animals such as fox, deer, and
squirrel.
Figure 22. Rain forest
destruction, Brazil.
Dark areas show
remaining rain forest.
Vegetation has been
destroyed by fires set
to clear lands for
slash-and-burn
agriculture. Original
image courtesy of
NASA's EarthRISE
database.
Tropical rain forests dominate in areas of high temperature,
high rainfall, plentiful sunlight, and high humidity at low
latitudes adjacent to the equator. These forests are
characterized by evergreen, broadleaf, hardwood trees such as
mahogany, teak, and ebony and are home to the most diverse
ecosystems on the planet. The forest is structured in three
distinct layers (high, middle, lower canopies) the tallest of
which tops out around 60 meters. The dense tree cover
prevents light or winds from penetrating to the forest floor so
much of the pollination is accomplished by insects, including
over 500 species of butterflies. The vertical character of the
forest has resulted in the evolution of arboreal (tree-dwelling)
species such as sloths, monkeys, and lemurs. Rapid weathering
rates ensure thick soils but the soils lack nutrients because the
vegetation doesn't lose leaves as in temperate environments.
Approximately half the original tropical rain forest has been
25
destroyed (Fig. 22) to open lands for slash-and-burn agriculture
that soon depletes the limited soil nutrients before cutting down
more forest cover. In comparison the U.S. has lost
approximately a third of its forested land area since the 1600s.
Deserts
Dry climates are characterized by hot and cold deserts and
semi-desert environments such as chaparral. These
environments are home to plants adapted to conserve water
(long roots, succulent tissues) and burrowing animals that
know enough to stay out of the sun. Hot deserts have high
temperatures throughout the year and are home to a few plants
that cling to life on a substrate of sand or rock (e.g., Sahara,
North Africa). Cacti are relatively common in temperate
deserts that have hot summers but cool winters (e.g., Mojave
Desert, southeast California; Fig. 23). Animals are rare in
desert environments but camels, scorpions, and the kangaroo
rat have adapted to life in such extreme conditions.
Chaparral, also known as Mediterranean shrubland, is
characterized by dry summers but has a similar total rainfall as
some temperate grasslands. It contains woody shrubs and may
have grassy woodlands with species such as the cork oak,
olive, and eucalyptus trees.
Mountains
Mountains represent a special case as their increasing
elevations have the same effect as increasing latitude.
Temperatures decline and precipitation increases as both
elevation and latitude increase. Equatorial mountain (alpine)
biomes are characterized by tundra vegetation at high
elevations and descend through coniferous (boreal), deciduous
(temperate), and tropical (rain) forests with decreasing altitude.
26
Figure 23. Saguaro
cactus, southwestern
U.S. Image courtesy of
USFWS.
Biodiversity & Species Preservation
•
•
•
Species loss is most commonly attributed to the destruction
of habitat.
There are three basic reasons to preserve species: (1)
Practical - species benefit mankind; (2) ecological - for
maintenance of biological systems; (3) aesthetic - to
preserve the natural environment for human enjoyment.
The Endangered Species Act was passed in attempt to
protect species of aesthetic, ecological, educational,
historical, recreational, and scientific value.
Loss of Biodiversity
Few biomes remain in their original state today as many have
been modified by a variety of human activities. Environmental
changes may be slow or rapid and may occur at local, regional
or national scales. Natural environments change as human
activity intrudes into an otherwise pristine environment (Fig.
24). Natural vegetation is cleared to create land for crops
and/or grazing. The loss of natural lands to agriculture or
urbanization inevitably results in habitat destruction and may
be accompanied by predator control efforts to reduce
populations of species (e.g., wolves) considered a threat to
domestic animals or the human population.
Figure 24. A
resourceful osprey
finds an
unconventional
nesting site on this
channel marker in
Kentucky Lake.
As human populations continue to increase, the impact on the
living resources of the biosphere may become more severe.
Westward expansion and ready access to rifles resulted in a
99.99% reduction in buffalo herds on the Great Plains in the
second half of the nineteenth century. Today overfishing has
depleted the majority of the world's commercial fisheries. Even
relatively benign activities such as tourism result in
environmental alterations as roads and buildings are
constructed to meet the demands of travelers. International
travel has aided the introduction of alien species into
environments where they replace native organisms. Human
activities such as farming, mining, or industrialization may
introduce pollutants into the environment that have a profound
effect on ecosystems. For example, the Exxon Valdez tanker
spilled millions of gallons of oil of the south coast of Alaska in
1989, destroying coastal marine habitats.
27
Preserving Endangered Species
We have long recognized that human development inevitably
leads to declines in wildlife populations. Awareness of
decreasing wildlife populations prompted the formation of
several conservation organizations among hunters and fueled
efforts to create federal regulations to prevent the exploitation
of game species. The federal government would later introduce
legislation to form wildlife refuges, bird reservations, forest
reserves, and wilderness areas (preserving habitat). More
recently, habitat destruction and pollution have replaced
hunting as the primary cause of species decline in many parts
of the world. The destruction of habitat is the prime reason why
species go into decline. Maintaining natural habitat for wildlife
became increasingly difficult as the human population grew
and as the nation’s transportation infrastructure expanded,
making almost the whole continent accessible. It is estimated
that we lose over 200 acres of land in the U.S. each hour to
development.
Given that there are millions of species on Earth and that more
than 99% of all species that ever lived are now extinct, it may
seem that the loss of a few additional organisms (Fig. 25) as a
result of human activity is a small and relatively insignificant
price to pay for progress. There are three basic reasons given to
counter this position:
•
Practical: Certain species benefit human beings by
providing resources essential for our present or future
existence. The most basic examples are species that are
harvested for food, but many others are used for
pharmaceuticals and those currently threatened by
extinction may have yet undiscovered benefits including
cures for human ailments. The rosy periwinkle, a plant
found only on Madagascar that contains organic
compounds that have been used to help treat cancer, is
today threatened by habitat destruction. There are
approximately 80,000 edible plant species and we use just
three (corn, wheat, rice) to provide half the world's food
supply. Wild relatives of common crops represent a
reservoir of genetic material that we can dip into to create
disease-resistant strains of domesticated species.
•
Ecological: Although some organisms have no direct
benefit to humans they are essential to maintaining the
order of the biosphere. The web of life is a complex system
with the life cycles of hundreds or thousands of species
28
tightly intertwined. Taking out one species is like removing
an organ from the human body. Sometimes we can
continue to operate efficiently with the loss because there is
some redundancy built into our anatomy (e.g., kidney,
lung) but a malfunctioning liver or heart can result in the
collapse of the whole body. Threatened or endangered
species such as the northern spotted owl and marbled
murrelet in the old-growth forests of the Pacific Northwest
may be considered a signal of a decline in this ecosystem
resulting from deforestation.
•
Aesthetic: Almost everyone derives some pleasure from
experiencing nature. The sales of nature calendars alone tell
us that people like to look at scenes that reflect the
processes of the biosphere. The 280 million people that
visit our national park system annually do so to appreciate
the physical environments created by natural processes.
Anyone who has ever paused to appreciate the changing
colors of a woodland on a fall day has recognized the
artistry of nature. Many recreational activities (e.g., hiking,
climbing, hunting, fishing, canoeing, swimming,
gardening) are intimately linked to the natural world around
us and would be diminished by the loss of the elements that
contribute to the biosphere.
Figure 25. Four
examples of
threatened or
endangered species
on the Endangered
Species List.
Clockwise from top
left: Karner Blue
butterfly; Leopard
Darter; Black-footed
ferret; and Dwarf lake
iris. Images courtesy of
U.S. FWS.
The primary law used to protect species today is the
Endangered Species Act (ESA), perhaps the most unpopular
piece of federal environmental legislation - at least among
some developers. The ESA, passed in 1973, declares it illegal
to harass, harm, pursue, shoot, wound, kill, trap, capture, or
collect listed species.
29
The goal of the ESA is to protect species long enough so that
they can undergo recovery, at which point they can be removed
from the Endangered Species List. Recent success stories for
the Pacific Gray whale and bald eagle have resulted from
restrictions on human activities including a ban on worldwide
commercial whaling and the elimination of the use of the
pesticide DDT in North America, respectively. However, such
successes are relatively rare. Less than 2% of species have
been removed from the endangered list and about a third of
those were declared extinct.
Summary
1. What is the difference between exponential and arithmetic
growth?
Exponential growth occurs at a constant rate (e.g., 5%) whereas
arithmetic growth increases by a constant amount (e.g., 5 acres
per year). Exponential growth will be the most rapid. Imagine
two people who each run 1 km a day for exercise. Person A
decides to increase the distance they run by 100 meters per day.
Person B decides to increase their distance by 10% of the
previous day's run (don't ask why, they just really love math).
After 10 days, A is running 2 km and B is running 2.6 km. Ten
days later A covers 3 km but B must run 6.7 km to keep on
pace. After another 10 days, A runs 4 km and is feeling pretty
good but B has to run over 17 km and must run nearly 2 km
further each day just to keep on their exponential pace. B gives
up and changes his exercise routine to include eating donuts
and watching football on his big-screen TV.
2. How many people are there in the world and where do they
live?
The current global population is a little over 6 billion and is
increasing by 78 million people each year. Most of these
people live in Asia. China and India have over 2.2 billion
people between them and 12 of the 20 most populous nations
are in Asia (3 in Africa, 1 in South America, 2 in Europe, 2 in
North America). The U.S. is a long way behind India but it is
the third most heavily populated country in the world.
3. How is population growth determined?
30
Population growth worldwide is the balance between global
birthrates and death rates. As births exceed deaths, population
is increasing at a rate of 1.3% per year. National growth rates
also include emigration (people leaving a country) and
immigration (people coming to a country). Approximately a
third of U.S. population growth is accounted for by
immigration. Political or economic refugees may cause shortterm fluctuations in a nation's population growth rate.
4. How have population growth rates changed with time?
Population growth rates typically have four possible
configurations: (a) birth rate and death rate are both high and
population growth rates are low; (b) birth rates are high and
death rates decline, growth rates increase; (c) birth rates decline
and death rates remain low, growth rates decline; (d) birth rate
equals death rate and population growth rate is zero. Global
population passed through the second stage during the first half
of this century and most of the world is now in the third stage.
Several nations (e.g., Italy, Japan) have reached the fourth
stage. Consequently, global population growth rates continue
to decline from a high of 2.2% during the early 1960s to a
current value of 1.3%.
5. What is the difference between birth rate, fertility rate, and
population growth rate?
Birth rate is the number of births per 1,000 people in a year.
Birth rates worldwide range from approximately 8 to 30/1,000.
The greater the difference between birth rate and death rate, the
more rapid the population growth rate. Fertility rate represents
the number of children born per woman. A fertility rate of 2
ensures parents are replaced by their children. Higher rates
result in population increases, lower rates cause population
declines.
6. Does a high population growth rate mean rapid population
growth?
Not necessarily. Population growth is dependent upon both the
size of the existing population and the growth rate. Thus annual
population growth was less (71 million) in the early 1960s than
today (78 million) even though growth rates were higher (2.2%
vs. 1.3%) because global populations were smaller (~3 vs. 6
billion).
7. Where are population growth rates highest and lowest?
Population growth rates are highest in the developing world
(e.g., Nigeria, 3%) and lowest in developed nations (e.g., Italy,
31
0%). Consequently, 95% of the people added to the world's
population will be born in developing nations.
8. What is the carrying capacity of Earth?
Good question. The carrying capacity is the maximum
population that Earth can support indefinitely. The answer is
probably anything between 6 to 20 billion depending upon the
standard of living we are willing to "enjoy." Human population
is expected to stabilize around 10 billion, essentially a selfimposed limit dictated by cultural standards.
9. Do we have a sustainable society now?
Do you use anything that is not renewable? The petroleum
products that allow us to drive our cars (or airplanes) or heat
our homes, the coal that is burned to generate our electricity;
both of these energy sources are non-renewable. Forests and
fish should be renewable on the human time-scale but both
have been substantially diminished in this century. The
Atlantic cod fishery off Nova Scotia collapsed due to
overfishing, despite warnings that it would happen. Tropical
rainforests are being burned as you read this, those trees are not
coming back any time soon. In short, we are some distance
away from being a sustainable society and we are not likely to
get close without the threat of more drastic shortages than is
apparent now.
10. Why didn't the people of Easter Island recognize what was
inevitably going to happen to their society?
Each generation focused on its own needs and ignored the
long-term consequences of their actions. Much of what they
did was influenced by the cultural norms of their society that
emphasized the need to make their big-headed statues.
11. What is "domesticated" land?
Over a third of Earth's land surface has been domesticated, that
is, it has been converted from its natural state to cropland,
pasture, forest plantations, or urban use.
12. What factors influence the degree of domestication?
Domestication is influenced by population density, the
character of the landscape and climate, and the rate of growth
of urban areas. Nations with relatively high population
densities typically have more domestication of land unless the
landscape is too rugged to support development or the climate
is too extreme to support human life. Lands surrounding
rapidly growing cities are most susceptible to domestication.
32
Urban growth rates are typically higher in major cities in
developing nations.
13. How has domestication influenced agricultural production?
Initially, natural lands were converted to agricultural lands and
production expanded. However, those same croplands and
pastures are now being consumed under expanding cities. The
land area available for grain production has decreased over the
last few decades but agricultural yields increased because of
improvements in technology and increased use of fertilizers.
Population has increased more rapidly than grain production so
production per person is now actually less than in the past.
14. What is the land ethic?
The land ethic defines each person as a member of the
ecological community of plants and animals, or collectively,
the land. Aldo Leopold suggested that just as in society, where
we have certain obligations and privileges, we have similar
constraints on our behavior as members of an ecological
community. The land ethic views our role as stewards of the
natural world.
15. What is the biosphere?
Earth is composed of several components, including the
hydrosphere (all waters, mainly the oceans), atmosphere (all
air, from Earth's surface to space), and the biosphere (all life on
Earth from bacteria to elephants). The biosphere is interwoven
with other components of the Earth system. Most life occurs in
the shallow oceans and at relatively low elevations on the land
surface but even in extreme conditions of the deep ocean floor
or the frozen poles, some organisms manage to survive.
16. What controls the distribution of life in the biosphere?
Life is limited by abiotic factors such as temperature and
precipitation that characterize the physical environment. These
conditions are independent of organic activity. Life is most
abundant in environments with plentiful supplies of key abiotic
components and is least abundant in regions with extreme
conditions.
17. How are energy and nutrients transferred from the physical
environment to plants and animals?
Plants use energy from the Sun to generate mass through
photosynthesis. They absorb key nutrients from water and
soils. Animals (consumers) eat the plants, thus receiving some
of the stored energy and nutrients, and may in turn be eaten by
33
other animals. Key elements cycle through the biosphere
through a series of biogeochemical cycles involving processes
such as respiration, photosynthesis, weathering, and rock
formation.
18. What is the relationship between biomass and biodiversity?
Biodiversity represents the variety of species and biomass is
the amount (mass) of material in part or all of the biosphere.
Increasing biodiversity typically is reflected in increasing
biomass, therefore, biomass per unit area can be used as an
approximate measure of biodiversity.
19. What is the difference between a biome and an ecosystem?
The difference is mainly one of scale. Biomes are bigger than
ecosystems. Ecosystems are communities of organisms that
inhabit specific physical environments, whereas, biomes
represent collections of ecosystems and are regional or
international in scale. Biomes are characterized by similar
association of species, comparable climates, and consistent soil
types.
20. What are the major biomes?
Biomes can be loosely divided into three major groups,
grasslands (savanna, temperate, tundra), forests (rain,
temperate, boreal), and deserts. These major groups are divided
by precipitation (more in forests, less in deserts) and are
subdivided by temperature (low temperature in boreal forests
and tundra; high temperatures in rain forests and savanna).
Some biomes may be transitional between the major biomes,
for example, chaparral occurs around the Mediterranean Sea
and is defined by dry summers and may contain grasslands or
shrublands.
21. What causes biodiversity to decrease?
Biodiversity can gradually decrease due to natural changes in
the physical environment but typically occurs more rapidly as a
result of human activity. Human actions such as agricultural
practices, commercial fishing, hunting, predator controls,
pollution, and tourism can destroy natural habitats and reduce
biodiversity.
22. What steps can be taken to preserve biodiversity?
Actions that ensure the preservation of natural habitats have the
greatest effect in preserving species because habitat destruction
is the primary cause of biodiversity loss. Habitat preservation
can occur through private or corporate programs, or through
34
government agencies that preserve forest lands (although these
agencies also oversee logging), national parks, and wildlife
refuges.
23. Why should we care to preserve biodiversity?
There are three principal reasons why we might consider it a
good idea to minimize species loss. First, for purely selfcentered reasons, we want to preserve species to help
ourselves. Domesticated plants and animals provide us with the
majority of our food resources. Related wild species represent a
genetic resource that can be used to improve domestic species.
The majority of species have yet to be formally described,
consequently, their potential for human benefit is unknown.
Relatively rare plant species provide us with treatments for
serious diseases. Second, we don't know what role most species
play in the functioning of the biosphere yet we know we like
the idea of having a biosphere so logic suggests we shouldn't
do anything to meddle with the natural order of life. Finally,
much human enjoyment is derived from the simple pleasures of
observing life on Earth. By preserving all elements of the
biosphere, we preserve a source of delight for the generations
that will follow us.
35