energy flow in the ecosystem - university of...
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Energy flow in the ecosystem
Educational Goals
• History and fundamental concepts pertaining to energy flux in ecosystems
• Ecosystem Concept and thermodynamic underpinnings
• Primary production and its measurement, limiting factors, and global patterns
• Secondary Production• Intratrophic transfers of energy• Ecological efficiency
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Ecosystem Ecology• Ecosystems as giant energy-transforming
machines
Background: Organizing Concepts
• Charles Elton – 1920’s revolutionary concept:– Organisms living in the same place have similar
tolerances of physical factors AND– Feeding relationships link organisms
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Food Webs
Conceptual OrganizationFocus on compartments and fluxes of energy
and materials
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A Simple Compartment Example• Pool size
• Flux rate (input = output)
• Residence Time
• Turnover Rate
Atmosphere
Evap.
Precip
Ecosystem Concept
A.G. Tansley (1935) – built on Elton’s ideas:• Coined the word “ecosystem”
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A.J. Lotka and Thermodynamics
The ecosystem as an energy-transforming machine:
• Set of equations representing exchanged of matter and energy
First law of thermodynamics
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Second Law of Thermodynamics
Synthesis – Lindeman (1942)
• Elton – food web structure• Tansley – fundamental unit• Lotka - thermodynamics
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Lindeman’s Foundations of Ecosystem Ecology
• The ecosystem is the fundamental unit of ecology.
• Food chain
• Trophic level
• Pyramid of energy
Trophic Structure
Functional Classification:• Producers (autotrophs)
• Reducers (heterotrophs)
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Trophic Pyramids
Odum’s Energy Flux Model• Used energy as currency in compartment
analysis
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Primary Production
• Primary producers capture light and transform it to energy– 6CO2 + 6H2O C6H12O6 + 6O2
– for each g of C assimilated, 39 kJ energy stored
Components of Primary Production
• gross primary production
• net primary production
• gross - net = respiration
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Components of Primary Production
Measurement of Primary Production
• harvest techniques
• gas exchange techniques
• Radioactive carbon (14C) may also determine net uptake of carbon by plants
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Application of Odum’s Approach
Gas Exchange in Aquatic Systems
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Limits to primary productivity?
• Photosynthetic efficiency – 1-2%
• Limits:– Light– Temperature– Water– Nutrients
Photosynthesis and Temperature
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Water limits primary productivity
Nutrients stimulate primary production
• Terrestrial systems may be nutrient limited
• Aquatic systems often strongly nutrient limited
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Primary Production varies among ecosystems
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Global Primary Productivity
Global Productivity
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More terminology…..
• Herbivore, carnivore, omnivore
• Trophic link
• Decomposers
Ecological Efficiency
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• Ingestion• Egestion• Assimilation• Excretion• Respiration• Production
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Fundamental Energy Relationships
Limits to Secondary Production?
• Correlated with Primary Production
• Biotic interactions - Predation, etc.
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Net Ecosystem ProductionIs there anything left over?
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:– 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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Energy moves through ecosystems at different rates.
Indices of energy cycling speeds– residence time
Energy moves through ecosystems at different rates.
Indices of energy cycling speeds– biomass accumulation ratio
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Ecosystem Energetics
Compare systems with different inputs:
• Autochthonous – produced within the system
• Allochthonous – produced outside the system
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 = 23yr
– forested terrestrial communities >20 yr
– planktonic aquatic ecosystems <20 days
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Residence Time for Litter• Decomposition of litter is dependent on
• Residence time
humid tropics
dry and montane tropics
southeastern US
boreal ecosystems
–3 mos
–1-2 yr
–4-16 yr
–>100 yr
Stream study…..
• Assimilation of energy by herbivores indicates subsidy
– autochthonous
– allochthonous
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