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Chapter 22: Energy in the
Ecosystem
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Background: Organizing Concepts
 In
1920s, English ecologist Charles Elton and others
promoted a revolutionary concept:


organisms living in the same place not only have similar
tolerances of physical factors, but
feeding relationships link these organisms into a single functional
entity
 This
system of feeding relationships is called a food
web.
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The Ecosystem Concept
 The
English ecologist A.G. Tansley took Elton’s ideas one
step further:

in 1935 Tansley coined the term ecosystem, the fundamental unit
of ecological organization
 the ecosystem concept: “the biological and physical parts of
nature together, unified by the dependence of animals and plants
on their physical surroundings and by their contributions to
maintaining the conditions and composition of the physical world.”
-R.E. Ricklefs
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Alfred J. Lotka, the Thermodynamic
Concept, and Lindeman’s concept
 Alfred
J. Lotka introduced the concept of the ecosystem
as an energy-transforming machine:

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described by a set of equations representing exchanges of
matter and energy among components, and
obeying thermodynamic principles that govern all energy
transformations
In 1942, Raymond Lindeman brought Lotka’s ideas of the ecosystem
as an energy-transforming machine to the attention of ecologists.
He incorporated:
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Lotka’s thermodynamic concepts
Elton’s concept of the food web as expression of the ecosystem’s
structure
Tansley’s concept of the ecosystem as the fundamental unit in ecology
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Lindeman’s Foundations of
Ecosystem Ecology
 The
ecosystem is the fundamental unit of ecology.
 Within
the ecosystem, energy passes through many steps
or links in a food chain.
 Each
link in the food chain is a trophic level (or feeding
level).
 Inefficiencies
in energy transformation lead to a pyramid
of energy in the ecosystem.
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Odum’s Energy Flux Model

Eugene P. Odum popularized ecology to a generation of
ecologists.

Odum further developed the emerging framework of
ecosystem ecology:
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
he recognized the utility of energy and masses of elements as
common “currencies” in comparative analysis of ecosystem
structure and function
Odum extended his models to incorporate nutrient cycling.
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Fluxes of energy and materials are closely linked in ecosystem
function. However, they are fundamentally different:
 energy enters ecosystems as light and is degraded into heat
 nutrients cycle indefinitely, converted from inorganic to organic
forms and back again
Studies of nutrient cycling provide an index to fluxes of energy.
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Simple
Ecosystem
Model
energy
input from
sun
PHOTOAUTOTROPHS
(plants, other producers)
nutrient
cycling
HETEROTROPHS
(consumers, decomposers)
energy output (mainly heat)
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Models of ecological energy flow
A single trophic level
A food chain
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An ecological pyramid of energy
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Only 5% to 20% of energy passes
between trophic levels.
 Energy
on:
reaching each trophic level depends
net primary production (base of food chain)
efficiencies of transfers between trophic levels
- More on this later 

 Plant
use between 15% and 70% of light
energy assimilated for maintenance – thus that
portion is unavailable to consumers
 Herbivores
and carnivores expend more
energy on maintenance than do plants:
production of each trophic level is only 5%
to 20% that of the level below it.
Energy:
how
many
lbs
of
grass
to
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support one hawk
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Ocean food pyramid – roughly
2500 lbs/1136 kg of phytoplankton
to support 0.5lb/0.23 kg of tuna
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Primary Production: reminder
Primary production is the process whereby plants,algae, and some bacteria
(primary producers) capture the energy of light and transform it into the energy
of chemical bonds in carbohydrate:
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its rate is referred to as primary productivity
6CO2 + 6H2O  C6H12O6 + 6O2
for each g of C assimilated, 39 kJ energy stored
The rate of primary production determines the rate of energy supply to the rest of
the ecosystem:
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gross primary production = total energy assimilated by primary
producers
net primary production = energy accumulated (in stored form)
by primary producers
gross - net = respiration, the energy consumed by producers for
maintenance and biosynthesis
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Measurement of Primary
Production 1
How much energy has been accumulated by net production?
 harvest
techniques determine dry mass accumulated
(net production)
 gas exchange techniques determine net uptake of CO2
in light (net production), production of CO2 in dark
(respiration) and gross production as their sum
 Radioactive carbon (14C) may also determine net uptake
of carbon by plants
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Measurements of Carbon Dioxide flux in dark and
light can provide an estimate of GPP
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Measurement of Primary
Production 2
 Aquatic
systems pose special problems:
 harvest
approach is not practical for small
organisms, such as phytoplankton
 carbon is too abundant for practical
measurement of small changes
 Alternatives
 light
for aquatic systems:
and dark bottles may be used to determine
changes in O2
 14C approach may also be used in unproductive
waters
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Use of Remote Sensing

Satellites can use spectral bands to infer amount of
chlorophyll in water or the near-infrared to red ratio on land
(NDVI index)

NDVI=Normalized Difference Vegetation Index
Fig. 47-10, p.850
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Effects of Light and Temperature 1

Plants are not usually light-limited in full sun.

Shading (by other leaves or plants) may reduce photosynthetic rate
below its maximum.
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Overall, photosynthetic efficiency of the ecosystem is typically 1-2%:
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remaining energy is either reflected or absorbed and dissipated
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Leaves reflect 25 to 75%
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Molecules other than photosynthetic pigments absorb remainder –
converted to heat and radiated, or conducted across leaf surface, or
transpired
Photosynthetic efficiency

Percentage of the energy in sunlight that is converted to net primary
production during the growing season
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Effects of Light and Temperature 2
 Optimum
temperature for photosynthesis
varies with system:
about 16oC for many temperate species
 as high as 38oC for some tropical species
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 Rate
of photosynthesis increases with
temperature, up to a point:
rate of respiration also increases with temperature
 net assimilation may thus decrease at high
temperatures
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Water limits primary production
(reminder)
 Photosynthesis
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under water stress, stomates close and gas exchange ceases,
stopping photosynthesis
 Transpiration
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in terrestrial systems is water-limited:
or water-use efficiency:
typically 2g production per kg of water transpired (4g for
drought-tolerant crops)
ecosystem-level efficiency may be an order of magnitude poorer
(0.2 g/kg)
Most precipitation is not taken up by plants
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Nutrients stimulate primary
production – terrestrial and aquatic.
 Terrestrial
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fertilizers stimulate crop production
N is the most common limiting element
 Aquatic
limited:
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production may be nutrient -limited:
systems are often strongly nutrient -
especially true of open ocean
inadvertent addition of nutrients may stimulate unwanted
production
+ Effects of fertilizer on plant growth
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Primary production varies among
ecosystems.
 Primary
production is maximum under favorable
combinations of:
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intense sunlight
warm temperatures
abundant rainfall
ample nutrients
 On
land, production is highest in humid tropics,
lowest in tundra and desert.
+ NPP among ecosystems
Credit: © Richard Herrmann/Visuals Unlimited
Pickelweed saltmarsh.
205379
Credit: © Theo Allofs/Visuals Unlimited
Temperate Rainforest showing moss-covered trees and ferns, Olympic National Park,
Washington.
283044
Credit: © Adam Jones/Visuals Unlimited
Fall foliage and view of Mt. LeConte, Great Smokey Mountains National Park, Tennessee.
212904
Credit: © Beth Davidow/Visuals Unlimited
Northern Boreal Forest of Spruce and Aspens and tundra ponds.
301419
Credit: © Joe McDonald/Visuals Unlimited
African Lioness (Panthera leo) and African Elephants, Masai Mara Game Reserve, Kenya.
300241
Credit: © Richard Herrmann/Visuals Unlimited
Chaparral vegetation.
205342
Credit: © Steve Maslowski/Visuals Unlimited
A Bison herd on the prairie.
210424
Credit: © Patrick J. Endres/Visuals Unlimited
Arctic tundra biome in summer, Alaska Range Mountains, Denali National Park, Alaska.
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Credit: © Richard Thom/Visuals Unlimited
Sonoran Desert scene with Creosote Bush, Saguaro, Cholla, and Paloverde.
307010
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Only 5% to 20% of energy passes
between trophic levels.
 Energy
on:


reaching each trophic level depends
net primary production (base of food chain)
efficiencies of transfers between trophic levels
 Plant
use between 15% and 70% of light
energy assimilated for maintenance – thus that
portion is unavailable to consumers
 Herbivores
and carnivores expend more
energy on maintenance than do plants:
production of each trophic level is only 5%
to 20% that of the level below it.
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Ecological Efficiency
 Ecological
efficiency (food
chain efficiency) is
the percentage of
energy transferred
from one trophic
level to the next:
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el
range of 5% to 20% is
typical, as we’ve seen
to understand this more
fully, we must study the
use of energy within a
trophic level
Undigested plant fibers in elephant dung
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Intratrophic Energy Transfers
 Intratrophic
transfers involve several
components:
ingestion (energy content of food ingested)
 egestion (energy content of indigestible materials
regurgitated or defecated) (the elephant dung)
 assimilation (energy content of food digested and
absorbed)
 excretion (energy content of organic wastes)
 respiration (energy consumed for maintenance)
 production (residual energy content for growth and
reproduction)
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Fundamental Energy Relationships
 Components
of an animal’s energy budget are
related by:
 ingested
energy - egested energy = assimilated
energy
 assimilated energy - respiration - excretion =
production
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Assimilation Efficiency
 Assimilation
efficiency =
assimilation/ingestion
 primarily
a function of food quality:
 seeds: 80%
 young
vegetation: 60-70%
 plant foods of grazers, browsers: 30-40%
 decaying wood: 15%
 animal foods: 60-90%
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Net Production Efficiency
 Net
production efficiency =
production/assimilation
 depends
largely on metabolic activity:
birds: <1%
 small mammals: <6%
 sedentary, cold-blooded animals: as much as 75%
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 Gross
production efficiency = assimilation
efficiency x net production efficiency =
production/ingestion, ranges from below 1%
(birds and mammals) to >30% (aquatic
animals).
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Active, warm-blooded animals – low net
production efficiencies; hummingbird: <1%
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Production Efficiency in Plants
 The
concept of production efficiency is
somewhat different for plants because
plants do not digest and assimilate food:
 net
production efficiency = net
production/gross production; varies between
30% and 85%
 rapidly growing plants in temperate zone have
net production efficiencies of 75-85%; their
counterparts in the tropics are 40-60% efficient
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Detritus Food Chains
 Ecosystems
support two parallel food
chains:
 herbivore-based
(relatively large animals feed
on leaves, fruits, seeds)
 detritus-based (microorganisms and small
animals consume dead remains of plants and
indigestible excreta of herbivores)
 herbivores consume:
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1.5-2.5% of net primary production in temperate forests
12% in old-field habitats
60-99% in plankton communities
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Exploitation Efficiency
 When
production and consumption are not
balanced, energy may accumulate in the
ecosystem (as organic sediments).
 Exploitation
efficiency = ingestion by one
trophic level/production of the trophic level
below it.
 To
the extent that exploitation efficiency is
<100%, ecological efficiency = exploitation
efficiency x gross production efficiency.
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Stop here
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Energy moves through ecosystems at
different rates.
 Other
indices address how rapidly energy cycles through
an ecosystem:
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residence time measures the average time a packet of energy
resides in storage:
 residence time (yr) = energy stored in biomass/net productivity
biomass accumulation ratio is a similar index based on biomass
rather than energy:
 biomass accumulation ratio (yr) = biomass/rate of biomass
production
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Biomass Accumulation Ratios
Biomass accumulation ratios
become larger as amount of
stored energy increases:
 humid tropical forests have
net production of 1.8
kg/m2/yr and biomass of 43
kg/m2, yielding biomass
accumulation ratio of 23yr
 ratios for forested terrestrial
communities are typically >20
yr
 ratios for planktonic aquatic
ecosystems are <20 days
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Residence Time for Litter
 Decomposition
of litter is dependent on conditions
of temperature and moisture.
 Index
is residence time = mass of litter
accumulation/rate of litter fall:
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3 months in humid tropics
1-2 yr in dry and montane tropics
4-16 yr in southeastern US
>100 yr in boreal ecosystems
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Ecosystem Energetics

Comparative studies of ecosystem energetics now
exist for various systems.
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Many systems are supported mainly by autochthonous
materials (produced within system).
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Some ecosystems are subsidized by input of
allochthonous materials (produced outside system).
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autochthonous production dominates in large rivers, lakes, marine
ecosystems
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allochthonous production dominates in small streams, springs, and
caves (100%)
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Cedar Bog Lake
Lindeman’s study of a small
lake in Minnesota uncovered
surprisingly low exploitation
efficiencies:
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herbivores: 20%
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carnivores: 33%
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residual production of plants
and herbivores accumulates
as bottom sediment
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Some General Rules
 Assimilation
efficiency increases at higher
trophic levels.
 Net
and gross production efficiencies decrease
at higher trophic levels.
 Ecological
efficiency averages about 10%.
 About
1% of net production of plants ends up as
production on the third trophic level: the
pyramid of energy narrows quickly.
 To
increase human food supplies means eating
lower on food chain!