Section 2
Energy Flow in Ecosystems
CHAPTER 16
Grade 10 Biology
Spring 2011
BELL RINGER
A robin, snake, hawk, frog, grasshopper,
mouse, and rabbit can all be found in an open
field.
 Draw arrows to show what eats what in this
field ecosystem.

OBJECTIVES
Distinguish between producers and consumers
 Compare food webs with food chains
 Describe why food chains are rarely longer than
three or four links

PRIMARY ENERGY SOURCES

Primary Productivity: the rate at which organic
material is produced by photosynthetic
organisms in an ecosystem
CHANGE OF ECOSYSTEMS OVER TIME

Producers: organisms that first capture energy
 Plants,
some bacteria, algae
 Make energy storing molecules
CHANGE OF ECOSYSTEMS OVER TIME

Consumers: organisms that consume plants or
other organisms to obtain energy necessary to
build their molecules
TROPHIC LEVELS

Trophic Level: an energy
level that is used to study
how energy is moved
through an ecosystem
 Organisms
are assigned to
trophic levels
 Energy moves from one
trophic level to another
SUN
PRODUCER
CONSUMER
CONSUMER
TROPHIC LEVELS

Food Chain: path of energy through the trophic
levels of an ecosystem
TROPHIC LEVELS

First Level: occupied by
producers
 Plants,
algae, bacteria
 Producers use energy of the
sun to build energy-rich
carbohydrates
 Many also incorporate key
nutrients (nitrogen) from the
environment into their
biological molecules
TROPHIC LEVELS

Second Level: herbivores eat the primary
producers from the first trophic level
 Primary

consumers
Herbivores: animals that eat plants or other
primary producers
TROPHIC LEVELS
Third Level: carnivores eat
the primary consumers
(herbivores) from the
second trophic level
 Carnivore: animals that eat
other animals
 Omnivore: both herbivores
and carnivores

TROPHIC LEVELS



Detritivores: consumer, obtain their energy from the organic
wastes and dead bodies that are produced at all trophic levels
 Bacteria, fungi, worms
Decomposers: cause decay
 Bacteria, fungi
This releases nutrients back into the environment
TROPHIC LEVELS

Fourth Level: carnivores that consume other
carnivores
 Tertiary
consumers, top carnivores
TROPHIC LEVELS

Food web: complicated, interconnected group
of food chains
 In
most ecosystems, energy does not follow simple
straight paths
 Individual animals often fed at several trophic
levels
LOSS OF ENERGY IN A FOOD CHAIN

Energy transfer: during
every transfer of energy
within an ecosystem,
energy is lost as heat
 Amount
of useful energy
available to do work
decreases as energy
passes through an
ecosystem
 Loss of energy limits
number of trophic levels
THE PYRAMID OF ENERGY

Energy Pyramid: diagram in which each trophic
level is represented by a block, and the blocks
are stacked on top of each other, with the
lowest trophic level on the bottom
 Pyramid
shaped
because each level
represents the amount
of energy stored in the
organisms at that level
LOSS OF ENERGY IN A FOOD CHAIN

At each trophic level, the energy stored by the
organisms in a level is about one-tenth of that
stored by the organisms in the level below
Fourth Trophic Level: 1J energy
Third Trophic Level: 10J energy
Second Trophic Level: 100J energy
First Trophic Level: 1,000J energy
LIMITATIONS OF TROPHIC LEVELS
Most ecosystems involve three or four levels
 Too much energy is lost at each level to allow
more
 Why can’t a large human population survive
from eating lions?

HUMANS EATING BREAD:
HUMANS EATING BEEF:

it takes a certain amount of
grain……

It takes 10 times more
grain…….

To produce enough bread……

To feed one cow……

To make enough beef…..

To provide one person with
the same amount of energy.

To provide one person with a
certain amount of energy.
LIMITATIONS OF TROPHIC LEVELS
LIMITATIONS OF TROPHIC LEVELS
Biomass: dry weight of tissue and other organic
matter found in a specific ecosystem
 Each higher level on the pyramid contains only
10% of biomass found in the trophic level below it

Used to determine the amount of energy present in
trophic levels
 Used because the number of
individuals in a trophic level
may not be an accurate
indicator of the amount of
energy in that level

 Due
to size of organisms
REVIEW
1.
2.
3.
How much energy is lost as you move up a
trophic level?
List the reasons why food chains do not tend
to exceed four links?
How are producers different from consumers.
ANSWERS
1.
2.
3.
90% of energy is lost at each trophic level
Food chains do not tend to exceed four links
because so much energy is lost at each
trophic level.
Producers use energy (usually from the sun) to
assemble food molecules, hence “producing
food”. Consumers must take in (or consume)
these food molecules to obtain their energy.