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Ecosystem Energetics (Chapter 6) A little physics Primary production

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Ecosystem Energetics (Chapter 6) A little physics Primary production Transfer of energy across trophic levels. 2 Laws of Thermodynamics Energy can neither be created nor destroyed, only transformed from one type to another conservation of energy

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slide1
Ecosystem Energetics (Chapter 6)
  • A little physics
  • Primary production
  • Transfer of energy across trophic levels
slide3
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
slide4
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
slide5
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
slide6
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
slide7
Plants use the products of photosynthesis in 2 ways
  • new proteins, tissues, cells, structures – growth & reproduction
  • fuel for the above processes
slide8

Assimilated Carbon

structures

fuel for biological processes

slide10
Gross primary production (GPP) – total amount of energy assimilated by photosynthesis
  • Net primary production (NPP) – energy actually stored as biomass
  • GPP – NPP = respiration
slide11

GPP

NPP

slide12

Gross primary production (GPP)

Respiration, maintenance

Net primary production (NPP)

slide13
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)
slide14
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
slide16
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
slide17
Water
    • transpiration (or water use) efficiency – amount of plant tissue produced per kilogram of water transpired
      • 2-4 g tissue/kg water
    • increasing precipitation -> increased NPP
    • some water is lost to runoff
slide18
Nutrients
    • Liebig’s law of the minimum…
    • most important in deserts, open ocean, agriculture
slide21
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
slide22
Ecological efficiency – proportion of the biomass of one trophic levels transformed into biomass at the next higher trophic level
slide23
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)
slide24
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
slide25

Gross primary production (GPP)

Respiration, maintenance

Net primary production (NPP)

Ingestion by herbivores

Exploitation efficiency =

Ingestion/NPP

slide26
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%
slide27

Gross primary production (GPP)

Respiration, maintenance

Net primary production (NPP)

Ingestion by herbivores

Indigestible

Assimilation efficiency =

Assimilation/Ingestion

Assimilation

slide28
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%
slide29
For plants, net production efficiency = NPP/GPP
    • fast-growing temperate plants – 75-85%
    • tropical species – 40-60%
slide30

Gross primary production (GPP)

Respiration, maintenance

Net primary production (NPP)

Ingestion by herbivores

Indigestible

Net Production Efficiency =

Growth/Assimilation

Respiration,

maintenance

Assimilation

Growth

slide31
Gross production efficiency = (biomass production)/(ingested energy)
  • 1-5% for warm-blooded animals
  • 5-15% for insects
  • up to 30% for aquatic animals
slide32

Gross primary production (GPP)

Respiration, maintenance

Net primary production (NPP)

Ingestion by herbivores

Indigestible

Gross production efficiency=

Growth/Ingestion

Respiration,

maintenance

Assimilation

Growth

slide33
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
slide34

Gross primary production (GPP)

Respiration, maintenance

Decomposition

Net primary production (NPP)

Ingestion by herbivores

Indigestible

Respiration,

maintenance

Assimilation

Growth

slide35

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

slide36

Gross primary production (GPP)

Respiration, maintenance

Decomposition

Net primary production (NPP)

Ingestion by herbivores

Indigestible

Respiration,

maintenance

Assimilation

Ingestion

by predators

Growth

slide37
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)