PRIMARY PRODUCTION
AND ENERGY FLOW
Chapter 18
Molles: Ecology 2nd Ed.
Chapter Concepts
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•
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Terrestrial Primary Production limited by
temperature and moisture
Aquatic Primary Production limited by
nutrient availability
Consumers can influence rates of primary
production in terrestrial and aquatic
ecosystems
Energy losses limit the number of trophic
levels found in ecosystems
Molles: Ecology 2nd Ed.
Fundamental Concepts
•
•
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Primary Production
Fixation of energy by autotrophs in an
ecosystem
Or, conversion of inorganic energy (light) into
organic forms (chemical potential energy)
Molles: Ecology 2nd Ed.
Rate of Primary Production
•
Amount of energy fixed during period of time
 Gross PP – Total amount of energy fixed
 Net PP – Amount of energy leftover after
autotrophs have met their metabolic needs
(subtract plant respiration)
 This is amount left over for consumption
by herbivores
Molles: Ecology 2nd Ed.
Fundamental Concepts
•
Trophic Level
 Position in food web determined by
number of energy transfers from primary
producers to current level:
 Primary producers = first level
 Primary consumers = second level
 Secondary consumers = third level
 Tertiary consumers = fourth level
Molles: Ecology 2nd Ed.
Primary producer
plant autotroph
Primary consumer
grazer
Secondary consumer
predator
Tertiary consumer
predator
Molles: Ecology 2nd Ed.
Evapotranspiration and Terrestrial Primary
Production
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Rosenzweig (1968)
 Estimated influence of moisture and
temperature on rates of primary production
 Plot annual net primary production and
annual actual evapotranspiration (AET)
Molles: Ecology 2nd Ed.
Plot annual net primary production and
annual actual evapotranspiration (AET)
 AET – annual amount of water that
evaporates and transpires
 Cold dry ecosystems tend to have low
Fig 18.2
AET
Molles: Ecology 2nd Ed.
Evapotranspiration and Terrestrial Primary
Production
 Positive relationship between net primary
production and AET
 Sala found east-west variation in primary
production correlated with rainfall
Molles: Ecology 2nd Ed.
So?
•
Rainfall or, rainfall and temperature
 Predict net primary productivity in
terrestrial ecosystems
QuickTime™ and a
GIF decompressor
are needed to see this picture.
Molles: Ecology 2nd Ed.
•
•
Soil Fertility and Terrestrial
Primary Production
Soil fertility is important too!
Shaver and Chapin
 Arctic net primary production twice as high
on fertilized plots compared to unfertilized
plots
Molles: Ecology 2nd Ed.
•
Bowman
 N is main nutrient limiting net primary
production in a dry tundra meadow;
 N and P jointly limit production in a wet
meadow
Molles: Ecology 2nd Ed.
Fig 18.5
Molles: Ecology 2nd Ed.
Patterns of Aquatic Primary Production
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•
Several studies found quantitative
relationship between phosphorus and
phytoplankton biomass
– nutrient availability controls rate of primary
production in freshwater ecosystems
Molles: Ecology 2nd Ed.
Figure 18.9
Molles: Ecology 2nd Ed.
Global Patterns of Marine Primary Production
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•
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Highest primary production by marine
phytoplankton –
 in areas with higher levels of nutrient
availability
= continental margins
 Nutrient run-off from land
 Sediment disturbance
Open ocean tends to be nutrient poor
 Vertical mixing main nutrient source
Molles: Ecology 2nd Ed.
A Comparative Analysis of Annual Primary Production (Kcal / m2 /yr)
Alfalfa
Field
Young Pine
Plantation
Middle-Aged
Oak Forest
Mature Tropical
Rain Forest
Total Solar
Radiation
2,091,000
2,091,000
2,091,000
2,415,000
Gross Primary
Production
24,400
12,200
11,400
45,000
Respiration
9,200
4,700
6,400
32,000
Net Primary
Production
15,200
7,500
5,000
13,000
GPP / Total
Solar Radiation
0.0117
(1.17%)
0.0058
(0.58%)
0.0055
(0.58%)
0.0186
(1.86%)
NPP / GPP
0.62
(62%)
0.61
(61%)
0.44
(44%)
0.29
(29%)
Summary of Comparisons of Ecosystem Primary
Production
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Efficiency of primary production is reduced by
drought or freezing-induced dormant seasons (sun
light going to waste)
•
Efficiency of primary production is reduced by high
temperatures that increase respiration w/o
increasing photosynthesis.
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Efficiency of primary production is reduced by large
amounts of non-photosynthetic plant biomass
(increase respiration w/o increasing
photosynthesis).
Association b/t Plant Biomass and Primary
Production
(There isn’t one)
Ecosystem
Tropical Rain Forest
Net Primary
Production
2000
Swamps & Marshes
2500
Algal Beds & Coral Reefs
2000
Boreal Conifer Forest
800
Savanna (Grass + Trees)
700
Ocean Upwelling Zone
500
Plant
Biomass
44
}
}
15
2
20
4
0.02
Consumer Influences
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Bottom-Up Controls
 Influences of physical and chemical factors
of an ecosystem
Top-Down Controls
 Influences of consumers
Molles: Ecology 2nd Ed.
Lake Primary Production
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Carpenter et al. (1985)
 Piscivores + planktivorous fish can cause
significant deviations in primary productivity
Carpenter and Kitchell (1988)
 Influence of consumers on lake primary
productivity propagate through food webs
 = Trophic Cascade Hypothesis
Molles: Ecology 2nd Ed.
Fig 18.11
Molles: Ecology 2nd Ed.
Fig. 18.13
Molles: Ecology 2nd Ed.
Carpenter + Kitchell
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Whole lake experiments
N. Wisconsin / UP Michigan
Molles: Ecology 2nd Ed.
UNDERC
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Three lakes: Peter, Paul, Tuesday
1985: reciprocal transplants of fish
Molles: Ecology 2nd Ed.
Carpenter and Kitchell experiment
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Three experimental lakes
 Two w/bass
 One winterkill lake; Tuesday – no bass
 Many planktivore fishes = minnows
Molles: Ecology 2nd Ed.
•
Experiment:
 Remove 90% bass from Peter lake
 Put into Paul lake
 Remove 90% planktivores from Paul lake
 Put into Peter lake
 Third lake = control (Tuesday)
Molles: Ecology 2nd Ed.
Lake Primary Production
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Carpenter and Kitchell
 Fewer planktivorous fish led to reduced rates
of primary production
 No planktivorous minnows resulted in more
predator invertebrates
 Abundant, large herbivorous zooplankton
increased, phytoplankton biomass and
primary productivity
Molles: Ecology 2nd Ed.
Summary
Fig 18.13
Molles: Ecology 2nd Ed.
Primary Production in the Serengeti
25,000 km2 grassland
 Millions of large mammals still present

Molles: Ecology 2nd Ed.
McNaughton (1985)
Serengeti grazers consume average of
66% of annual primary production
 Rate of primary production in Serengeti
positively correlated with rainfall quantity

Molles: Ecology 2nd Ed.
Primary Production in the Serengeti
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Found that grazers can increase primary
production
 Increased growth rate =
 Compensatory Growth is plant response
 Lower respiration rate due to lower
biomass
 Reduced self-shading
 Improved water balance due to
reduced leaf area
Molles: Ecology 2nd Ed.
Fig 18.14
Molles: Ecology 2nd Ed.
Primary Production in the Serengeti
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In addition, McNaughton found compensatory
growth highest at intermediate grazing
intensities
 Light grazing insufficient to produce
compensatory growth
 Heavy grazing reduces plant’s capacity to
recover
Molles: Ecology 2nd Ed.
Fig. 18.15
Molles: Ecology 2nd Ed.
Trophic Dynamic View of Ecosystems
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Lindeman (1942)
 Ecosystem concept fundamental to study
of energy transfer within an ecosystem
 Suggested grouping organisms within an
ecosystem into trophic levels
 Each feeds on level immediately below
 As energy is transferred from one
trophic level to another, energy is
degraded
Molles: Ecology 2nd Ed.
Trophic Dynamic View of Ecosystems

As energy is transferred from one trophic
level to another, energy is degraded:
 Limited assimilation
 Consumer respiration
 Heat production
 Energy quality decreases with each
successive trophic level
– Pyramid-shaped energy distribution
Molles: Ecology 2nd Ed.
Fig. 18.16
Molles: Ecology 2nd Ed.
Trophic Pyramids
Why are big, fierce animals
(top carnivores) rare ?
Bio-magnification
Because there is not
enough energy to support
large populations at the
highest trophic levels.
Human Trophic Pyramid
10 6
10 7
10 10
How Will We Feed a Growing Human
Population ?
After 30 Years of Increase, Grain Production per Person
Has Stabilized
Green Revolution
Agricultural production
increases faster than
human population
Agricultural production
just keeping pace with
human population
growth
What was the “Green Revolution”
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Genetic improvement of crop species to increase
grain yields and resistance to pests, disease, wind
damage.
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Increased use of fertilization, herbicide, pesticide,
and irrigation.
All the “easy” gains in production have been obtained.
Further gains will come slowly, if at all.
Negative effects of fertilization, pesticides and irrigation
are beginning to affect production
The human population continues to grow
Acreage of Productive Farmland
Is Declining In the USA
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From 1992 – 1997 six million acres of farm
land were converted to developed use.
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We lost farm and ranch land 51% faster in the
1990’s than in the 1980’s.
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Prime agricultural land is being converted
30% faster than non-prime rural lands.
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We must depend more on food production
from marginal land that requires more
irrigation and fertilization.
“Farming On the Edge” (American Farmland Trust)
Losing Farm Land (USA)
25% of U.S. food supply
comes from the California
Central Valley
Losing Farm Land (Indiana)
•Wasteful land use is the
problem, not growth itself.
Losing Farm Land (Indiana)
•Wasteful land use is the
problem, not growth itself.
•From 1982-1997 the U.S.
population grew by 17%,
while urbanized land grew
by 47%.
Losing Farm Land (Indiana)
•Wasteful land use is the
problem, not growth itself.
•From 1982-1997 the U.S.
population grew by 17%,
while urbanized land grew
by 47%.
•Over the past 20 years,
the acreage per person for
new housing almost
doubled .
Losing Farm Land (Indiana)
•Wasteful land use is the
problem, not growth itself.
•From 1982-1997 the U.S.
population grew by 17%,
while urbanized land grew
by 47%.
•Over the past 20 years,
the acreage per person for
new housing almost
doubled .
•Since 1994, 10+ acre
housing lots have
accounted for 55% of land
developed.
Energy Flow In the Human Food Chain
Energy In Corn Grain (8.2 million Kcal)
Maybe humans should
just eat the corn
Not assimilated by cow
Energy Assimilated by Feedlot Cattle (7.0 million Kcal)
Respiration
Energy in the Body of Mature Feedlot Cattle (1.2 million Kcal)
Waste in meat processing
Energy in Meat from Feedlot Cattle (0.4 million Kcal)
5% of Energy in Corn is Available In Beef
Meat Production Continues to Rise
All Endothermic Animals
More Efficient Meat Production
Type of Meat
Pounds of Grain per
Pound of Meat Produced
Beef
7
Pork
4
Chicken
2
Farm-grown Fish
???
How to Feed a Growing Human Population With a Finite or
Shrinking Agricultural Land Base
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Increase the proportion of calories in the
diet provided by grains.
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Increase the proportion of protein in the diet
provided by energy-efficient animals (poultry
and fish).
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Protect prime farm land from development.
The End