Ecosystem Energetics (Chapter 6)

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• Ecosystem Energetics (Chapter 6)
• A little physics
• Primary production
• Transfer of energy across trophic levels

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• 2 Laws of Thermodynamics
• Energy can neither be created nor destroyed, only
  transformed from one type to another
• conservation of energy
• In any transfer of energy, some energy is lost
• entropy

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• 1st law all energy in an ecosystem ultimately
  comes from
• the sun
• chemical compounds (hydrogen sulfide)
• 2nd law energy is constantly being lost to the
  environment as heat

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• Primary production capturing light energy and
  storing it in chemical bonds of carbon compounds
• primary productivity the rate of primary
  production
• primary producers photosynthetic autotrophs

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• Energy 6CO2 6H2O ? C6H12O6 6O2
• Each gram of C assimilated 39 kilojoules (kJ)
  of energy stored
• Joules unit of energy
• Biomass is dominated by carbon
• biomass and energy are equivalent

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• Plants use the products of photosynthesis in 2
  ways
• new proteins, tissues, cells, structures growth
  reproduction
• fuel for the above processes

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Assimilated Carbon
structures
fuel for biological processes
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• Gross primary production (GPP) total amount of
  energy assimilated by photosynthesis
• Net primary production (NPP) energy actually
  stored as biomass
• GPP NPP respiration

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GPP
NPP
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Gross primary production (GPP)
Respiration, maintenance
Net primary production (NPP)
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• Measuring NPP in nature
• Units energy per unit area per year
• kJ per m2 per yr, or W per m2
• 1 g C assimilated 39 kJ energy
• can use plant biomass or CO2 uptake as an
  estimate of energy
• Ignoring roots annual aboveground net
  productivity (AANP)

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• Measuring NPP in nature
• Measure amount of CO2 absorbed by leaves
• extrapolate from a very small area
• Use radioactive isotopes to measure total uptake
  of C
• In aquatic systems, measure changes in O2
  concentration

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Light Dark
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• Limits on Primary Production
• Light
• in shade, forest understory
• photosynthetic efficiency percentage of the
  energy in sunlight converted to NPP
• averages just 1 to 2
• Temperature
• most plants have an optimum temperature
• respiration increases with temperature

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• Water
• transpiration (or water use) efficiency amount
  of plant tissue produced per kilogram of water
  transpired
• 2-4 g tissue/kg water
• increasing precipitation -gt increased NPP
• some water is lost to runoff

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• Nutrients
• Liebigs law of the minimum
• most important in deserts, open ocean, agriculture

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• Transfer of energy across trophic levels
• All energy used by higher trophic levels
  originates with primary producers
• With each step in the food chain, 80-95 of
  energy is lost

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• Ecological efficiency proportion of the biomass
  of one trophic levels transformed into biomass at
  the next higher trophic level

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• For heterotrophs, ecological efficiencies average
  5-20
• Why?
• indigestible tissues
• hair, feathers, insect exoskeletons, cartilage,
  bone
• cellulose, lignin
• maintenance costs
• loss of energy as heat (entropy)

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• Exploitation efficiency proportion of
  production on one trophic level consumed by the
  next higher level
• usually less than 100
• Not all food consumed by heterotrophs is
  transformed into biomass

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Gross primary production (GPP)
Respiration, maintenance
Net primary production (NPP)
Ingestion by herbivores
Exploitation efficiency Ingestion/NPP
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• Amount of energy actually absorbed from food is
  assimilated energy
• Assimilation efficiency proportion of ingested
  energy actually absorbed by the body
• seeds 80
• young vegetation 60-70
• grazing/browsing 30-40
• wood 15
• animals 60-90

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Gross primary production (GPP)
Respiration, maintenance
Net primary production (NPP)
Ingestion by herbivores
Indigestible
Assimilation efficiency Assimilation/Ingestion
Assimilation
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• Growth and reproduction in heterotrophs adds
  biomass
• Net production efficiency (biomass
  production)/(assimilated energy)
• the proportion of energy not used for maintenance
  and not lost as heat
• birds 1
• small mammals 6
• cold-blooded animals 75

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• For plants, net production efficiency NPP/GPP
• fast-growing temperate plants 75-85
• tropical species 40-60

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Gross primary production (GPP)
Respiration, maintenance
Net primary production (NPP)
Ingestion by herbivores
Indigestible
Net Production Efficiency Growth/Assimilation
Respiration, maintenance
Assimilation
Growth
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• Gross production efficiency (biomass
  production)/(ingested energy)
• 1-5 for warm-blooded animals
• 5-15 for insects
• up to 30 for aquatic animals

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Gross primary production (GPP)
Respiration, maintenance
Net primary production (NPP)
Ingestion by herbivores
Indigestible
Gross production efficiency Growth/Ingestion
Respiration, maintenance
Assimilation
Growth
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• Detritus (dead stuff)
• Assimilation efficiency of herbivores is only
  30-70
• most plant tissue is not digested by animals and
  ends up as detritus
• Two independent food chains
• herbivores
• most important in plankton communities
• detritivores
• terrestrial communities

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Gross primary production (GPP)
Respiration, maintenance
Decomposition
Net primary production (NPP)
Ingestion by herbivores
Indigestible
Respiration, maintenance
Assimilation
Growth
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Gross primary production (GPP)
Respiration, maintenance
Decomposition
Net primary production (NPP)
Ingestion by herbivores
Indigestible
Ecological Efficiency Biomass (higher
level)/ Biomass (lower level)
Respiration, maintenance
Assimilation
Growth
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Gross primary production (GPP)
Respiration, maintenance
Decomposition
Net primary production (NPP)
Ingestion by herbivores
Indigestible
Respiration, maintenance
Assimilation
Ingestion by predators
Growth
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• Residence time average time that energy spends
  on one trophic level
• (energy stored in biomass)/(net productivity)
• Biomass accumulation ratio residence time based
  on biomass rather than energy
• (biomass)/(rate of biomass production)

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