YARN SUNBEAM

(adapted from Bretzfelder Park Naturalist Program for BES)

 

Objective: Students will:

1. Understand how energy flows through an ecosystem; it does not cycle.
2. Understand that energy does not disappear, but changes form from more ordered, concentrated forms (such as light energy and chemical energy in food) to less ordered forms (such as heat energy).
3. Understand that the number of levels in food pyramids is limited because the amount of available energy for living thing sis limited.

 

Time:  Approx. 30 minutes

 

Materials:       25 feet of yellow string or yarn (easily breakable or cut with scissors)

                        Poster board labeled “Unavailable heat energy” to label pile of yarn that will form

                        Copies of 7 cards (following) – on tag board, or attached to 5” X 8” cards

                       

Procedure:

1. Ask the group to sit in a semi-circle approximately 25 feet long. 
2. Holding up the yarn, tell the group you hold the sun’s energy in your hands and that you will follow it as it flows through natural communities.
3. Ask one person to act as the sun and the rest to represent the natural community.  Hand out cards, #1 to the sun and the rest in order spaced around the circle.
4. Give the yarn sunbeam, to the sun.  He/She will read the first card. 
5. The group will be able to follow the fate of the yarn sunbeam by reading each card and breaking off the appropriate fraction of the remaining yarn to throw in the center pile.  You may need to help estimate the fraction or cut the yarn.
6. Encourage dramatic reading and questions during the activity.  There should be a surprisingly small piece left by the end.

 

Follow-up questions:

1.  In school many of us memorized that “energy is neither created nor destroyed.”  If energy is never destroyed, why don’t plants use that indestructible energy over and over instead of continually trapping more light energy?

      While energy may not be destroyed, this activity shows that living things change it from ordered, concentrated, useable forms such as light and food to less ordered, less concentrated, and less useable forms such as heat.  So green plants need to continually trap the sun’s light energy to get energy in a useable form.

 

2.  Could we use the sun’s energy in green plants more efficiently by being vegetarians or by being meat-eaters?  Why?

      As the activity illustrates, more energy becomes unavailable at each step in the food chain because more useable food energy is converted to respiration (oxidation) into unusable heat energy.  So if we ate wheat from the start of the food chain, more energy would be available to us than beef cattle fed on that wheat because cattle would have converted a considerable amount of the energy stored in wheat to unavailable heat energy.

 

CARD #1

 

I am the sun and all of you living things need my energy to survive.

If I walked around this circle 100 times handing out yarn, that could represent the amount of energy that reaches the earth.  But 90% of that is reflected as heat or light (remember how bright the earth looks to the astronauts?)

So is I walked around 10 times, the yarn I handed out would represent the energy the earth absorbs.  Most of it is absorbed by rocks, air and water.  They heat up and what happens?  Ocean currents, wind rain – so I cause weather!

But I’ll pass out one strand of yarn – only 1% of the energy I beam toward the earth.  This energy is not reflected away from the earth, or absorbed by non-living rocks, air or water.  Who gets this precious 1%?

     

CARD #2

 

We do!

We are the green plants – pine trees, hobblebush, ferns and many others in our woods.  Because we have green pigments, we can absorb this light energy from the sun and turn it into food energy.

We use some of this energy ourselves to grow, move sap, make seeds – in short, to live.  When we use the concentrated food energy, we change it into less concentrated heat energy that other living things can’t use.  To show this, we’ll now throw 1/5 of the total we captured from the sun onto the “Unavailable Heat Energy” pile.

But 4/5 of the energy we originally trapped, represented by the yarn sunbeam we’re still holding, is still stored in our leaves, roots, seeds and all other parts of us as food energy that other living things can use.  Who gets it?

 

 

CARD #3

 

          We get part of it!

          We are the decomposers of leaves, logs, and all plant material.  You would recognize some of us as mushrooms and molds; many of us, such as bacteria, are too small to see without a microscope. 

          We have no green pigments, so we can’t trap the sun’s energy.  That means we’re dependent on the green plants – but usually not until they’ve died and turned brown.  Think of all of the dead leaves and logs in the woods that no animal has eaten.  What happens to them?  We decompose them!  As we use the stored food energy to live, we give off heat energy. (Did you ever feel how hot a pile of rotting leaves gets inside?)

          To represent the energy we use, we’ll throw ¾ of the remaining yarn on the “Unavailable Heat Energy” pile.

 

 

CARD #4

          

We get the rest of the energy from green plants!  You know many of us – large plant-eaters such as deer, and small herbivores such as caterpillars.  All of us herbivores depend on green plants for energy to survive.

          We much and digest green plants to get a lot of energy.  We need it to survive and grow, like green plants themselves, but unlike them, we need a larger fraction of what we eat to move around.  Think about how much food energy is changed to heat energy when we run!

          So we’ll throw 1/3 of the remaining energy from green plants on the “Unavailable Heat Energy” pile.  The rest is stored in our bodies as tasty muscle and fat.  Who gets some of it?

CARD #5

          

We are the decomposers of herbivores.  You probably haven’t seen many of us, since many of us are bacteria and too small to be seen.  You may be prejudiced against us because of what you’ve smelled of a road-killed woodchuck or other herbivore.  But the smell is just gases released as we do our important work of decomposition.

          We use a good ½ of the energy stored in the bodies of plant-eaters.  AS we use that energy to live, grow and reproduce, we change it into unavailable heat energy.  So here it goes onto the pile.

 

CARD #6

 

          Even if you don’t see us very often, you would recognize us as animal eaters.  Some of us are spiders, weasels, and owls.  Although we sometimes eat other carnivores, we depend mostly on herbivores for our food.  So now we’ll eat up the energy stored in their tasty little bodies.

          Think of the difference between the amount of energy a weasel needs to chase a mouse, and the amount a mouse needs to nibble seeds.  Which is greater?  Obviously we use a larger fraction of the energy available to us than the herbivores did.  So we’ll throw 6/10 of this energy on the “Unavailable Heat Energy” pile.

          The rest of the energy we got from eating herbivores is stored in our bodies.  Who gets what’s left?

 

CARD #7

 

           We’re the decomposers of carnivores; many of us are bacteria.  Energy that came all the way through the food chain from the sun to carnivores is what we live on.  We obtain it by decaying carnivores such as bobcats or sharks.

          As we use this last bit of energy, we change it into (you guessed it!) Unavailable Heat Energy.  So here it goes on the pile!