LECTURE 21 CH 6 ENERGY IN THE ECOSYSTEM
Pg. 125-135
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. Hazardous agents may accumulate in an inverse pyramid and cause environmental
diseases.
6. Secondary production by consumers depends on NPP, efficiencies of transfer, and
residence time. Secondary production = assimilated energy – respiration – excretion.
7. Energy use by consumers varies with body size (allometry) and rate of metabolic
activity.
8, Stable isotope analysis is used to trace energy flow, trophic level of organisms, and
diet in ecosystems.
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
Energy flows through food chains
Food chains represent energy relationships
Trophic level = one link in a chain
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
Hazardous agents may accumulate in an inverse pyramid and cause environmental
diseases.
Primary production of plants
Gross PP (assimilation) – respiration = Net PP (accumulation)
How measure?
Environmental effects: light, temperature, precipitation, nutrients, CO2; species
composition and # functional groups
Nutrient limitations of PP in aquatic systems
Great variation among ecosystems: land vs. aquatic; per area vs. total
Photosynthetic efficiency = % of energy from sun converted to NPP = 1-2%
Net production efficiency = NPP/GPP; greater in temperate than tropics
Secondary production of consumers
Assimilation and accumulation of energy by consumers
Depends on NPP, efficiencies of transfer, residence time
Intertrophic energy transfer
Energy (food chain) efficiency = production of level n / production of n-1
5-20%; lower on land than aquatic
Limits to length of food chain
Hypothesis 1: energetics
Hypothesis 2: dynamic stability
Intratrophic energy transfer
Energy use varies with body size (allometry)
Smaller organisms: > energy/unit mass; larger organisms: > total energy
Exploitation efficiency = ingestion by level n / production of level n-1
Assimilated energy = ingestion – egestion = absorbed into blood stream
Assimilation efficiency = assimilation/ingestion
Secondary production = assimilated energy – respiration – excretion
Net production efficiency = production/assimilation
Warm-blooded>cold blooded because of > metabolic activity