MIT Department of Biology
7.014 Introductory Biology, Spring 2005
Lecture 18 Productivity and
Food Webs
March 16, 2005
Photosynthesis and Respiration on ecosystem
scales
¾ Analysis of global productivity
¾
¾ Energy flow through trophic levels and
ecosystems
¾ Three weeks in the life of a food web (DVD)
World Net Primary Productivity
(Rounded Estimates)
Ecosystem
Desert
Grassland, etc.
Cultivated Land
Moist Forest
Tropical Forest
NPP
(g m-2 y-1)
World NPP
(x1015 g y-1)
50
500
1000
1000
2000
2
25
10
40
40
MARINE TOTAL
720
4000
1000
30,000
45,000
177
LAND TOTAL
Estuaries
Continental Shelf
Open Ocean
Biomass World Biomass
(g m-2 )
(x1015 g )
2000
500
100
4
10
40
54
15
125
14
900
750
1804
1500
20
3
2.6
.3
1.0
3.9
World NPP = NPP × ∑Area
World Biomass = Biomass × ∑Area
1
Marine Food Web
Shore Birds
Sea Bass
Blue Crab
Barnacle
Sea Worms
Zooplankton
Shore Shrimp
Phytoplankton
Detritus
All photos taken from the National Oceanic and Atmospheric Administration at: http://www.noaa.gov/.
Flow of Carbon (Energy) Through a
Trophic Level (or organism)
Pn-1 = Productivity of trophic level n-1
Dn
= Portion of Pn-1 not consumed
In
= Amount Ingested
Fn
= Fecal matter produced
An
= Amount assimilated and available
Rn
= Respiration
Bn
= Biomass in trophic level n
Pn
= Productivity at trophic level n
An
In
Pn -1
Rn
Trophic
Level n
Pn
Bn
Dn
Dead
Organic
Matter
Fn
Living
Biomass
To Detritus
Food Web
2
Rn
Ecological Efficiencies
Trophic
Level n
Exploitation Efficiency (EE)
EE = In / Pn-1 × 100
In
Pn -1
An
Pn
Bn
Assimilation Efficiency
(AE)
AE = An / In × 100
Production Efficiency (PE)
Dn
Living
Biomass
Fn
Dead
Organic
Matter
PE = Pn / An × 100
To Detritus
Food Web
Ecological Efficiency = (EE)(AE)(PE)
= Pn / Pn-1 × 100
≈ 10 % to 20 %
Marine Food Web
Shore Birds
Sea Bass
Blue Crab
Barnacle
Sea Worms
Zooplankton
Shore Shrimp
Phytoplankton
Detritus
All photos taken from the National Oceanic and Atmospheric Administration at: http://www.noaa.gov/.
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Grazing Food Web
RA
2nd Carnivore
RH
RH
Pgross
Pnet
D
I
A
A
I
P
RH
D
F
F
POC = Particulate Organic
Carbon
Heat
Net Ecosystem Production
= GPP - RA - ∑ RH
DOC = Dissolved Organic
Carbon
RH
Detritus
RH
DOC
POC
Etc.
Detrivores
Etc.
Microorganisms
“Refractory” Carbon
Microbial Food Web
Detrivore Food Web
DOC = Dissolved Organic Carbon
POC = Particulate Organic Carbon
Pyramids of Biomass and Energy
Biomass
Energy Flow
(grams/m2)
(calories/m2/day)
Grassland
Biomass in green
plants, accessible to
herbivores
Forest
Biomass tied up in
wood & unavailable
to herbivores
Open Ocean
Inverted Pyramid
of Biomass
Producers
Herbivores
Carnivores
4
Figure by MIT OCW.
Medium%
Largest%
Detritivores
Small%
Human Appropriation of Photosynthesis
Herbivores
Primary
Carnivores
Secondary
Carnivores
Between 10 and 55%
has been appropriated
(difficult to estimate)
RESPIRATION
Direct Use (Food, Fuel, Fiber, Timber)
Diverted Productivity (Crop Waste, Lumber Waste, Burning etc)
Productive Capacity Lost by Land Conversion & Desertification or Overuse
(Open Land to Cities, Forests to Crop land etc.)
Figure by MIT OCW.
5
Blue Planet DVD
Three weeks in the life of a
Marine Food Web
Think About:
• The massive amount of energy and
carbon the phytoplankton must be
processing to support such a diversity and
biomass at higher trophic levels
• How the information in DNA could be
controlling all of these complex processes
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