Ecology and

Ecosystems

•How energy flows through the ecosystem by understanding the terms in bold that relate to food chains and food webs.

•The difference between gross primary productivity and net primary productivity.

•The carbon and nitrogen biogeochemical cycles.

Ecosystems, Energy, and Matter

Ecosystems, Energy, and Matter

• An ecosystem consists of all the organisms living in a community – As well as all the

abiotic factors with which they interact

Ecosystems can range from a

microcosm, such as an

aquarium

•To a large area such as a

lake or forest

Life Depends on the Sun Energy from the sun enters an ecosystem when a

plant uses sunlight to make sugar molecules

(carbohydrates) by a process called photosynthesis. 6CO2 + 6H2O + solar energy C6H12O6 + 6O2

Carbohydrates are energy-rich molecules which organisms use to carry out daily activities, such as movement, growth, and repair.

As organisms consume food and use energy from carbohydrates, the energy travels from one organism to another.

An exception to the Rule: Deep-Ocean Ecosystems

Bacteria use the hydrogen sulfide present in hot water that escapes from cracks in the ocean floor to produce their own food. These bacteria are eaten by the other underwater organisms, thus supporting a thriving ecosystem in darkness.

An organism’s body breaks down its food to obtain the

energy stored in the food by a process called cellular

repiration.

C6H12O6 + 6O2 6CO2 + 6H2O + energy

When cellular respiration occurs, the carbon-carbon bonds found in carbohydrates are broken and the carbon is combined with oxygen to form carbon dioxide. This process releases the energy, which is either used by the organism (to move its muscles, digest food, excrete wastes, think, etc.) or the energy may be lost as heat.

Ecosystem ecology emphasizes energy

flow and chemical cycling

• Ecosystem ecologists view

ecosystems

– As transformers of energy and

processors of matter

• Ecosystems are not

supernatural so…

– They abide by..

• Thermodynamics

• The Law of Conservation of

Energy

• The Law of Conservation of

Matter

Trophic Relationships • Energy and

nutrients pass from primary producers (autotrophs) – To primary

consumers (herbivores) and then to secondary consumers (carnivores)

Trophic Relationships

• Only ~10-20% of energy flows from one trophic

level to the next.

• Energy flows through an ecosystem

– Entering as light and exiting as heat

• Nutrients cycle within an ecosystem

Figure 54.2

Microorganisms

and other

detritivores

Detritus

Primary producers

Primary consumers

Secondary

consumers

Tertiary

consumers

Heat

Sun

Key

Chemical cycling

Energy flow

Decomposition

• Decomposition

– Connects all trophic levels

•Detritivores, mainly

bacteria and fungi,

recycle essential

chemical elements •By decomposing

organic material and

returning elements to

inorganic reservoirs

Figure 54.2

Microorganisms

and other

detritivores

Detritus

Primary producers

Primary consumers

Secondary

consumers

Tertiary

consumers

Heat

Sun

Key

Chemical cycling

Energy flow Detritivores obtain energy from

nonliving organic matter called

Detritus.

Physical and chemical factors limit

primary production in ecosystems

• Primary production in an ecosystem – Is the

amount of light energy converted to chemical energy by autotrophs during a given time period

Earth is bombarded with 1022 joules of solar energy every day.

•This is enough energy to meet human demands for 24years at our 2004

consumption level.

Gross and Net Primary Production

• Total primary production

in an ecosystem

– Is known as that

ecosystem’s gross

primary production

(GPP)

• Not all of this production

– Is stored as organic

material in the growing

plants

Gross and Net Primary Production

• Net primary production (NPP)

– Is equal to GPP minus the energy used by the primary producers for respiration

• Only NPP

– Is available to consumers

NPP = GPP – R (respiration)

Different ecosystems vary considerably

in their net primary production

And in their contribution to the total NPP on Earth

Lake and stream

Open ocean

Continental shelf

Estuary

Algal beds and reefs

Upwelling zones

Extreme desert, rock, sand, ice

Desert and semidesert scrub

Tropical rain forest

Savanna

Cultivated land

Boreal forest (taiga)

Temperate grassland

Tundra

Tropical seasonal forest

Temperate deciduous forest

Temperate evergreen forest

Swamp and marsh

Woodland and shrubland

0 10 20 30 40 50 60 0 500 1,000 1,500 2,000 2,500 0 5 10 15 20 25

Percentage of Earth’s net

primary production

Key

Marine

Freshwater (on continents)

Terrestrial

5.2

0.3

0.1

0.1

4.7

3.5

3.3

2.9

2.7

2.4

1.8

1.7

1.6

1.5

1.3

1.0

0.4

0.4

125

360

1,500

2,500

500

3.0

90

2,200

900

600

800

600

700

140

1,600

1,200

1,300

2,000

250

5.6

1.2

0.9

0.1

0.04

0.9

22

7.9

9.1

9.6

5.4

3.5

0.6

7.1

4.9

3.8

2.3

0.3

65.0 24.4

Figure 54.4a–c

Percentage of Earth’s

surface area (a) Average net primary

production (g/m2/yr) (b)

(c)

Lake and stream

Open ocean

Continental shelf

Estuary

Algal beds and reefs

Upwelling zones

Extreme desert, rock, sand, ice

Desert and semidesert scrub

Tropical rain forest

Savanna

Cultivated land

Boreal forest (taiga)

Temperate grassland

Tundra

Tropical seasonal forest

Temperate deciduous forest

Temperate evergreen forest

Swamp and marsh

Woodland and shrubland

0 10 20 30 40 50 60 0 500 1,000 1,500 2,000 2,500 0 5 10 15 20 25

Percentage of Earth’s net

primary production

Key

Marine

Freshwater (on continents)

Terrestrial

5.2

0.3

0.1

0.1

4.7

3.5

3.3

2.9

2.7

2.4

1.8

1.7

1.6

1.5

1.3

1.0

0.4

0.4

125

360

1,500

2,500

500

3.0

90

2,200

900

600

800

600

700

140

1,600

1,200

1,300

2,000

250

5.6

1.2

0.9

0.1

0.04

0.9

22

7.9

9.1

9.6

5.4

3.5

0.6

7.1

4.9

3.8

2.3

0.3

65.0 24.4

Figure 54.4a–c

Percentage of Earth’s

surface area (a)

Average net primary

production (g/m2/yr

Overall, terrestrial ecosystems Contribute about two-thirds of global NPP

and marine ecosystems about one-third

Figure 54.5

180 120W 60W 0 60E 120E 180

North Pole

60N

30N

Equator

30S

60S

South Pole

Primary Production in Marine and

Freshwater Ecosystems

• In marine and freshwater ecosystems

– Both light and nutrients are important in

controlling primary production

Limiting Nutrients

• A limiting nutrient is the element that must

be added

– In order for production to increase in a particular

area

• Nitrogen and phosphorous

– Are typically the nutrients that most often limit

marine production

Life Depends on the Sun Energy from the sun enters an ecosystem when a

plant uses sunlight to make sugar molecules

(carbohydrates) by a process called photosynthesis. 6CO2 + 6H2O + solar energy C6H12O6 + 6O2

Carbohydrates are energy-rich molecules which organisms use to carry out daily activities, such as movement, growth, and repair.

As organisms consume food and use energy from carbohydrates, the energy travels from one organism to another.

An exception to the Rule: Deep-Ocean Ecosystems

Bacteria use the hydrogen sulfide present in hot water that escapes from cracks in the ocean floor to produce their own food. These bacteria are eaten by the other underwater organisms, thus supporting a thriving ecosystem in darkness.

An organism’s body breaks down its food to obtain the

energy stored in the food by a process called cellular

repiration.

C6H12O6 + 6O2 6CO2 + 6H2O + energy

When cellular respiration occurs, the carbon-carbon bonds found in carbohydrates are broken and the carbon is combined with oxygen to form carbon dioxide. This process releases the energy, which is either used by the organism (to move its muscles, digest food, excrete wastes, think, etc.) or the energy may be lost as heat.

Nutrient enrichment experiments

Confirmed that nitrogen was limiting phytoplankton growth in an area of the ocean

EXPERIMENT Pollution from duck farms concentrated near

Moriches Bay adds both nitrogen and phosphorus to the coastal water

off Long Island. Researchers cultured the phytoplankton Nannochloris

atomus with water collected from several bays.

Figure 54.6

Coast of Long Island, New York.

The numbers on the map indicate

the data collection stations.

Shinnecock

Bay

Moriches Bay

Atlantic Ocean

30 21

19 15

11 5

4

2

Nutrient Enrichment Experiments

Figure 54.6

(a) Phytoplankton biomass and phosphorus concentration (b) Phytoplankton response to nutrient enrichment

Great

South Bay

Moriches

Bay

Shinnecock

Bay

Starting

algal

density

2 4 5 11 30 15 19 21

30

24

18

12

6

0

Unenriched control

Ammonium enriched Phosphate enriched

Station number

Ph

yto

pla

nkto

n

(mill

ion

s o

f ce

lls p

er

mL

)

8

7

6

5

4

3

2

1

0 2 4 5 11 30 15 19 21

8

7

6

5

4

3

2

1

0

Ino

rga

nic

ph

osp

horu

s

(g a

tom

s/L

)

Ph

yto

pla

nkto

n

(mill

ion

s o

f ce

lls/m

L)

Station number

CONCLUSION Since adding phosphorus, which was already in rich supply, had no effect on Nannochloris growth, whereas adding nitrogen increased algal density dramatically, researchers concluded that nitrogen was the nutrient limiting phytoplankton growth in this ecosystem.

Phytoplankton

Inorganic

phosphorus

RESULTS Phytoplankton abundance parallels the abundance of phosphorus in the water (a).

Nitrogen, however, is immediately taken up by algae, and no free nitrogen is measured in the

coastal waters. The addition of ammonium (NH4) caused heavy phytoplankton growth in bay

water, but the addition of phosphate (PO43) did not induce algal growth (b).

Experiments in another ocean region

Showed that iron limited primary production

Table 54.1

Limiting Nutrients

• The addition of large amounts of nutrients

to lakes

– Has a wide range of ecological impacts

Limiting Nutrient • In some areas, sewage runoff

– Has caused eutrophication of lakes, which can

lead to the eventual loss of most fish species

from the lakes

Figure 54.7

Eutrophication Video 1

Eutrophication Video 2

Eutrophication Video 3

Primary Production in Terrestrial and

Wetland Ecosystems • In terrestrial and wetland

ecosystems climatic factors – Such as temperature

and moisture, affect primary production on a large geographic scale

• The contrast between wet and dry climates – Can be represented by

a measure called actual evapotranspiration

Actual evapotranspiration

• Is the amount of water annually transpired by plants and evaporated from a landscape

– Is related to net primary production

Figure 54.8

Actual evapotranspiration (mm H2O/yr)

Tropical forest

Temperate forest

Mountain coniferous forest

Temperate grassland

Arctic tundra

Desert

shrubland

Ne

t p

rim

ary

pro

du

ction

(g

/m2/y

r)

1,000

2,000

3,000

0

500 1,000 1,500 0

•Figure 54.8

shows…

•Tropical Forest

has the

greatest NPP

•Desert

Shrubland has

the least NPP

Soil Productivity

On a more local scale

A soil nutrient is often the limiting factor in primary production

EXPERIMENT

Over the summer of 1980, researchers added

phosphorus to some experimental plots in the salt marsh, nitrogen

to other plots, and both phosphorus and nitrogen to others. Some

plots were left unfertilized as controls. RESULTS

Experimental plots receiving just

phosphorus (P) do not outproduce

the unfertilized control plots.

CONCLUSION

Liv

e,

above-g

round b

iom

ass

(g d

ry w

t/m

2)

Adding nitrogen (N)

boosts net primary

production.

300

250

200

150

100

50

0 June July August 1980

N P

N only

Control

P only

These nutrient enrichment experiments

confirmed that nitrogen was the nutrient limiting plant growth in

this salt marsh.

Energy transfer between trophic levels is

usually less than 20% efficient

• The secondary production of an ecosystem – Is the amount of chemical energy in consumers’ food

that is converted to their own new biomass during a

given period of time

Figure 54.10

Plant material

eaten by caterpillar

Cellular

respiration

Growth (new biomass)

Feces 100 J

33 J

200 J

67 J

•When a caterpillar feeds on a

plant leaf

•Only about one-sixth of the

energy in the leaf is used for

secondary production

The production efficiency of an organism

– Is the fraction of energy stored in food that is

not used for respiration

Trophic Efficiency and

Ecological Pyramids • Trophic efficiency

– Is the percentage of production transferred from one trophic level

to the next

– Usually ranges from 5% to 20%

Figure 54.11

Tertiary

consumers

Secondary

consumers

Primary

consumers

Primary

producers

1,000,000 J of sunlight

10 J

100 J

1,000 J

10,000 J

•This loss of energy with

each transfer in a food

chain •Can be represented by a

pyramid of net production

Pyramids of Biomass

• One important ecological consequence of low trophic efficiencies – Can be represented in a

biomass pyramid

• Most biomass pyramids

– Show a sharp decrease

at successively higher

trophic levels

(a) Most biomass pyramids show a sharp decrease in biomass at

successively higher trophic levels, as illustrated by data from

a bog at Silver Springs, Florida.

Trophic level Dry weight

(g/m2)

Primary producers

Tertiary consumers

Secondary consumers

Primary consumers

1.5

11

37

809

Exceptions to the Biomass Pyramid

• Certain aquatic ecosystems

– Have inverted biomass pyramids

Figire 54.12b

Trophic level

Primary producers (phytoplankton)

Primary consumers (zooplankton)

(b) In some aquatic ecosystems, such as the English Channel,

a small standing crop of primary producers (phytoplankton)

supports a larger standing crop of primary consumers

(zooplankton).

Dry weight

(g/m2)

21

4

Pyramids of Numbers

• A pyramid of numbers

– Represents the number of individual

organisms in each trophic level

Figure 54.13

Trophic level Number of

individual organisms

Primary producers

Tertiary consumers

Secondary consumers

Primary consumers

3

354,904

708,624

5,842,424

Humans and Energy Efficiency

• The dynamics of energy flow through

ecosystems

– Have important implications for the human

population

• Eating meat

– Is a relatively inefficient way of tapping

photosynthetic production

Worldwide agriculture could

successfully feed many more people

If humans all fed more efficiently, eating only

plant material

Figure 54.14

Trophic level

Secondary

consumers

Primary

consumers

Primary

producers

The Green World Hypothesis • According to the green world hypothesis

– Terrestrial herbivores consume relatively little plant biomass

because they are held in check by a variety of factors

• Most terrestrial ecosystems have large standing crops despite the

large numbers of herbivores

Figure 54.15

The green world hypothesis

• proposes several factors that keep

herbivores in check

– Plants have defenses against herbivores

– Nutrients, not energy supply, usually limit

herbivores

– Abiotic factors limit herbivores

– Intraspecific competition can limit herbivore

numbers

– Interspecific interactions check herbivore

densities

Biological and geochemical processes move

nutrients between organic and inorganic

parts of the ecosystem

• Life on Earth

– Depends on the recycling of essential chemical

elements

• Nutrient circuits that cycle matter through an

ecosystem

– Involve both biotic and abiotic components and

are often called biogeochemical cycles

A General Model of Chemical

Cycling • Gaseous forms of carbon, oxygen, sulfur, and nitrogen

– Occur in the atmosphere and cycle globally

• Less mobile elements, including phosphorous, potassium, and calcium – Cycle on a more local level

A general model of

nutrient cycling

Includes the main

reservoirs of elements

and the processes that

transfer elements

between reservoirs Figure 54.16

Organic

materials

available

as nutrients

Living

organisms,

detritus

Organic

materials

unavailable

as nutrients

Coal, oil,

peat

Inorganic

materials

available

as nutrients

Inorganic

materials

unavailable

as nutrients

Atmosphere,

soil, water Minerals

in rocks Formation of

sedimentary rock

Weathering,

erosion

Respiration,

decomposition,

excretion Burning

of fossil fuels

Fossilization

Reservoir a Reservoir b

Reservoir c Reservoir d

Assimilation,

photosynthesis

Biogeochemical Cycles

• All elements

– Cycle between organic and inorganic

reservoirs

Biogeochemical Cycles

• The water cycle and the carbon cycle

Figure 54.17

Transport

over land

Solar energy

Net movement of

water vapor by wind

Precipitation

over ocean Evaporation

from ocean

Evapotranspiration

from land

Precipitation

over land

Percolation

through

soil

Runoff and

groundwater

CO2 in atmosphere

Photosynthesis

Cellular

respiration

Burning of

fossil fuels

and wood Higher-level

consumers Primary

consumers

Detritus Carbon compounds

in water

Decomposition

THE WATER CYCLE THE CARBON CYCLE

Water Cycle (Read Page 1196)

The Carbon Cycle (Read Page 1196)

The Nitrogen Cycle (Read Page 1197)

The Phosphorus Cycle (Read Page 1197)

Decomposition and Nutrient

Cycling Rates • Decomposers (detritivores) play a key role

– In the general pattern of chemical cycling

Figure 54.18

Consumers

Producers

Nutrients

available

to producers

Abiotic

reservoir

Geologic

processes

Decomposers

•The rates at which

nutrients cycle in

different ecosystems

•Are extremely

variable, mostly as

a result of

differences in rates

of decomposition

Vegetation and Nutrient Cycling: The Hubbard

Brook Experimental Forest

• Nutrient cycling – Is strongly regulated by

vegetation

• Long-term ecological research projects – Monitor ecosystem dynamics

over relatively long periods of time

• The Hubbard Brook Experimental Forest – Has been used to study nutrient

cycling in a forest ecosystem since 1963

• The research team

constructed a dam on the site

– To monitor water and

mineral loss

(a) Concrete dams and weirs built across

streams at

the bottom of watersheds enabled

researchers to monitor the outflow of

water and nutrients from the ecosystem.

Experiment: Human Disturbances and how

they effect nutrient cycles

• In one experiment, the trees in one valley

were cut down

– And the valley was sprayed with herbicides

Figure 54.19b (b) One watershed was clear cut to study the effects of the loss

of vegetation on drainage and nutrient cycling.

Experiment: Human Disturbances and how

they effect nutrient cycles

• Net losses of water and minerals were studied

– And found to be greater than in an undisturbed area

• These results showed how human activity

– Can affect ecosystems

Figure 54.19c

(c) The concentration of nitrate in runoff from the deforested

watershed was 60 times greater than in a control (unlogged)

watershed.

Nitra

te c

on

ce

ntr

atio

n in r

un

off

(mg/L

)

Deforested

Control

Completion of

tree cutting

1965 1966 1967 1968

80.0

60.0

40.0

20.0

4.0

3.0

2.0

1.0

0

The human population is disrupting

chemical cycles throughout the

biosphere

• As the human population has grown in size – Our activities

have disrupted the trophic structure, energy flow, and chemical cycling of ecosystems in most parts of the world

Nutrient Enrichment

• In addition to transporting nutrients from

one location to another

– Humans have added entirely new materials,

some of them toxins, to ecosystems

Nutrient Enrichment • In addition to transporting nutrients from one

location to another

– Humans have added entirely new materials, some of

them toxins, to ecosystems

• Agriculture

constantly removes

nutrients from

ecosystems

– That would

ordinarily be cycled

back into the soil

Figure 54.20

Nutrient Enrichment

• Nitrogen is the main nutrient lost through

agriculture

– Thus, agriculture has a great impact on the

nitrogen cycle

• Industrially produced fertilizer is typically

used to replace lost nitrogen

– But the effects on an ecosystem can be

harmful

Contamination of Aquatic

Ecosystems • The critical load

for a nutrient – Is the amount

of that nutrient that can be absorbed by plants in an ecosystem without damaging it

• When excess nutrients

are added to an

ecosystem, the critical

load is exceeded

– And the remaining

nutrients can

contaminate

groundwater and

freshwater and

marine ecosystems

• Sewage runoff

contaminates

freshwater (and

saltwater)

ecosystems

– Causing cultural

eutrophication,

excessive algal

growth, which can

cause significant

harm to these

ecosystems

This is happening in your

backyard!!!!

Click Picture to learn about

the NY/NJ Harbor Estuary

studies and action plans

Acid Precipitation • Combustion of fossil fuels

– Is the main cause of acid precipitation

• North American and

European ecosystems

downwind from

industrial regions

– Have been damaged

by rain and snow

containing nitric and

sulfuric acid

Figure 54.21

4.6

4.6

4.3

4.1 4.3

4.6

4.6 4.3

Europe

North America

Acid Precipitation

• By the year 2000

– The entire contiguous United States was affected by acid precipitation

Figure 54.22

Field pH

5.3 5.2–5.3

5.1–5.2

5.0–5.1 4.9–5.0

4.8–4.9

4.7–4.8

4.6–4.7

4.5–4.6 4.4–4.5

4.3–4.4 4.3

Toxins in the Environment

• Humans release an immense variety of

toxic chemicals

– Including thousands of synthetics previously

unknown to nature

• One of the reasons such toxins are so

harmful

– Is that they become more concentrated in

successive trophic levels of a food web

Biological Magnification – Toxins

concentrate at higher trophic levels because at these levels biomass tends to be lower

Co

nce

ntr

ation

of P

CB

s

Herring

gull eggs

124 ppm

Zooplankton

0.123 ppm Phytoplankton

0.025 ppm

Lake trout

4.83 ppm

Smelt

1.04 ppm

Polychlorinated Biphenyl (PCB)

PCBs belong to a broad family of man-made organic

chemicals known as chlorinated hydrocarbons. PCBs were

domestically manufactured from 1929 until their

manufacture was banned in 1979. They have a range of

toxicity and vary in consistency from thin, light-colored

liquids to yellow or black waxy solids. Due to their non-

flammability, chemical stability, high boiling point, and

electrical insulating properties, PCBs were used in

hundreds of industrial and commercial applications

including electrical, heat transfer, and hydraulic equipment;

as plasticizers in paints, plastics, and rubber products; in

pigments, dyes, and carbonless copy paper; and many

other industrial applications.

Atmospheric Carbon Dioxide • One pressing problem caused by human

activities

– Is the rising level of atmospheric carbon

dioxide

• Due to the increased burning of fossil fuels and other human activities

– The concentration of atmospheric CO2 has been steadily increasing

Figure 54.24

CO

2 c

on

ce

ntr

atio

n (

pp

m)

390

380

370

360

350

340

330

320

310

300 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

1.05

0.90

0.75

0.60

0.45

0.30

0.15

0

0.15

0.30

0.45

Te

mp

era

ture

va

ria

tio

n (C

)

Temperature

CO2

Year

National Geographic Video on

Global Warming

Click on picture for 3min

video

The Greenhouse Effect and

Global Warming • The greenhouse effect is caused by many

gases, but atmospheric CO2 plays a major

role

– But is necessary to keep the surface of the

Earth at a habitable temperature

• Increased levels of atmospheric CO2 are

magnifying the greenhouse effect

– Which could cause global warming and

significant climatic change

Depletion of Atmospheric

Ozone • Life on Earth is protected from the

damaging effects of UV radiation

– By a protective layer or ozone molecules (O3)

present in the atmosphere

• Satellite studies of the atmosphere

– Suggest that the ozone layer has been gradually thinning since 1975

Figure 54.26

Ozone laye

r th

ickness (

Dobson u

nits)

Year (Average for the month of October)

350

300

250

200

150

100

50

0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

The destruction of atmospheric

ozone

Probably results from chlorine-releasing

pollutants produced by human activity

Figure 54.27

1

2

3

Chlorine from CFCs interacts with ozone (O3),

forming chlorine monoxide (ClO) and

oxygen (O2).

Two ClO molecules

react, forming

chlorine peroxide (Cl2O2).

Sunlight causes

Cl2O2 to break

down into O2

and free

chlorine atoms.

The chlorine

atoms can begin

the cycle again. Sunlight

Chlorine O3

O2

ClO

ClO

Cl2O2

O2

Chlorine atoms

• What Is the Ecosystem Approach to Ecology?

• 1.Describe the relationship between autotrophs and heterotrophs in an ecosystem.

• 2.Explain how decomposition connects all trophic levels in an ecosystem.

• 3.Explain how the first and second laws of thermodynamics apply to ecosystems.

• Primary Production in Ecosystems

• 4.Explain why the amount of energy used in photosynthesis is so much less than the amount of solar energy that reaches Earth.

• 5.Define and compare gross primary production and net primary production. 6.Define and compare biomass and standing crop.

• 7.Compare primary productivity in marine, freshwater, and terrestrial ecosystems.

• Secondary Production in Ecosystems

• 8.Explain why energy is said to flow rather than cycle within ecosystems. Use the example of insect caterpillars to illustrate energy flow.

• 9.Define, compare, and illustrate the concepts of production efficiency and trophic efficiency.

• 10.Distinguish between energy pyramids and biomass pyramids. Explain why both relationships are in the form of pyramids. Explain the special circumstances of inverted biomass pyramids.

• How Specific Immunity Arises

• 11.Explain why food pyramids usually have only four or five trophic levels 12.Define the pyramid of numbers.

• 13.Explain why worldwide agriculture could feed more people if all humans consumed only plant material.

• 14.Explain the green-world hypothesis. Describe six factors that keep herbivores in check.

• The Cycling of Chemical Elements in Ecosystems

• 15.Describe the four nutrient reservoirs and the processes that transfer the elements between reservoirs.

• 16.Explain why it is difficult to trace elements through biogeochemical cycles.

• 17.Describe the hydrologic water cycle.

• 18.Describe the nitrogen cycle and explain the importance of nitrogen fixation to all living organisms.

• 19.Describe the phosphorus cycle and explain how phosphorus is recycled locally in most ecosystems.

• 20.Explain how decomposition affects the rate of nutrient cycling in ecosystems.

• 21.Describe the experiments at Hubbard Brook that revealed the key role that plants play in regulating nutrient cycles.

• Human Impact on the Chemical Dynamics of the Biosphere

• 22.Describe how agricultural practices can interfere with nitrogen cycling.

• 23.Explain how "cultural eutrophication" can alter freshwater ecosystems.

• 24.Describe the causes and consequences of acid precipitation.

• 25.Explain why toxic compounds usually have the greatest effect on top-level carnivores.

• 26.Describe how increased atmospheric concentrations of carbon dioxide could affect Earth.

• 27.Describe how human interference might alter the biosphere.