LECTURE 24 CH 22 ENERGY IN THE ECOSYSTEM
MAJOR CONCEPTS
1. Ecosystems are energy-transforming machines that obey thermodynamic principles.
2. Primary production (PP) is energy assimilation and production of organic matter by
photosynthesis.
3. Terrestrial PP is generally limited by temperature and moisture; aquatic PP by
nutrients. Top-down control by consumers can also influence PP.
4. Energy flows through food chains with only 5-20% passing to each higher trophic
level. Energy pyramids result from this loss.
5. Energy losses limit the number of trophic levels in ecosystems.
6. Secondary production by consumers depends on NPP, efficiencies of transfer, and
residence time. Secondary production = assimilated energy – respiration – excretion.
7. Net ecosystem production (C gain – loss) = potential to store C and not accelerate
greenhouse warming.
I. Ecosystem ecology: Study of interactions between biotic and abiotic factors 464-465
Two themes = energy flows and matter cycles
Ecosystem as energy-transforming machine
Obeys thermodynamic principles
Law 1: Conservation of energy  none unaccounted for; have to ‘balance books’
Law 2: Inefficient transfer of energy  heat loss in biochemical transformations
II. Universal model of energy flow 465-6 22.2
Energy ingested  reduced by excretion + respiration  less energy at next trophic level
Biochemical transformations lead to energy loss as heat
Energy flows through ecosystem and is lost; must be replenished by photosynthesis capturing
more (Nutrients cycle through ecosystem; never lost; re-used)
III Primary production of plants 466-67
Gross PP (assimilation) – respiration = Net PP (accumulation) 22.3
How measure? 467-468 22.5
Productivity is limited:
Photosynthetic efficiency = % of energy from sun converted to NPP = 1-2% 470
Net production efficiency = NPP/GPP; greater in temperate than tropics
Environmental limitations: light, temperature, precipitation, nutrients, CO2; species
composition and # functional groups 469-472 22.7, 22.11, 22.8, 22.9
Nutrient limitations of PP in aquatic systems:
P in freshwater; N near coast and Fe/Si in open ocean
Global and habitat variation in NPP is great: land vs. aquatic 472-474 22.10; 22.12
IV Energy flows between trophic levels via food chains 465 22.1
Food chains represent energy relationships
Trophic level = one link in a chain
Two parallel food chains: plant-based and decomposer-based 477
V. Secondary production of consumers 474-476
Assimilation and accumulation of energy by consumers (non-photosynthesizers)
Depends on NPP, efficiencies of transfer, assimilation, and residence time
1. Intertrophic energy transfer (resulting in energy pyramid)
Energy (food chain) efficiency = production of level n / production of n-1
5-20%; lower on land than aquatic 474-5
Limits to length of food chain
Hypothesis 1: energetics
Hypothesis 2: dynamic stability
2. Intratrophic energy transfer results in:
Secondary production = assimilated energy – respiration – excretion
3. Some generalities:
Assimilation efficiency increases at higher trophic levels.
Net and gross production efficiencies decrease at higher trophic level.
Ecological efficiency averages about 10%.
About 1% of NPP ends up as production on third trophic level.
The pyramid of energy narrows quickly.
VI. Net ecosystem production = carbon gain – carbon loss 478-79
Measures net carbon accumulation -->
carbon ‘sequestered’ in organic compounds in soil and living biomass 
no ‘greenhouse’ warming effect
Positive NEP represents a carbon sink that removes CO2 from the atmosphere
Summary 1-12; 15. 17
QUANTIFICATION OF ENERGY FLOW
WITHIN PLANTS
Photosynthetic Efficiency = NPP/energy from sun in same time interval
Gross Primary Production (GPP) = Net Primary Production (NPP) + Respiration
Net Production Efficiency = NPP/GPP
BETWEEN TROPHIC LEVELS
Ecological Efficiency = Net production of level n/net production of level n-1
WITHIN ANIMAL TROPHIC LEVEL
Secondary Production = Assimilated Energy - Respiration - Excretion
WITHIN ECOSYSTEM
Net Ecosystem Production (NEP) = carbon gain – carbon lost