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“All flesh is grass”
-Isaiah 40:6
Three hundred trout are needed to support one man
for a year. The trout, in turn, must consume 90,000
frogs, that must consume 27 million grasshoppers
that live off of 1,000 tons of grass.
++++++++++++++++++++++++++
-- G.
Tyler Miller, Jr., American Chemist (1971)
+++++++++++++++++
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Energy Flow and Geochemical Cycling
in an Ecosystem
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Regardless of an ecosystem’s size, its
dynamics involve two main processes: energy
flow and chemical cycling
• Energy flows through ecosystems while matter
cycles within them
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Conservation of Energy –
The first law of thermodynamics
• States that energy cannot be created or
destroyed, only transformed
• Energy enters an ecosystem as solar radiation,
is conserved, and is lost from organisms as
heat
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• The second law of thermodynamics states that
every exchange of energy increases the entropy
of the universe
•
Entropy is commonly understood as a measure of molecular disorder
within a macroscopic system.
• In an ecosystem, energy conversions are not
completely efficient, and some energy is always
lost as heat
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Conservation of Mass
• The law of conservation of mass
states that matter cannot be
created or destroyed
• Chemical elements are continually
recycled within ecosystems
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
•In a forest ecosystem, most nutrients
enter as dust or solutes in rain and
are carried away in water
•Ecosystems are open systems,
absorbing energy and mass and
releasing heat and waste products
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Energy, Mass, and Trophic Levels
• Autotrophs build molecules themselves using
photosynthesis or chemosynthesis as an
energy source.
• Heterotrophs depend on the biosynthetic
output of other organisms
• Trophic level – hierarchy describing how
organisms obtain their energy
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
(reactants)
Energy and nutrients
Energy
and nutrients pass from
pass from primary
producers
primary producers (autotrophs)
(autotrophs) to
to
primary
consumers
primary
consumers
(herbivores)
to
(herbivores)
to secondary
secondary
consumers
(carnivores) to
consumers
(carnivores)
to tertiary
tertiary consumers
(carnivores
consumers
that
feed
on
other
carnivores)
(carnivores that feed
on other carnivores)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Detritivores, or decomposers, are consumers
that derive their energy from detritus, nonliving
organic matter
• Prokaryotes and fungi are important
detritivores
• Decomposition connects all trophic levels
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 55-3
Concept 55.2: Energy and other limiting factors
control 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Global Energy Budget
• The amount of solar radiation reaching the
Earth’s surface limits photosynthetic output of
ecosystems
• Only a small fraction of solar energy actually
strikes photosynthetic organisms, and even
less is of a usable wavelength
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 55.3: Energy transfer between trophic
levels is typically only 10% efficient
• Secondary production of an ecosystem is the
amount of chemical energy in food converted
to new biomass during a given period of time
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Production Efficiency
• When a caterpillar feeds on a leaf, only about
one-sixth of the leaf’s energy is used for
secondary production
• An organism’s production efficiency is the
fraction of energy stored in food that is not
used for respiration
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 55-9
*one-sixth of the leaf’s
energy is used for
secondary production
*Energy transfer
between trophic
levels is typically
only 10% efficient
100 J
Feces
Plant material
eaten by caterpillar
200 J
67 J
33 J
Growth (new biomass)
Cellular
respiration
Trophic Efficiency and Ecological Pyramids
• Trophic efficiency is the percentage of
production transferred from one trophic level to
the next
• It usually ranges from 5% to 20%
• Trophic efficiency is multiplied over the length
of a food chain
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Approximately 0.1% of chemical energy fixed
by photosynthesis reaches a tertiary consumer
• A pyramid of net production represents the loss
of energy with each transfer in a food chain
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 55-10
Tertiary
consumers
Secondary
consumers
100 J
1000 J
Primary
consumers
10,000 J
Primary
producers
100,000J
1,000,000 J of sunlight
• Each tier represents the dry weight of all
organisms in one trophic level
• Shows sharp decrease at successively higher
trophic levels
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 55.4: Biological and geochemical
processes cycle nutrients between organic and
inorganic parts of an ecosystem
• Life depends on recycling chemical elements
• Nutrient circuits in ecosystems involve biotic
and abiotic components and are often called
biogeochemical cycles
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Biogeochemical Cycles
• Gaseous carbon, oxygen, sulfur, and nitrogen
occur in the atmosphere and cycle globally
• Less mobile elements such as phosphorus,
potassium, and calcium cycle on a more local
level
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 55-14a
Transport
over land
Solar energy
Net movement of
water vapor by wind
Precipitation Evaporation
over ocean
from ocean
Precipitation
over land
Evapotranspiration
from land
Percolation
through
soil
Runoff and
groundwater
The Carbon Cycle
• Carbon-based organic molecules
are essential to all organisms
• Carbon reservoirs include fossil fuels, soils and
sediments, solutes in oceans, plant and animal
biomass, and the atmosphere
• CO2 is taken up and released through
photosynthesis and respiration; additionally,
volcanoes and the burning of fossil fuels
contribute CO2 to the atmosphere
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 55-14c
N2 in atmosphere
Assimilation
NO3–
Nitrogen-fixing
bacteria
Decomposers
Ammonification
NH3
Nitrogen-fixing
soil bacteria
Nitrification
NH4+
NO2–
Nitrifying
bacteria
Denitrifying
bacteria
Nitrifying
bacteria
The Terrestrial Nitrogen Cycle
• The pathway in which nitrogen moves
through the ecosystem
• Nitrogen is a component of amino acids,
proteins, nucleic acids (DNA), ATP
• The main reservoir of nitrogen (N2) is the
atmosphere, 78% . N2 gas must be converted
to other forms.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
4 processes of the Nitrogen Cycle
Nitrogen Fixation - process converts N2 into
NH4+ (ammonium ions). Done by nitrogen fixing
bacteria found in soil or nodules on roots of
legumes
Examples of legumes: peas, beans, alfalfa, peanuts,
clover
• NH4+ by ammonification, and NH4+ is decomposed
to NO2- and often to NO3– by nitrification
• Nitrifying bacteria convert NH4+ in soils
• Plants absorb nitrates through roots
Bashan, Y. and Levanony, H. (1987)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Ammonification
• Breakdown of nitrogen compounds back into
ammonia products
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Denitrification
• Conversion of NH4+, NO2-, NO3- back to
N2 gas (bacteria do this).
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Energy Flow and Geochemical Cycling
in an Ecosystem
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 55-17
Fig. 55-18
Winter
Summer
Fig. 55-16
(a) Concrete dam
and weir
Nitrate concentration in runoff
(mg/L)
(b) Clear-cut watershed
80
60
40
20
4
3
2
1
0
Deforested
Completion of
tree cutting
1965
Control
1966
(c) Nitrogen in runoff from watersheds
1967
1968
Fig. 55-14b
CO2 in atmosphere
Photosynthesis
Photosynthesis
Cellular
respiration
Burning of
fossil fuels Phytoand wood plankton
Higher-level
consumers
Primary
consumers
Carbon compounds
in water
Detritus
Decomposition
Toxins in the Environment
• Humans release many toxic chemicals,
including synthetics previously unknown
to nature
• In some cases, harmful substances persist for
long periods in an ecosystem
• One reason toxins are harmful is that they
become more concentrated in successive
trophic levels
• Biological magnification concentrates toxins
at higher trophic levels, where biomass is lower
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• PCBs and many pesticides such as DDT are
subject to biological magnification in
ecosystems
• In the 1960s Rachel Carson
brought attention to the
biomagnification of DDT in
birds in her book Silent Spring
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 55-20
Herring
gull eggs
124 ppm
Lake trout
4.83 ppm
Smelt
1.04 ppm
Zooplankton
0.123 ppm
Phytoplankton
0.025 ppm
Depletion of Atmospheric Ozone
• Life on Earth is protected from damaging
effects of UV radiation by a protective layer of
ozone molecules in the atmosphere
• Satellite studies suggest that the ozone layer
has been gradually thinning since 1975
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 55-23
Ozone layer thickness (Dobsons)
350
300
250
200
100
0
1955 ’60
’65
’70
’75
’80 ’85
Year
’90
’95 2000 ’05
• Destruction of atmospheric ozone probably
results from chlorine-releasing pollutants such
as CFCs produced by human activity
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 55-24
Chlorine atom
O2
Chlorine O3
ClO
O2
ClO
Cl2O2
Sunlight
• Scientists first described an “ozone hole” over
Antarctica in 1985; it has increased in size as
ozone depletion has increased
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 55-25
(a) September 1979
(b) September 2006