FOOD CHAINS, FOOD WEBS AND ECOLOGICAL PYRAMIDS
In an ecosystem, plants use the energy from the sun to make oxygen and a type of sugar called glucose. This process is
called ​photosynthesis
​
.
Photosynthesis is only the beginning of a chain of energy transfer. There are many types of animals that will eat the products
of the photosynthesis process. Examples are deer eating shrub leaves, rabbits eating carrots, or worms eating grass. When
these animals eat these plant products, ​food ​energy​ is ​transferred​ from the plants to the animals​. These animals are in turn
eaten by other animals, again ​transferring energy​. Examples would be lions eating zebras, foxes eating rabbits, or birds
eating worms.
This chain of ​energy transferring​ from one species to
another can go on and on many more times, but it it does
end. It ends with the dead animals that are broken down
and used as food or nutrition by bacteria and fungi.
These bacteria and fungi are called decomposers.
Decomposers play a very important part in this world
because they take care of breaking down (cleaning) many
dead material. There are more than 100,000 different
types of decomposers! Once they break down the tissues,
the chain starts all over again.
Producers:​ Organisms, such as plants, that make their
own food are called ​producers​. They are called
producers because all of the species of the ecosystem depend on them. Another scientific word for producers is autotroph.
Auto means “by itself” and troph means “an organism with nutritional requirements.”
Consumers:​ All the organisms that can not make
their own food (and need producers) are called
consumers. They are called consumers because
they depend on others. They get their food (and
energy) by eating other organisms. Remember
when we talked about the levels of an ecosystem?
That was the easy part. There are different levels
of consumers, too. There are primary consumers,
secondary consumers, tertiary consumers and
quaternary (1st, 2nd, 3rd and 4th levels). I noticed
that some of you already have learned this back in
elementary school! That’s fantastic! Organisms that
feed on (eat) primary consumers are called
secondary consumers. Those who feed on
secondary consumers are tertiary (pronounced
ter’shiary) consumers. Another scientific word for
consumer is ​heterotroph​. The prefix “hetero”
means “different”. In the figure above, the snake
acts as a secondary consumer and the hawk as a
tertiary consumer. Some organisms, like the
squirrel are at different levels. When the squirrel
eats acorns or fruits (which are plant product), it is
a primary consumer; however, when it eats insects
or nestling birds, it is a tertiary consumer. Consumers are also classified depending on what they eat; they can be herbivores,
carnivores, omnivores or scavengers.
When we talk about an energy pyramid, energy is transferred from one level to the next. These levels are called ​trophic
levels​. Energy is passed from one ​trophic level​ to the next. When we figure out and calculate how much energy is passed
from one level to the next, we can use a pyramid. We call this an ​energy pyramid​. This pyramid helps us to see the fact
that in an ecosystem, there needs to be many producing organisms at the bottom of the pyramid to be able to sustain and
support just a few of the organisms at the top. In looking at the pyramid, can you guess how much larger the volume of each
layer is as compared to the one just above it? Take a guess. It might not look like it but they are close to 10 times larger.
A basic pyramid shape often shows a typical food chain or food web. The pyramid represents the decrease in the amount of
energy and the number of organisms from the producer to the high - order consumer levels. The decrease in the numbers is
due to energy loss, because fewer organisms can be supported at each trophic level.
Pyramid of Energy
Energy is lost between each link in a food chain.
So, much of the energy never really makes it to
the next level. Where does the energy go as it
moves through a food chain? ​Some of the
energy is used for carrying out the
organism’s life processes like hunting or
foraging, moving, running, walking, playing,
mating, and/or predator/prey behavior, etc​.
Producers make their own food source directly
from sunlight by the process of photosynthesis.
In order to carry out life functions, consumers get
their energy through the 'burning' or breaking
down of food molecules they consume (eat). This
“burning up” of energy is called ​therma​l energy otherwise known as heat - is created as a result
of the burning of these food molecules. ​More
than half of the energy from each food
molecule is lost as heat.​ Only about 10% of
energy at each trophic level is available to pass
on to the next level. Scientists call this, ​The 10%
Rule​.
Pyramid of Numbers
The loss of energy at each trophic level also explains why there are usually fewer organisms in each higher trophic level.
The total number of plants in a particular area would usually be higher than the number of herbivores that the plants support
and the number of herbivores would be higher than the number of higher order carnivores.
Human Impact on Food Chains and Webs
Humans have the ability to have a great impact and affect on ecosystems. Living things are a huge part of any ecosystem,
therefore any activity that affects an ecosystem is also likely to affect the organisms within that ecosystem. If the organisms
are affected, the food chains and webs that the organisms are a part of will also feel the affects.
Name:______________________________________________
Period: _______
Ecological Pyramids Worksheet
1.
Give an example of a ​food chain​ in nature - use 3-5 organisms. You may write it
OR draw it on the line below.
________________________________________________________________
2.
In an ecological pyramid, what happens to the energy, ​AND​ the number of
species as you move up the energy pyramid?
3.
In an ecosystem, can there be more carnivores (meat eaters) than herbivores
(plant eaters)? ​Explain your thought.
4.
Where does the energy in an organism, as you move up the energy pyramid?
How is it used?
5.
Brainstorm at least 2 things humans sometimes do that can affect ecosystems,
food chains and food webs. How does the activity you named, affect the
ecosystem?
6.
Label​ the ecological pyramid below with the following words: p
​ roducers, tertiary
consumer, secondary consumer, primary consumer, decomposers, hawk,
grass, chicken, grasshopper.
Also label the percent of ​energy​ as you move up the energy pyramid. Look at
the text! There is a diagram!
7.
What level do you think humans hold on an ecological pyramid? Explain what
could happens to humans if some types of poisons enters into ​OUR​ food web.