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Chapter 18
Energy flux in the biopshere is unidirectional
Primary production on land is limited by moisture and temperature
Primary production in water is limited by nutrients
Primary production can be influenced by and consumers
Energy flow must obey the laws of thermodynamics
Moisture and Heat in Terrestrial Systems (Fig. 18.2, 18.3)
• Actual Evapotranspiration (AET)
• Amount of water that evaporates and transpires off the landscape
• Combines heat and moisture
Nutrients in Terrestrial Systems (Fig. 18.4)
• If light and moisture are controlled, nutrients can also influence productivity
Nutrients in Aquatic Systems (Fig. 18.6)
• Given that moisture is rarely limited in aquatic systems, nutrients (phosphorus
and nitrogen) are the most common controlling factor
Whole Lake Manipulations (Fig. 18.8)
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Limits on Primary Productivty
Graphs suggest that limits may be related to variation in:
– Temperature
– Radiation
– Moisture
– Nutrients (depending on the system)
In reality, primary productivity is limited by a succession of factors over the
course of a year.
Trophic Cascades
Focus on primary productivity is “bottom-up” explanation
Can also look at top-down controls
Aquatic Systems (Fig. 18.11, 18.13)
Grazing Ungulates (Fig. 18.14, 18.15)
Consumer Pathways
“Secondary Production”
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Live-consumer systems
– Herbivores consume plants (primary consumers)
– Carnivores consume herbivores (secondary consumers)
Detritivore/Decomposer systems
– Bacteria and fungi break down non-living organic material
– Detritivores consume both non-living organic material and its associated
microbial community
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Live-consumer Systems
Secondary productivity depends on primary productivity.
In general, systems with high primary productivity have high secondary
productivity as well.
Levels of secondary productivity depend on “efficiencies” of energy transfer.
Secondary Production
Energy Transfer Efficiencies
Consumption Efficieny
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Consumption efficiency
– Percentage of production at trophic level n-1 that is consumed at trophic
level n
– Varies markedly across systems and consumers
• Forest herbivores may consume 5% of NPP
• Grassland herbivores may consume 25% of NPP
• Zooplankton and planktivorous fish may consume over 50% of
freshwater aquatic NPP
• Invertebrate predators may consume 25% of invertebrate prey
Energy Transfer Efficiencies
Assimilation Efficiency
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Assimilation efficiency
– Percent of ingested energy that is transferred across digestive system wall
– Assimilated energy available for growth or maintenance
• Microbes?
– Also highly variable
• Depends on food source quality
• Depends on digestive capabilities of consumer
Energy Transfer Efficiencies
Production Efficiency
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Production efficiency
– Percent of assimilated energy that is incorporated into new biomass
– Assimilated energy minus energy used in respiration
– Efficiency varies according to method of internal heat regulation
• Ectotherms range from 10 to 40%
• Endotherms range from 1 to 3%
Energy Transfer Efficiencies
Ecological Efficiency
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Ecological Efficiency
– Consumption efficiency × Assimilation efficiency × Production efficiency
– Production at trophic level n divided by production at trophic level n - 1
• Herbivore production / NPP
• Carnivore production / Herbivore production
– Average ecological efficiency is 10% (range goes from 2% to 24%)