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Food Webs
Investigation: Marine Food Webs
Galapagos Marine Coastal Organisms
Objectives:
Students will:
 Understand how food webs illustrate the ways in which
organisms depend on each other for energy and
nutrients.
 Make food webs showing the feeding relationships among
organisms encountered in the Galapagos coastal waters.
Background:
Producers-Organisms that get their energy directly
from the sun. (plants)Plants convert light energy into
chemical energy (photosynthesis) Producers produce
their own energy.
Primary Consumer (): Organisms that get their energy
from eating producers.
Carnivores: Primarily eat meat- cats, wolves, sharks
Herbivores: Primarily eat plants- cows, zebras,
giraffes,
Omnivores: Eat both- humans, bears, dogs,
Secondary Consumer (carnivore) Organisms that get
their energy primarily from eating primary herbivores.
Tertiary Consumer (carnivore) Organisms that eat other
consumers
Decomposers- Organisms that get their energy by eating
dead things.
Decomposers are found at every level and are vital to
replenish nutrients to producers.
Ex. Worms, fungi, bacteria
Autotrophs (producers) – produce energy by themselves
(auto=self,troph=nourish)
Heterotrophs- (consumers) gets energy by consuming
organisms (hetero=different, troph=nourish)
Omnivores: eat both plants and animals
Lesson
Food Web vs. Food Chain
Demo on whiteboard
Example:
Seaweed  Turtle  Shark
Phytoplankton  Zooplankton  Plankton-eating fish 
penguin  Sea Lion  Shark
1. Have students review the diets of each organism, then
form groups and create a colored poster of a Galapagos
food web.
2. Have groups present posters, being able to correctly
describe the flow of energy, and examples of
producers, consumers, herbivores, and carnivores.
3. Each student answers class discussion questions on a
sheet of paper.
Class Discussion:
1. At which levels of the food web would one expect to
find:
a. The largest adult population?
b. The smallest adult population?
2. Which organisms might be described as “top
carnivores?”
a. The largest adult organism?
b. The smallest adult organism?
1
.
At which levels of the food web would one would expect to
find:
Q. The largest population? The smallest population?
A.
Q. The largest adult organism? The smallest adult organism?
A.
2
.
Q. Which organisms might be described as “top carnivores”
and why?
A.
3. Discuss how changes in the environment could affect
populations of organisms. Below are two examples of
how environmental changes can have severe consequences
on life in the Galapagos. Use these examples to
elaborate how a food web is dependent on many
different factors:
El Niño
Marine iguanas primarily eat ulva, a type of green
algae that looks like lettuce (a producer in trophic
level 1), which grows on rocks in the cool, shallow
waters around the islands. If, during an El Niño, the
ocean temperatures were to rise, the ulva could
disappear. As other types of algae are not as
nutritious for the iguanas, iguanas would suffer from
malnutrition and weaken, dying from disease and
predation before they could die of hunger.
Upwellings
As the wind moves surface water away from the coast,
cold subsurface waters are brought up to the surface in
a process known as “upwelling.” Upwellings bring cold,
mineral- and nutrient-rich water from the deeper sea
levels close to shore. Because algae thrive on such
nutrient-rich water, their population would expand
greatly and clear water would turn greenish from the
population explosion. Herbivores that eat the algae,
such as the marine iguana, would thrive, as would
algae-eating sea urchins and bivalves. However, unlike
the iguana, these organisms wear down the reefs on
which they feed. Large reefs, therefore, cannot survive
in an area in which upwellings occur. If something were
to cause an upwelling in the vicinity of a large reef,
that reef would eventually be worn down, and the other
life forms that need a large reef to survive would have
to move elsewhere or die.