1

Ecosystem Energetics

• Lindeman, R.L. 1942. The trophic-dynamic aspect of ecology. Ecology 23: 399-418.

• Productivity and biomass by trophic level – Eltonian

Pyramid (Charles Elton, 1927)

Primary Production

Primary Consumer

Secondary Consumer

Tertiary Consumer

Primary production efficiency• How much energy is available?

– Solar constant 1366 watts/m2 (1.96 cal cm-2 min-1)

• What percentage converted to usable carbohydrate energy?

– 1-2% on land 3-4% for algae

– Efficient solar panels = 15-30%

Net Global Primary productivity Ecosystem Productivity per unit Biomass

2

Why so inefficient?

• Laws of thermodynamics:

– First law of thermodynamics – conservation of energy

– Second law of thermodynamics – entropy

• Plant productivity limited by:

– Limited wavelengths used

– Energy spent conducting photosynthesis or respiration (~30%)

– Most ecosystem energy “lost” as kinetic energy or heat

– Limitations to productivity

• Patchy distribution of water and nutrients

• Growing season length

• Temperature

• Soil depth

Fate of energy

• General positive relationship between primary and secondary productivity

• Secondary typically order of magnitude less than primary

• Slope of line reflects transfer (in)efficiency

Energy Transfer → herbivore inefficiency

• Plant defenses

– Physical (spines, silica)

– Chemical (alkaloids, terpenoids, tannins, nicotine, caffeine)

– Low food quality

• Selective pressure on [specialist] herbivores to increase efficiency

• Consumption efficiency

• Where

– In – energy consumed at

level n

– Pn-1 –energy available at

level n-1

CEI

P

n

n

=

−1

100*

3

• Assimilation efficiency

• Production Efficiency

• An – energy assimilated and available for use at level n

• Pn – energy incorporated into new biomass

AEA

I

n

n

= *100

Ingested Energy

Feces

Absorbed Energy

UrineAvailable Metabolic

EnergySpecific Dynamic

Action

Maintenance Work Storage

Growth -somatic

ReproductiveHeat

PEP

I

n

n

= *100

• Trophic level

transfer efficiency

TLTEP

P

n

n

=

−1

100*

4

Other physiological considerations

• Endotherms - active over a wide range of temperatures, up to 30% of energy

spent heating body, low assimilation efficiency

• Ectotherms - more efficient under ideal conditions,

restricted range of temperatures, high assimilation efficiency

Ingested Energy

Feces

Absorbed Energy

UrineAvailable Metabolic

EnergySpecific Dynamic

Action

Maintenance Work Storage

Growth -somatic

ReproductiveHeat