16 - Ecosystem Energetics

8/3/2019 16 - Ecosystem Energetics

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Ecosystem Energetics (Chapter 6)

1. A little physics

2. Primary production3. Transfer of energy across trophic

levels


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2 Laws of Thermodynamics

1. Energy can neither be created nordestroyed, only transformed from onetype to another

conservation of energy2. 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 lostto the environment as heat


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Primary production capturing light

energy and storing it in chemical bonds ofcarbon compounds

primary productivity the rate of primaryproduction

primary producers photosyntheticautotrophs


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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 ways1. new proteins, tissues, cells, structures

growth & reproduction

2. fuel for the above processes


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Assimilated Carbon


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Gross primary production (GPP)

total amount of energy assimilated byphotosynthesis

Net primary production (NPP)energy actually stored as biomass

GPP NPP = respiration


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GPP

NPP


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Gross primaryproduction (GPP)

Net primaryproduction

(NPP)

Respiration,maintenance


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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 asan estimate of energy

Ignoring roots annual abovegroundnet productivity (AANP)


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Measuring NPP in nature

1. Measure amount of CO2 absorbed byleaves

extrapolate from a very small area

2. Use radioactive isotopes to measuretotal uptake of C

3. 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 percentageof 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) efficiencyamount of plant tissue produced perkilogram of water transpired

2-4 g tissue/kg water

increasing precipitation -> 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 thebiomass of one trophic levelstransformed into biomass at the nexthigher 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 ofproduction on one trophic level

consumed by the next higher level usually less than 100%

Not all food consumed by heterotrophsis transformed into biomass


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(NPP)


Ingestion byherbivores

Exploitation efficiency =Ingestion/NPP


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Amount of energy actually absorbedfrom food is assimilated energy

Assimilation efficiency proportion ofingested 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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(NPP)



Indigestible

Assimilation

Assimilation efficiency =Assimilation/Ingestion


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Growth and reproduction inheterotrophs adds biomass

Net production efficiency = (biomass

production)/(assimilated energy) the proportion of energy notused formaintenance and notlost 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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(NPP)



Indigestible

Assimilation


Growth

Net Production Efficiency =

Growth/Assimilation


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Gross production efficiency = (biomassproduction)/(ingested energy)

1-5% for warm-blooded animals

5-15% for insects

up to 30% for aquatic animals


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(NPP)



Indigestible

Assimilation


Growth

Gross production efficiency=Growth/Ingestion


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Detritus (dead stuff)

Assimilation efficiency of herbivores isonly 30-70%

most plant tissue is not digested byanimals and ends up as detritus

Two independent food chains

herbivores

most important in plankton communities

detritivores

terrestrial communities


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(NPP)



Indigestible

Assimilation

Decomposition


Growth


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(NPP)



Indigestible

Assimilation

Decomposition


Growth

Ecological Efficiency =

Biomass (higher level)/Biomass (lower level)


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(NPP)



Indigestible

Assimilation

Decomposition


Growth

Ingestionby predators


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Residence time average time thatenergy spends on one trophic level

= (energy stored in biomass)/(netproductivity)

Biomass accumulation ratioresidence time based on biomass ratherthan energy

= (biomass)/(rate of biomass production)


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16 - Ecosystem Energetics

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