Env Sci 120
Chapter 3 Notes: HOW ECOSYSTEMS WORK
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Read section 3.1 on pp. 54-61, and then copy the following notes
Section 3.1 Energy Flow In Ecosystems
• All energy on planet earth comes from the sun. It is used by the
plants in photosynthesis to make food energy and oxygen, which is
then passed on to consumers.
• photosynthesis - the chemical process carried out by green plants,
using light to produce sugar (an energy rich food).
sunlight
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carbon dioxide + water ----------> glucose + oxygen
CO2(g) + H2O(l) ----------> C6H12O6(s) + O2(g)
from air
from soil
food material
Only 1-5% of the sunlight hitting the surface of planet Earth gets used
by plants (producers) in photosynthesis to make energy.
producer - an organism that makes its own food (green plant).
consumer - an organism that gets its food energy by eating other
organisms (all animals and a few plants).
herbivore - organism that eats only plants (e.g. cow, moose, rabbit)
carnivore - organism that eats only animals (e.g. wolf, coyote, lion)
omnivore - organism that eats both plants and animals (e.g. humans,
bears)
top carnivore - the top organism on a food chain; no other organism
relies on it for its food source, while alive. (e.g. humans, bears, eagles)
decomposers - microscopic organisms that feed off and break down
waste and dead organisms (e.g. bacteria, fungi). When they do this,
they provide nutrients for the plants to use again. (There are more
decomposers than all other organisms on Earth combined.)
cellular respiration - the process carried out by consumers to digest
or break down food to provide energy to live. It is the opposite of
photosynthesis:
glucose
+ oxygen —> carbon dioxide + water (+ energy)
C6H12O6(s) +
O2(g) —> CO2(g) + H2O(g)
Notice the balanced system – producers inhale CO2(g) and breathe out
O2(g), while consumers inhale O2(g) and breathe out CO2(g).
Combustion or “burning” of any carbon-based fuel produces the same
products – CO2(g) + H2O(g)
Env Sci 120
Chapter 3 Notes: HOW ECOSYSTEMS WORK
Page 2 of 3
• food chain - a step-by-step order linking organisms that feed on each
other, starting with a green plant.
clover —> rabbit —> fox —> coyote
producer –> 1st order consumer –> 2nd order consumer –> 3rd order consumer
producer –> primary consumer –> secondary consumer –> tertiary consumer
food web - two or more interconnected food chains
• Most of the energy an organism consumes is used for it to live – to
carry out its life functions in its cells. Only a small portion or
percentage of the energy gets passed along from one feeding level to
the next in a food chain (typically no more than 10%).
• trophic level - feeding level in a food chain
clover —>
rabbit
—>
fox
—> coyote
st
nd
producer –> 1 order consumer –> 2 order consumer –> 3rd order consumer
1st trophic –> 2nd trophic level –> 3rd trophic level
–> 4th trophic level
100% energy –> 10% of original energy –> 1% energy –> 0.1% energy
• autotroph - makes own energy; green plant.
• heterotroph - cannot make own energy; consumer; animal or nongreen plant.
Env Sci 120
Chapter 3 Notes: HOW ECOSYSTEMS WORK
Page 3 of 3
energy pyramid - a diagram that shows where energy exists in the
trophic levels of a food chain.
• In the above diagram, if an owl died and decayed (consumed by
decomposers), what percentage of the original energy would the
decomposers be getting?
• Using the principle of trophic levels and energy pyramids, explain
why a section of forest would commonly have more rabbits and
squirrels than hawks and owls.
Do Exercise p. 61, questions 1-5. (Answer in sentence form.)