Study Guide

Chapter 4

THE ATMOSPHERE 1

4.1 THE ATMOSPHERE

4.1.1 INTRODUCTION

The atmosphere , the gaseous layer that surrounds the earth, formed over four billion years ago. During

the evolution of the solid earth, volcanic eruptions released gases into the developing atmosphere. Assuming

the outgasing was similar to that of modern volcanoes, the gases released included: water vapor (H2O),

carbon monoxide (CO), carbon dioxide (CO2), hydrochloric acid (HCl), methane (CH4), ammonia (NH3),

nitrogen (N2) and sulfur gases. The atmosphere was reducing because there was no free oxygen. Most of

the hydrogen and helium that outgassed would have eventually escaped into outer space due to the inability

of the earth's gravity to hold on to their small masses. There may have also been signicant contributions

of volatiles from the massive meteoritic bombardments known to have occurred early in the earth's history. Water vapor in the atmosphere condensed and rained down, eventually forming lakes and oceans. The

oceans provided homes for the earliest organisms which were probably similar to cyanobacteria. Oxygen

was released into the atmosphere by these early organisms, and carbon became sequestered in sedimentary

rocks. This led to our current oxidizing atmosphere, which is mostly comprised of nitrogen (roughly 71

percent) and oxygen (roughly 28 percent). Water vapor, argon and carbon dioxide together comprise a

much smaller fraction (roughly 1 percent). The atmosphere also contains several gases in trace amounts,

such as helium, neon, methane and nitrous oxide. One very important trace gas is ozone, which absorbs

harmful UV radiation from the sun.

4.1.2 ATMOSPHERIC STRUCTURE

The earth's atmosphere extends outward to about 1,000 kilometers where it transitions to interplanetary

space. However, most of the mass of the atmosphere (greater than 99 percent) is located within the rst

40 kilometers. The sun and the earth are the main sources of radiant energy in the atmosphere. The

sun's radiation spans the infrared, visible and ultraviolet light regions, while the earth's radiation is mostly

infrared. The vertical temperature prole of the atmosphere is variable and depends upon the types of radiation

that aect each atmospheric layer. This, in turn, depends upon the chemical composition of that layer

(mostly involving trace gases). Based on these factors, the atmosphere can be divided into four distinct

layers: the troposphere, stratosphere, mesosphere, and thermosphere. The troposphere is the atmospheric layer closest to the earth's surface. It extends about 8 - 16 kilometers

from the earth's surface. The thickness of the layer varies a few km according to latitude and the season of

the year. It is thicker near the equator and during the summer, and thinner near the poles and during the 1

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CHAPTER 4. THE ATMOSPHERE

winter. The troposphere contains the largest percentage of the mass of the atmosphere relative to the other

layers. It also contains some 99 percent of the total water vapor of the atmosphere. The temperature of the troposphere is warm (roughly 17 ºC) near the surface of the earth. This is due to

the absorption of infrared radiation from the surface by water vapor and other greenhouse gases (e.g. carbon

dioxide, nitrous oxide and methane) in the troposphere. The concentration of these gases decreases with

altitude, and therefore, the heating eect is greatest near the surface. The temperature in the troposphere

decreases at a rate of roughly 6.5 ºC per kilometer of altitude. The temperature at its upper boundary is

very cold (roughly -60 ºC).

Because hot air rises and cold air falls, there is a constant convective overturn of material in the tropo-

sphere. Indeed, the name troposphere means region of mixing. For this reason, all weather phenomena

occur in the troposphere. Water vapor evaporated from the earth's surface condenses in the cooler upper

regions of the troposphere and falls back to the surface as rain. Dust and pollutants injected into the tro-

posphere become well mixed in the layer, but are eventually washed out by rainfall. The troposphere is

therefore self cleaning. A narrow zone at the top of the troposphere is called the tropopause. It eectively separates the

underlying troposphere and the overlying stratosphere. The temperature in the tropopause is relatively

constant. Strong eastward winds, known as the jet stream, also occur here.

The stratosphere is the next ma jor atmospheric layer. This layer extends from the tropopause (roughly

12 kilometers) to roughly 50 kilometers above the earth's surface. The temperature prole of the stratosphere

is quite dierent from that of the troposphere. The temperature remains relatively constant up to roughly

25 kilometers and then gradually increases up to the upper boundary of the layer. The amount of water

vapor in the stratosphere is very low, so it is not an important factor in the temperature regulation of the

layer. Instead, it is ozone (O3) that causes the observed temperature inversion. Most of the ozone in the atmosphere is contained in a layer of the stratosphere from roughly 20 to 30

kilometers. This ozone layer absorbs solar energy in the form of ultraviolet radiation (UV), and the energy is

ultimately dissipated as heat in the stratosphere. This heat leads to the rise in temperature. Stratospheric

ozone is also very important for living organisms on the surface of the earth as it protects them by absorbing

most of the harmful UV radiation from the sun. Ozone is constantly being produced and destroyed in

the stratosphere in a natural cycle. The basic reactions involving only oxygen (known as the " Chapman

Reactions ") are as follows: Figure 4.1

The

production of ozone from molecular oxygen involves the absorption of high energy UV radiation

(UVA) in the upper atmosphere. The destruction of ozoneby absorption of UV radiation involves

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moderate and low energy radiation (UVB and UVC). Most of the production and destruction of ozone

occurs in the stratosphere at lower latitudes where the ultraviolet radiation is most intense. Ozone is very unstable and is readily destroyed by reactions with other atmospheric species such nitrogen,

hydrogen, bromine, and chlorine. In fact, most ozone is destroyed in this way. The use of chlorouorocar-

bons (CFCs) by humans in recent decades has greatly aected the natural ozone cycle by increasing the

rate of its destruction due to reactions with chlorine. Because the temperature of the stratosphere rises with

altitude, there is little convective mixing of the gases. The stratosphere is therefore very stable. Particles

that are injected (such as volcanic ash) can stay aloft for many years without returning to the ground. The

same is true for pollutants produced by humans. The upper boundary of the stratosphere is known as the

stratopause , which is marked by a sudden decrease in temperature.

The third layer in the earth's atmosphere is called the mesosphere. It extends from the stratopause

(about 50 kilometers) to roughly 85 kilometers above the earth's surface. Because the mesosphere has

negligible amounts of water vapor and ozone for generating heat, the temperature drops across this layer.

It is warmed from the bottom by the stratosphere. The air is very thin in this region with a density about

1/1000 that of the surface. With increasing altitude this layer becomes increasingly dominated by lighter

gases, and in the outer reaches, the remaining gases become stratied by molecular weight.

The fourth layer, the thermosphere, extends outward from about 85 kilometers to about 600 kilometers.

Its upper boundary is ill dened. The temperature in the thermosphere increases with altitude, up to 1500 º

C or more. The high temperatures are the result of absorption of intense solar radiation by the last remaining

oxygen molecules. The temperature can vary substantially depending upon the level of solar activity. The lower region of the thermosphere (up to about 550 kilometers) is also known as the ionosphere.

Because of the high temperatures in this region, gas particles become ionized. The ionosphere is important

because it reects radio waves from the earth's surface, allowing long-distance radio communication. The

visual atmospheric phenomenon known as the northern lights also occurs in this region. The outer region

of the atmosphere is known as the exosphere. The exosphere represents the nal transition between the

atmosphere and interplanetary space. It extends about 1000 kilometers and contains mainly helium and

hydrogen. Most satellites operate in this region. Solar radiation is the main energy source for atmospheric heating. The atmosphere heats up when

water vapor and other greenhouse gases in the troposphere absorb infrared radiation either directly from the

sun or re-radiated from the earth's surface. Heat from the sun also evaporates ocean water and transfers heat

to the atmosphere. The earth's surface temperature varies with latitude. This is due to uneven heating of

the earth's surface. The region near the equator receives direct sunlight, whereas sunlight strikes the higher

latitudes at an angle and is scattered and spread out over a larger area. The angle at which sunlight strikes

the higher latitudes varies during the year due to the fact that the earth's equatorial plane is tilted 23.5 º

relative to its orbital plane around the sun. This variation is responsible for the dierent seasons experienced

by the non-equatorial latitudes.

4.1.3 WIND

Convecting air masses in the troposphere create air currents known as winds, due to horizontal dierences

in air pressure. Winds ow from a region of higher pressure to one of a lower pressure. Global air movement

begins in the equatorial region because it receives more solar radiation. The general ow of air from the

equator to the poles and back is disrupted, though, by the rotation of the earth. The earth's surface travels

faster beneath the atmosphere at the equator and slower at the poles. This causes air masses moving to the

north to be deected to the right, and air masses moving south to be deected to the left. This is known as

the " Coriolis Eect ." The result is the creation of six huge convection cellssituated at dierent latitudes.

Belts of prevailing surface winds form and distribute air and moisture over the earth. Jet streams are extremely strong bands of winds that form in or near the tropopause due to large air

pressure dierentials. Wind speeds can reach as high as 200 kilometers per hour. In North America, there

are two main jet streams: the polar jet stream, which occurs between the westerlies and the polar easterlies,

and the subtropical jet stream, which occurs between the trade winds and the westerlies.

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CHAPTER 4. THE ATMOSPHERE

4.1.4 WEATHER

The term weatherrefers to the short term changes in the physical characteristics of the troposphere. These

physical characteristics include: temperature, air pressure, humidity, precipitation, cloud cover, wind speed

and direction. Radiant energy from the sun is the power source for weather. It drives the convective mixing

in the troposphere which determines the atmospheric and surface weather conditions. Certain atmospheric conditions can lead to extreme weather phenomena such as thunderstorms, oods,

tornadoes and hurricanes. A thunderstormforms in a region of atmospheric instability, often occurring

at the boundary between cold and warm fronts. Warm, moist air rises rapidly (updraft) while cooler air

ows down to the surface (downdraft). Thunderstorms produce intense rainfall, lightning and thunder. If

the atmospheric instability is very large and there is a large increase in wind strength with altitude (vertical

wind shear), the thunderstorm may become severe. A severe thunderstorm can produce ash oods, hail,

violent surface winds and tornadoes.

Floods can occur when atmospheric conditions allow a storm to remain in a given area for a length of

time, or when a severe thunderstorm dumps very large amounts of rainfall in a short time period. When

the ground becomes saturated with water, he excess runo ows into low-lying areas or rivers and causes

ooding. Atornado begins in a severe thunderstorm. Vertical wind shear causes the updraft in the storm to

rotate and form a funnel. The rotational wind speeds increase and vertical stretching occurs due to angular

momentum. As air is drawn into the funnel core, it cools rapidly and condenses to form a visible funnel

cloud. The funnel cloud descends to the surface as more air is drawn in. Wind speeds in tornadoes can

reach several hundred miles per hour. Tornadoes are most prevelant in the Great Plains region of the United

States, forming when cold dry polar air from Canada collides with warm moist tropical air from the Gulf of

Mexico. Acyclone is an area of low pressure with winds blowing counter-clockwise (Northern Hemisphere) or

clockwise (Southern Hemisphere) around it. Tropical cyclones are given dierent names depending on their

wind speed. The strongest tropical cyclones in the Atlantic Ocean (wind speed exceeds 74 miles per hour)

are called hurricanes . These storms are called typhoons(Pacic Ocean) or cyclones (Indian Ocean) in

other parts of the world. Hurricanes are the most powerful of all weather systems, characterized by strong

winds and heavy rain over wide areas. They form over the warm tropical ocean and quickly lose intensity

when they move over land. Hurricanes aecting the continental United States generally occur from June

through November.

4.1.5 OCEAN CURRENTS

The surface of the earth is over 71 percent water, so it is not surprising that oceans have a signicant eect

on the weather and climate. Because of the high heat capacity of water, the ocean acts as a temperature

buer. That is why coastal climates are less extreme than inland climates. Most of the radiant heat from

the sun is absorbed by ocean surface waters and ocean currents help distribute this heat. Currents are the movement of water in a predictable pattern. Surface ocean currents are driven mostly

by prevailing winds. The "Coriolis Eect" causes the currents to ow in circular patterns. These currents

help transport heat from the tropics to the higher latitudes. Two large surface currents near the United

States are the California current along the west coast and the Gulf Stream along the east coast. Deep ocean

currents are driven by dierences in water temperature and density. They move in a convective pattern. The less dense (lower salinity) warm water in the equatorial regions rises and moves towards the polar

regions, while more dense (higher salinity) cold water in the polar regions sinks and moves towards the

equatorial regions. Sometimes this cold deep water moves back to the surface along a coastline in a process

known as upwelling . This cold deep water is rich in nutrients that support productive shing grounds.

About every three to seven years, warm water from the western equatorial Pacic moves to the eastern

equatorial Pacic due to weakened trade winds. The eastern Pacic Ocean thus becomes warmer than usual

for a period of about a year. This is known as El Niño. El Niño prevents the nutrient-rich, cold-water

upwellings along the western coast of South America. It also impacts the global weather conditions. Some

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regions receive heavier than usual rainfall, while other regions suer drought conditions with lower than

usual rainfall. Probably the most important part of weather is precipitationas rainfall or snowfall. Water from the

vast salty oceans evaporates and falls over land as fresh water. It is rainfall that provides fresh water for

land plants, and land animals. Winter snowfall in mountainous regions provides a stored supply of fresh

water which melts and ows into streams during the spring and summer. Atmospheric clouds are the generators of precipitation. Clouds form when a rising air mass cools

and the temperature and humidity are right for condensation to occur. Condensation does not occur spon-

taneously, but instead requires a condensation nuclei. These are tiny (less than 1 m) dust or smoke

particles. The condensation droplet is small enough (about 20 m) that it is supported by the atmosphere

against the pull of gravity. The visible result of these condensation droplets is a cloud. Under the right conditions, droplets may continue to grow by continued condensation onto the droplet

and/or coalescence with other droplets through collisions. When the droplets become suciently large

they begin to fall as precipitation. Typical raindrops are about 2 mm in diameter. Depending upon the

temperature of the cloud and the temperature prole of the atmosphere from the cloud to the earth's surface,

various types of precipitation can occur: rain, freezing rain, sleet or snow. Very strong storms can produce

relatively large chunks of ice called hailstones.

4.1.6 CLIMATE

Climate can be thought of as a measure of a region's average weather over a period of time. In dening a

climate, the geography and size of the region must be taken into account. A micro-climate might involve a

backyard in the city. A macroclimate might cover a group of states. When the entire earth is involved, it

is a global climate. Several factors control large scale climates such as latitude (solar radiation intensity),

distribution of land and water, pattern of prevailing winds, heat exchange by ocean currents, location of

global high and low pressure regions, altitude and location of mountain barriers. The most widely used scheme for classifying climate is the K

o ppen System . This scheme uses average

annual and monthly temperature and precipitation to dene ve climate types: 1. tropical moist climates: average monthly temperature is always greater than 18

C 2. dry climates: decient precipitation most of the year

3. moist mid-latitude climates with mild winters

4. moist mid-latitude climates with severe winters

5. polar climates: extremely cold winters and summers.

Table 4.1

Using the K

o ppen system and the seasonal dominance of large scale air masses (e.g., maritime or conti-

nental), the earth's climate zones can be grouped as follows: 1. tropical wet

2. tropical wet and dry

3. tropical desert

4. mid-latitude wet

5. mid-latitude dry summer

6. mid-latitude dry winter

7. polar wet

8. dry and polar desert

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CHAPTER 4. THE ATMOSPHERE

Table 4.2

Los Angeles has a mid-latitude dry summer climate, whereas New Orleans has a mid-latitude wet climate.

Data from natural climate records (e.g. ocean sediments, tree rings, Antarctic ice cores) show that the

earth's climate constantly changed in the past, with alternating periods of colder and warmer climates.

The most recent ice age ended only about 10,000 years ago. The natural system controlling climate is very

complex. It consists of a large number of feedback mechanisms that involve processes and interactions within

and between the atmosphere, biosphere and the solid earth.

Some of the natural causes of global climate change include plate tectonics (land mass and ocean current

changes), volcanic activity (atmospheric dust and greenhouse gases), and long-term variations in the earth's

orbit and the angle of its rotation axis (absolute and spatial variations in solar radiation). More recently, anthropogenic (human) factors may be aecting the global climate. Since the late 19th

century, the average temperature of the earth has increased about 0.3 to 0.6 ºC. Many scientists believe

this global warming trend is the result of the increased release of greenhouse gases (e.g., CO2) into the

atmosphere from the combustion of fossil fuels.

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THE BIOSPHERE 1

5.1 THE BIOSPHERE

5.1.1 INTRODUCTION

The biosphere is the region of the earth that encompasses all living organisms: plants, animals and bacte-

ria. It is a feature that distinguishes the earth from the other planets in the solar system. "Bio" means life,

and the term biosphere was rst coined by a Russian scientist (Vladimir Vernadsky) in the 1920s. Another

term sometimes used is ecosphere("eco" meaning home). The biosphere includes the outer region of the

earth (the lithosphere ) and the lower region of the atmosphere (the troposphere). It also includes the

hydrosphere , the region of lakes, oceans, streams, ice and clouds comprising the earth's water resources.

Traditionally, the biosphere is considered to extend from the bottom of the oceans to the highest moun-

taintops, a layer with an average thickness of about 20 kilometers. Scientists now know that some forms of

microbes live at great depths, sometimes several thousand meters into the earth's crust.

Nonetheless, the biosphere is a very tiny region on the scale of the whole earth, analogous to the thickness

of the skin on an apple. The bulk of living organisms actually live within a smaller fraction of the biosphere,

from about 500 meters below the ocean's surface to about 6 kilometers above sea level. Dynamic interactions occur between the biotic region(biosphere) and the abiotic regions(atmosphere,

lithosphere and hydrosphere) of the earth. Energy, water, gases and nutrients are exchanged between the

regions on various spatial and time scales. Such exchanges depend upon, and can be altered by, the envi-

ronments of the regions. For example, the chemical processes of early life on earth (e.g. photosynthesis,

respiration, carbonate formation) transformed the reducing ancient atmosphere into the oxidizing (free oxy-

gen) environment of today. The interactive processes between the biosphere and the abiotic regions work

to maintain a kind of planetary equilibrium. These processes, as well as those that might disrupt this

equilibrium, involve a range of scientic and socioeconomic issues. The study of the relationships of living organisms with one another and with their environment is the

science known as ecology. The word ecology comes from the Greek words oikos and logos, and literally means

"study of the home." The ecology of the earth can be studied at various levels: an individual (organism),

a population , acommunity , anecosystem , abiome or the entire biosphere. The variety of living

organisms that inhabit an environment is a measure of its biodiversity.

5.1.2 ORGANISMS

Life evolved after oceans formed, as the ocean environment provided the necessary nutrients and support

medium for the initial simple organisms. It also protected them from the harsh atmospheric UV radiation.

As organisms became more complex they eventually became capable of living on land. However, this could 1

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CHAPTER 5. THE BIOSPHERE

not occur until the atmosphere became oxidizing and a protective ozone layer formed which blocked the

harmful UV radiation. Over roughly the last four billion years, organisms have diversied and adapted to all

kinds of environments, from the icy regions near the poles to the warm tropics near the equator, and from

deep in the rocky crust of the earth to the upper reaches of the troposphere. Despite their diversity, all living organisms share certain characteristics: they all replicate and all use

DNA to accomplish the replication process. Based on the structure of their cells, organisms can be classied

into two types: eukaryotes and prokaryotes. The main dierence between them is that a eukaryotehas a nucleus, which contains its DNA, while a

prokaryote does not have a nucleus, but instead its DNA is free-oating in the cell. Bacteria are prokary-

otes , and humans are eukaryotes. Organisms can also be classied according to how they acquire energy.

Autotrophs are "self feeders" that use light or chemical energy to make food. Plants are autotrophs.

Heterotrophs (i.e. other feeders ) obtain energy by eating other organisms, or their remains. Bacteria

and animals are heterotrophs. Groups of organisms that are physically and genetically related can be clas-

sied into species. There are millions of species on the earth, most of them unstudied and many of them

unknown. Insects and microorganisms comprise the ma jority of species, while humans and other mammals

comprise only a tiny fraction. In an ecological study, a single member of a species or organism is known as

an individual .

5.1.3 POPULATIONS AND COMMUNITIES

A number of individuals of the same species in a given area constitute a population. The number typically

ranges anywhere from a few individuals to several thousand individuals. Bacterial populations can number

in the millions. Populations live in a place or environment called a habitat. All of the populations of species

in a given region together make up a community. In an area of tropical grassland, a community might be

made up of grasses, shrubs, insects, rodents and various species of hoofed mammals. The populations and communities found in a particular environment are determined by abiotic and biotic

limiting factors . These are the factors that most aect the success of populations. Abiotic limiting factors

involve the physical and chemical characteristics of the environment. Some of these factors include: amounts

of sunlight, annual rainfall, available nutrients, oxygen levels and temperature. For example, the amount of

annual rainfall may determine whether a region is a grassland or forest, which in turn, aects the types of

animals living there. Each population in a community has a range of tolerancefor an abiotic limiting factor. There are

also certain maximum and minimum requirements known as tolerance limits, above and below which

no member of a population is able to survive. The range of an abiotic factor that results in the largest

population of a species is known as the optimum rangefor that factor. Some populations may have a

narrow range of tolerance for one factor. For example, a freshwater sh species may have a narrow tolerance

range for dissolved oxygen in the water. If the lake in which that sh species lives undergoes eutrophication,

the species will die. This sh species can therefore act as an indicator species, because its presence or

absence is a strict indicator of the condition of the lake with regard to dissolved oxygen content. Biotic limiting factors involve interactions between dierent populations, such as competition for food

and habitat. For example, an increase in the population of a meat-eating predator might result in a decrease

in the population of its plant-eating prey, which in turn might result in an increase in the plant population

the prey feeds on. Sometimes, the presence of a certain species may signicantly aect the community make

up. Such a species is known as a keystone species. For example, a beaver builds a dam on a stream and

causes the meadow behind it to ood. A starsh keeps mussels from dominating a rocky beach, thereby

allowing many other species to exist there.

5.1.4 ECOSYSTEMS

An ecosystem is a community of living organisms interacting with each other and their environment.

Ecosystems occur in all sizes. A tidal pool, a pond, a river, an alpine meadow and an oak forest are all

examples of ecosystems. Organisms living in a particular ecosystem are adapted to the prevailing abiotic

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and biotic conditions. Abiotic conditions involve both physical and chemical factors (e.g., sunlight, water,

temperature, soil, prevailing wind, latitude and elevation). In order to understand the ow of energy and

matter within an ecosystem, it is necessary to study the feeding relationships of the living organisms within

it. Living organisms in an ecosystem are usually grouped according to how they obtain food. Autotrophs

that make their own food are known as producers, while heterotrophs that eat other organisms, living

or dead, are known as consumers. The producers include land and aquatic plants, algae and microscopic

phytoplankton in the ocean. They all make their own food by using chemicals and energy sources from their

environment. For example, plants use photosynthesis to manufacture sugar (glucose) from carbon dioxide and water.

Using this sugar and other nutrients (e.g., nitrogen, phosphorus) assimilated by their roots, plants produce

a variety of organic materials. These materials include: starches, lipids, proteins and nucleic acids. Energy

from sunlight is thus xed as food used by themselves and by consumers. The consumers are classed into dierent groups depending on the source of their food. Herbivores(e.g.

deer, squirrels) feed on plants and are known as primary consumers. Carnivores(e.g. lions, hawks, killer

whales) feed on other consumers and can be classied as secondary consumers. They feed on primary

consumers. Tertiary consumers feed on other carnivores. Some organisms known as omnivores(e.g.,

bears, rats and humans) feed on both plants and animals. Organisms that feed on dead organisms are called

scavengers (e.g., vultures, ants and ies). Detritivores(detritus feeders, e.g. earthworms, termites, crabs)

feed on organic wastes or fragments of dead organisms. Decomposers (e.g. bacteria, fungi) also feed on organic waste and dead organisms, but they digest the

materials outside their bodies. The decomposers play a crucial role in recycling nutrients, as they reduce

complex organic matter into inorganic nutrients that can be used by producers. If an organic substance can

be broken down by decomposers, it is called biodegradable.

In every ecosystem, each consumer level depends upon lower-level organisms (e.g. a primary consumer

depends upon a producer, a secondary consumer depends upon a primary consumer and a tertiary consumer

depends upon a secondary consumer). All of these levels, from producer to tertiary consumer, form what

is known as a food chain. A community has many food chains that are interwoven into a complex food

web . The amount of organic material in a food web is referred to as its biomass. When one organism eats

another, chemical energy stored in biomass is transferred from one level of the food chain to the next. Most

of the consumed biomass is not converted into biomass of the consumer. Only a small portion of the useable

energy is actually transferred to the next level, typically 10 percent. Each higher level of the food chain

represents a cumulative loss of useable energy. The result is a pyramid of energy ow, with producers

forming the base level. Assuming 10 percent eciency at each level, the tertiary consumer level would use only 0.1 percent of

the energy available at the initial producer level. Because there is less energy available high on the energy

pyramid, there are fewer top-level consumers. A disruption of the producer base of a food chain, therefore,

has its greatest eect on the top-level consumer. Ecosystem populations constantly uctuate in response to changes in the environment, such as rainfall,

mean temperature, and available sunlight. Normally, such changes are not drastic enough to signicantly

alter ecosystems, but catastrophic events such as oods, res and volcanoes can devastate communities and

ecosystems. It may be long after such a catastrophic event before a new, mature ecosystem can become

established. After severe disturbance the make up of a community is changed. The resulting community

of species changes, as early, post disturbance, fast-growing species are out-competed by other species. This

natural process is called ecological succession. It involves two types of succession: primary succession

and secondary succession .

Primary succession is the development of the rst biota in a given region where no life is found. An

example is of this is the surrounding areas where volcanic lava has completely covered a region or has built

up a new island in the ocean. Initially, only pioneer speciescan survive there, typically lichensand

mosses , which are able to withstand poor conditions. They are able to survive in highly exposed areas

with limited water and nutrients. Lichen, which is made up of both a fungus and an alga, survives by

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CHAPTER 5. THE BIOSPHERE

mutualism. The fungus produces an acid, which acts to further dissolve the barren rock. The alga uses

those exposed nutrients, along with photosynthesis, to produce food for both. Grass seeds may land in the

cracks, carried by wind or birds. The grass grows, further cracking the rocks, and upon completing its own

life cycle, contributes organic matter to the crumbling rock to make soil. In time, larger plants, such as

shrubs and trees may inhabit the area, oering habitats and niches to immigrating animal life. When the

maximum biota that the ecosystem can support is reached, the climax communityprevails. This occurs

after hundreds if not thousands of years depending on the climate and location. Secondary succession begins at a dierent point, when an existing ecosystem's community of species is

removed by re, deforestation, or a bulldozer's work in a vacant lot, leaving only soil. The rst few centimeters

of this soil may have taken 1000 years to develop from solid rock. It may be rich in humus, organic waste,

and may be stocked with ready seeds of future plants. Secondary succession is also a new beginning, but

one with a much quicker regrowth of organisms. Depending on the environment, succession to a climax

community may only require 100 to 200 years with normal climate conditions, with communities progressing

through stages of early plantandanimal species ,mid-species andlate successional species . Some

ecosystems, however, can never by regained.

5.1.5 BIOMES

The biosphere can be divided into relatively large regions called biomes. A biome has a distinct climate

and certain living organisms (especially vegetation) characteristic to the region and may contain many

ecosystems. The key factors determining climate are average annual precipitation and temperature. These

factors, in turn, depend on the geography of the region, such as the latitude and elevation of the region,

and mountainous barriers. The ma jor types of biomes include: aquatic,desert ,forest ,grassland and

tundra . Biomes have no distinct boundaries. Instead, there is a transition zone called an ecotone, which

contains a variety of plants and animals. For example, an ecotone might be a transition region between a

grassland and a desert, with species from both. Water covers a ma jor portion of the earth's surface, so aquatic biomes contain a rich diversity of plants

and animals. Aquatic biomes can be subdivided into two basic types: freshwaterandmarine .

Freshwater has a low salt concentration, usually less than 1 percent, and occurs in several types of regions:

ponds and lakes, streams and rivers, and wetlands. Ponds and lakesrange in size, and small ponds may

be seasonal. They sometimes have limited species diversity due to isolation from other water environments.

They can get their water from precipitation, surface runo, rivers, and springs. Streams and riversare

bodies of owing water moving in one general direction (i.e., downstream). Streams and rivers start at their

upstream headwaters, which could be springs, snowmelt or even lakes. They continue downstream to their

mouths, which may be another stream, river, lake or ocean. The environment of a stream or river may

change along its length, ranging from clear, cool water near the head, to warm, sediment-rich water near

the mouth. The greatest diversity of living organisms usually occurs in the middle region. Wetlands are

places of still water that support aquatic plants, such as cattails, pond lilies and cypress trees. Types of

wetlands include marshes, swamps and bogs. Wetlandshave the highest diversity of species with many

species of birds, fur-bearing mammals, amphibians and reptiles. Some wetlands, such as salt marshes, are

not freshwater regions.

Marine regions cover nearly three-fourths of the earth's surface. Marine bodies are salty, having

approximately 35 grams of dissolved salt per liter of water (3.5 percent). Oceansare very large marine

bodies that dominate the earth's surface and hold the largest ecosystems. They contain a rich diversity of

living organisms. Ocean regions can be separated into four ma jor zones: intertidal,pelagic ,benthic and

abyssal . The intertidal zone is where the ocean meets the land. Sometimes, it is submerged and at other

times exposed, depending upon waves and tides. The pelagic zone includes the open ocean further away

from land. The benthic zone is the region below the pelagic zone, but not including the very deepest parts

of the ocean. The bottom of this zone consists of sediments. The deepest parts of the ocean are known as

the abyssal zone. This zone is very cold (near freezing temperatures), and under great pressure from the

overlying mass of water. Mid-ocean ridges occur on the ocean oor in abyssal zones. Coral reefsare found

in the warm, clear, shallow waters of tropical oceans around islands or along continental coastlines.

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They are mostly formed from calcium carbonate produced by living coral. Reefs provide food and shelter

for other organisms and protect shorelines from erosion. Estuaries are partially enclosed areas where fresh

water and silt from streams or rivers mix with salty ocean water. They represent a transition from land to

sea and from freshwater to saltwater. Estuaries are biologically very productive areas and provide homes for

a wide variety of plants, birds and animals.

Deserts are dry areas where evaporation usually exceeds precipitation. Rainfall is low less than 25

centimeters per year and can be highly variable and seasonal. The low humidity results in temperature

extremes between day and night. Deserts can be hot or cold. Hot deserts(e.g. the Sonovan) are very hot

in the summer and have relatively high temperatures throughout the year and have seasonal rainfall. Cold

deserts (e.g. the Gobi) are characterized by cold winters and low but year-round precipitation. Deserts

have relatively little vegetation and the substrate consists mostly of sand, gravel or rocks. The transition

regions between deserts and grasslands are sometimes called semiarid deserts(e.g. the Great Basin of the

western United States). Grasslands cover regions where moderate rainfall is sucient for the growth of grasses, but not enough

for stands of trees. There are two main types of grasslands: tropical grasslands(savannas) andtemperate

grasslands . Tropical grasslands occur in warm climates such as Africa and very limited regions of Australia.

They have a few scattered trees and shrubs, but their distinct rainy and dry seasons prevent the formation of

tropical forests. Lower rainfall, more variable winter-through-summer temperatures and a near lack of trees

characterize temperate grasslands. Prairies are temperate grasslands at fairly high elevation. They may be

dominated by long or short grass species. The vast prairies originally covering central North America, or the

Great Plains, were the result of favorable climate conditions created by their high elevation and proximity

to the Rocky Mountains. Because temperate grasslands are treeless, relatively at and have rich soil, most

have been replaced by farmland. Forests are dominated by trees and can be divided into three types: tropical forests,temperate

forests andboreal forests . Tropical forests are always warm and wet and are found at lower latitudes.

Their annual precipitation is very high, although some regions may have distinct wet and dry seasons.

Tropical forests have the highest biodiversity of this biome. Temperate forests occur at mid-latitudes (i.e.,

North America), and therefore have distinct seasons. Summers are warm and winters are cold. The temperate

forests have suered considerable alteration by humans, who have cleared much of the forest land for fuel,

building materials and agricultural use. Boreal forests are located in higher latitudes, like Siberia, where

they are known as " taiga." They have very long, cold winters and a short summer season when most of the

precipitation occurs. Boreal forests represent the largest biome on the continents. Very low temperatures, little precipitation and low biodiversity characterize tundra. Its vegetation is

very simple, with virtually no trees. The tundra can be divided into two dierent types: arctic tundra

and alpine tundra . The arctic alpine occurs in polar regions. It has a very short summer growing season.

Water collects in ponds and bogs, and the ground has a subsurface layer of permanently frozen soil known

as permafrost. Alpine tundra is found at high elevations in tall mountains. The temperatures are not as low

as in the arctic tundra, and it has a longer summer growing season.

5.1.6 EVOLUTION OF LIFE

Wherever they are found in the biosphere, living organisms are necessarily linked to their environment.

Ecosystems are dynamic and communities change over time in response to abiotic or biotic changes in the

environment. For example, the climate may be become warmer or colder, wetter or drier, or the food chain

may be disrupted by the loss of a particular population or the introduction of a new one. Species must

be able to adapt to these changes in order to survive. As they adapt, the organisms themselves undergo

change. Evolution is the gradual change in the genetic makeup of a population of a species over time. It

is important to note that it is the population that evolves, rather than individuals. A species evolves to a particular niche either by adapting to use a niche's environment or adapting to

avoid competition with another species. Recall that no two species can occupy the exact same niche in an

ecosystem. The availability of resources is pivotal.

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CHAPTER 5. THE BIOSPHERE

In the case of ve warbler species which all consume insects of the same tree, to survive each species

needs to gather its food (insects) in dierent parts of that tree. This avoids competition and the possible

extinction of one or more species. Therefore, one of the bird species will adapt to hunting at the treetops;

another the lowest branches; another the mid-section. In this way, these species have evolved into dierent,

yet similar, niches. All ve species in this way can survive by adapting to a narrow niche. Organisms with

a narrow niche are called specialized species. Another example is a species that may evolve to a narrow

niche by consuming only one type of leaf, such as the Giant Panda, which consumes bamboo leaves. This strategy allows it to co-exist with another consumer by not competing with it. In both cases, species

with a narrow niche are often vulnerable to extinction because they typically cannot respond to changes in

the environment. Evolving to a new niche would take too much time for the specialized species under the

duress of a drought, for example. On the other hand, a species that can use many foods and locations in which to hunt or gather are known

as generalized species . In the event of a drought, a generalized species such as a cockroach may be more

successful in nding alternative forms of food, and will survive and reproduce. Yet another form of evolution is co-evolution, where species adapt to one another by interacting closely.

This relationship can be a predator-prey type of interaction. Prey is at risk, but as a species it has evolved

chemical defenses or behaviors. On the other hand, co-evolution can be a mutualistic relationship, often

characterized by the ants and an acacia tree of South America. The acacia provides ants with food and a

habitat, and its large pro jecting thorns provides protection from predators. The ants, in turn, protect the

tree by attacking any animal landing on it and by clearing vegetation at its base. So closely evolved are the

species that neither can exist without the other. Similar ecosystems may oer similar niches to organisms, that are adapted or evolved to that niche.

Convergent evolution is the development of similar adaptations in two species occupying dierent yet

similar ecosystems. Two species evolve independently to respond to the demands of their ecosystem, and

they develop the same mechanism to do so. What emerges are adaptations that resemble look-alikes: Wings

of birds and bats are similar, but evolved separately to meet the demands of ying through air. The dolphin,

a mammal, shares adaptations that allow for movement through water with the extinct reptile ichthyosaur.

They have similar streamlined shapes of ns, head, and nose, which make the bodies better suited for

swimming. Natural selection is another process that depends on an organism's ability to survive in a changing

environment. While evolution is the gradual change of the genetic makeup over time, natural selection is

the force that favors a benecial set of genes. For example, birds migrating to an island face competition for the insects on a tropical tree. One genetic

pool of a new generation may include a longer beak, which allows the bird to reach into a tropical ower

for its nectar. When high populations of birds compete for insects, this ability to use the niche of collecting

nectar favors that bird's survival. The long-beaked gene is passed to the next generation and the next,

because birds can coexist with the insect-gathering birds by using a dierent niche. Through reproduction

of the surviving longer-beaked birds, natural selection favors its adaptability. A species, family or larger group of organisms may eventually come to the end of its evolutionary line.

This is known as extinction. While bad news for those that become extinct, it's a natural occurrence that

has been taking place since the beginning of life on earth. Extinctions of species are constantly occurring

at some background rate, which is normally matched by speciation. Thus, in the natural world, there is a

constant turnover of species. Occasionally large numbers of species have become extinct over a relatively short geologic time period.

The largest mass extinction event in the earth's history occurred at the end of the Permian period, 245

million years ago. As many as 96 percent of all marine species were lost, while on land more than 75

percent of all vertebrate families became extinct. Although, the actual cause of that extinction is unclear,

the consensus is that climate change, resulting from sea level change and increased volcanic activity, was

an important factor. The most famous of all mass extinctions occurred at the boundary of the Cretaceous

and Tertiary periods, 65 million years ago. About 85 percent of species became extinct, including all of the

dinosaurs. Most scientists believe that the impact of a small asteroid near the Yucatan Peninsula in Mexico

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triggered that extinction event. The impact probably induced a dramatic change in the world climate. The most serious extinction of mammals occurred about 11,000 years ago, as the last Ice Age was

ending. Over a period of just a few centuries, most of the large mammals around the world, such as the

mammoth, became extinct. While climate change may have been a factor in their extinction, a new force had

also emerged on the earth - modern humans. Humans, aided by new, sharp-pointed weapons and hunting

techniques, may have hurried the demise of the large land mammals. Over the years, human activity has

continued to send many species to an early extinction. The best known examples are the passenger pigeon

and the dodo bird, but numerous other species, many of them unknown, are killed o by over harvesting

and other human-caused habitat destruction, degradation and fragmentation.

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CHAPTER 5. THE BIOSPHERE

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