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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.
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