FOOD CHAINS and FOOD WEBS
FOOD CHAINS
Are designed to show
the direction that
energy flows from
producers to top
carnivores, in a
simplified single
pathway.
ENERGY IN FOOD CHAINS
energy passed to consumer
Producer 10%
Primary consumer 10%
90%
2o consumer
90%
energy used to live
10% Rule: only 10% of the energy in an organism is
passed on to the next trophic level
1 unit
100 units
10 units
10 units
100 units
1000 units
FOOD WEBS & TROPHIC LEVELS
1. TROPHIC LEVEL - the place in a food chain that an
organism feeds
a. FIRST TROPHIC LEVEL - producers or autotrophs
that feed themselves
*Any organisms found at the second trophic level or
higher are considered to be heterotrophs i.e. they
do not produce their own food
b. SECOND TROPHIC LEVEL - herbivores or
primary [1o] consumers that eat producers
c. THIRD TROPHIC LEVEL – carnivores or
secondary [2o] consumers that eat herbivores
d. FOURTH TROPHIC LEVEL – carnivore or tertiary
[3o] consumers that eat other carnivores
2.
FOOD WEB - a series of
interconnected food chains
We will use this simpler food web to answer a series of questions
HAWK
BOBCAT
WEASEL
FROG
MOUSE
GRASSHOPPER
SEEDS
VOLE
GRASS
FALLEN LEAVES
HAWK
BOBCAT
WEASEL
FROG
MOUSE
GRASSHOPPER
SEEDS
Identify an autotroph
VOLE
GRASS
FALLEN LEAVES
produces its own
food
Identify a primary consumer eats a producer
HAWK
BOBCAT
WEASEL
FROG
MOUSE
GRASSHOPPER
SEEDS
producer
2o consumer
VOLE
GRASS
Identify a secondary consumer
a carnivore that eats herbivores
3o consumer
1o consumer
FALLEN LEAVES
1o Consumer
Identify a tertiary consumer
a carnivore that eats other carnivores
HAWK
BOBCAT
WEASEL
FROG
MOUSE
GRASSHOPPER
SEEDS
VOLE
GRASS
FALLEN LEAVES
Identify an organism at the fourth trophic level
[show how it is at this level]
HAWK
BOBCAT
WEASEL
FROG
MOUSE
GRASSHOPPER
SEEDS
Identify a decomposer
Identify an omnivore
VOLE
GRASS
FALLEN LEAVES
something that eats dead things
something that eats both plants & animals
HAWK
BOBCAT
WEASEL
FROG
MOUSE
GRASSHOPPER
SEEDS
VOLE
GRASS
FALLEN LEAVES
Explain, what would be the effect on grasshoppers if fewer leaves
fewer fallen leaves means less food for voles
voles now eat more grass
voles eat more grass
less grass for grasshoppers to eat
fewer grasshoppers as they leave area as
less food
Over a longer time
less food means some voles leave
fewer voles means less food for bobcat
bobcats now eat more frogs
fewer frogs means weasels eat more
grasshoppers fewer grasshoppers
HAWK
BOBCAT
WEASEL
FROG
MOUSE
GRASSHOPPER
SEEDS
VOLE
GRASS
FALLEN LEAVES
Explain what would be the effect on bobcats if fewer grasshoppers
Fewer grasshoppers means less grass & seeds
being eaten and less food for frog, weasels &
mice
Less food for frogs means frogs leave
If fewer frogs, less food for bobcats 
fewer bobcats
Over a longer time
If more grass, more food for voles
If more voles, more food for
bobcats  more bobcats
HAWK
BOBCAT
WEASEL
FROG
MOUSE
GRASSHOPPER
SEEDS
VOLE
GRASS
FALLEN LEAVES
Explain what would be the effect on hawk if fewer bobcats.
Fewer bobcats means more voles and frogs
Over a longer time
more frogs means more grasshoppers eaten
More voles eat more grass
Fewer grasshoppers means less food for mice
& weasels [although weasels can eat frogs]
Less grass for grasshoppers & mice
Overall, less food for hawks  fewer hawks
Fewer grasshoppers & mice, less
food for hawks  fewer hawks
HAWK
BOBCAT
WEASEL
FROG
MOUSE
GRASSHOPPER
SEEDS
VOLE
GRASS
FALLEN LEAVES
Explain what would be the effect on voles if fewer hawks
Fewer hawks means more weasels and more mice
More mice & weasels eat more grasshoppers
So now fewer grasshoppers so they eat less grass
Now more grass, so more food for voles  more voles
References
• Investigating Science – Pearson
• A. Dercho