As energy flows through ecosystems, the amount of available energy decreases at each trophic level. At each level, some energy is lost as heat. Food chains and webs show the transfer of energy as organisms consume other organisms. The second law of thermodynamics explains that as energy is transferred and changed between trophic levels, some is lost as heat. Productivity decreases with each increasing trophic level, following the typical 10% efficiency rule. Primary productivity refers to the energy captured by producers, with net primary productivity being the energy left over after producer needs.

1. 3-3 Energy in Ecosystems

2. Concept to Understand
As energy flows through
ecosystems in food chains and
webs, the amount of chemical
energy available to organisms at
each succeeding feeding level
decreases.

3. Fig. 3-8, p. 46
Trophic Levels
At each level energy is transferred to the
next but some energy is “lost” in the form
of heat

4. Energy Flow in Ecosystems
Food chain
Sequence of organisms, each
of which serves as a source of
food for the next
Food web
Network of
interconnected food
chains
More complex than a food
chain

5. Usable Energy by Trophic Level
Energy flow follows the second law of
thermodynamics, which is……
2nd Law of Thermodynamics: Energy
cannot be created or destroyed, but
changed.
But as energy is changed, some
energy is lost in the form of heat.

7. Usable Energy by Trophic Level
Biomass – dry weight of all organic
matter in plants and animals in an
ecosystem.
Biomass
decreases with
increasing
trophic level

8. Fig. 3-10, p. 47
Ecological efficiency – typically 10%
(10% rule)

9. Two Kinds of Primary Productivity
Gross primary productivity (GPP) – The rate at which
ecosystems producers convert solar energy into chemical
energy in the form of biomass
So the amount of
solar energy
converted into
cells
kilocalories/square
meter/year

11. Two Kinds of Primary Productivity
Net primary productivity (NPP) – is what energy is left over
for the consumers in the ecosystem.
GPP minus the energy the producer need to survive

12. Two Kinds of Primary Productivity
Planet’s NPP limits number of consumers
Humans use, waste, or destroy 10-55% of earth’s total
potential NPP
Human population is less than 1% of total biomass of
earth’s consumers

13. Temperate forest
Aquatic Ecosystems
Open ocean
Continental shelf
Lakes and streams
Estuaries
Terrestrial Ecosystems
Extreme desert
Desert scrub
Tundra (arctic and alpine)
Temperate grassland
Woodland and shrubland
Agricultural land
Savanna
Northern coniferous
forest (taiga)
Swamps and marshes
Tropical rain forest
Fig. 3-11, p. 48
Average net primary productivity (kcal/m2/yr)
800 1,600 2,400 3,200 4,000 4,800 5,600 6,400 7,200 8,000 8,800 9,600