LECTURE 24 CH 22 ENERGY IN THE ECOSYSTEM
MAJOR CONCEPTS
1. Ecosystems are energy-transforming machines that obey thermodynamic principles.
2. Primary production is the assimilation of energy 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.
Ecosystem ecology: 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
Universal model of energy flow 22.2
Energy ingested  reduced by excretion + respiration  less energy at next trophic level
Biochemical transformations lead to energy oloss as heat
Energy flows through ecosystem and is lost; must replenished by photosynthesis capturing more
(Nutrients cycle through ecosystem; never lost; re-used)
Primary production of plants
Gross PP (assimilation) – respiration = Net PP (accumulation) 22.3
How measure? 22.5
Productivity is limited:
Photosynthetic efficiency = % of energy from sun converted to NPP = 1-2%
Net production efficiency = NPP/GPP; greater in temperate than tropics
Environmental limitations: light, temperature, precipitation, nutrients, CO2; species
composition and # functional groups 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 22.10; 22.12
Energy flows between trophic levels via food chains 22.1
Food chains represent energy relationships
Trophic level = one link in a chain
Two parallel food chains: plant-based and decomposer-based
Pyramids of:
Energy – can’t be inverted; energy lost to heat makes less available at higher levels
Biomass – may be inverted
Numbers – may be inverted; depends on size of organisms
Secondary production of consumers
Assimilation and accumulation of energy by consumers (non-photosynthesizers)
Depends on NPP, efficiencies of transfer, assimilation, and residence time
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 pg. 474-5
Limits to length of food chain
Hypothesis 1: energetics
Hypothesis 2: dynamic stability
Intratrophic energy transfer results in:
Secondary production = assimilated energy – respiration – excretion
Some general rules:
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.
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