Food Web Energy

Catalyst:
 Imagine
that a bear wanders
into the food web (shown at
right) and eats all the
mountain lions. What
happens to the elk
population?
 Write two different food
chains in this food web.
Write in complete sentences!
Don’t talk during the Catalyst!
Objectives
By the end of today, all SWBAT…
 Explain why ecosystems require
continuous energy from the Sun
 Evaluate the efficiency of the flow
of energy through a food chain
 Explain why Mr. LY is a vegetarian
Homework Review
Catalyst Review


Imagine that a bear
wanders into the food
web (shown at right) and
eats all the mountain
lions. What happens to
the elk population?
Label two different food
chains in this food web.
Food webs need the Sun!

Key Point #1: Ecosystems require continuous
input of energy from the Sun
 All
of the energy in a food web ALWAYS begins with
the Sun.
 What if the energy from the Sun were removed?
 Without
energy from the Sun, producers die.
 If producers die, primary consumers die.
 If primary consumers die, secondary consumers die.
 The
bottom line: ecosystems require energy from the Sun,
or else everything dies!!!
Guided Practice (GP)
What would happen
to the producers in
this ecosystem if
the Sun stopped
shining?
Guided Practice (GP)
What would happen
to the primary
consumers in this
ecosystem if the
Sun stopped
shining?
Guided Practice (GP)
What would happen
to the secondary
consumers in this
ecosystem if the
Sun stopped
shining?
Guided Practice (GP)
Why does this food
web require
continuous input of
energy from the
Sun?
Energy Ineffiency


Trophic level = step in a food
chain
Key Point #2: Only 10% of
the energy in a food web is
passed from one trophic level
to the next.
rest is lost as heat.
 Called the “10% rule.”
 The higher up in the food chain
you go, the more food an
organism must eat to get the
same amount of energy.
1%
0. 1%
1%
 The
10% 10%
100%
Environmental Vegetarianism
1%
10%
Humans
Cows
10%
100%
Wheat

You have to eat 10kg of beef
to get the same amount of
energy as 1kg of wheat
What % of energy does each organism receive?

The taiga is one of the coldest ecosystems on Earth: Winter
lasts 5-6 months, and hardly any sunlight reaches it during
the rest of the year. Fur trees and grass use this meager
sunlight to grow. Mice feed on the grass and, though they
are rare, a few owls feed upon the mice.
Fur
100%
1%
Grass
Mice
10%
Owl
What % of energy does each organism receive?

In the tropical savanna, large amounts of sunlight allow for
an enormous, unbroken layer of grass to grow. Because of
the abundance of grass, there are many grazing animals,
such as sheep, goats, and cattle. A few lions roam the
grassland, eating these delicious mammals.
Sheep
10%
100%
Grass
Goats
Cattle
1%
Lions
Guided Practice (GP)
In the first box of your “Drawing Time!”
worksheet, make a food web.
 Trade your worksheet to the right. When you
get a new worksheet, draw a single food
chain from this food web.

 Label

the percentage of energy received.
Once you get your worksheet back, check the
answers.
 If
your partners made a mistake, you lose points!
Closing

Ecosystems require continuous input of energy from
the Sun
 Sun
provides energy for producers
 Producers provide energy for consumers
 No Sun  no producers  no consumers

Only 10% of the energy in a food web is passed
from one trophic level to the next
 100%
 10%  1%  0.1%
Exit Questions: Answer BOTH


AIn your own words,
explain WHY
ecosystems require a
continuous input of
energy from the Sun.
Hint: Use the words
“producer,” “primary
consumer,” “secondary
consumer.”
B
Read the following passage to answer
the questions below:
The ranches of the American Midwest
are full of grass. The grass uses
sunlight to grow. Cows and rabbits
graze on the grass. However, the
grass provides these animals little
shelter from wolves, which eat the
cows and rabbits.



Where does all the energy that
enters this food web begin?
What percent of the food web’s
energy does the grass receive?
What percent of the food web’s
energy does the wolves receive?