8.4 3.1.5 - energy flow

Energy flow and nutrient cycles

Organisms such as plants and animals need energy for growth, movement and reproduction. They get this in the form of nutrients from the food they eat.The main source of energy for life on earth is the sun. The sun provides energy to producers that use photosynthesis to grow and become food for consumers.Consumers include herbivores, carnivores and omnivores. Decomposers break down discarded plant and animal (organic) materials into simpler substances, whichreturns nutrients to the soil and atmosphere for new plants to use to grow.

Food chain

A food chain is a series of nutrients and energy moving through a chain of organisms. Below is an example of a simple food chain in a grassland. Thearrows show the movement of energy from one organism to another.

IMAGES sourced from: http://www.flickr.com/photos/blueridgekitties/4625665988/sizes/o/in/photostream/

http://www.flickr.com/photos/zest-pk/924783392/sizes/m/in/photostream/

http://www.flickr.com/photos/e3000/5922771249/sizes/l/in/photostream/

Activity 1:

Can you trace a food chain of the vegetables, fruit, cheese, eggs or meat that you had for breakfast or will have for dinner?

Activity 2:

1. In the food chain shown above which of the three organisms is the
   1. Herbivore
   2. Carnivore
   3. Producer
2. Draw a food chain showing at least 4 organisms.
3. Producers use sunlight to manufacture their own food. Write a word equation as well as a balanced equation to depict this process.
4. Draw in the decomposers in the above food chain. Ensure that the direction of the arrows is correct.
5. What animal will feed on the leopard?

Food web

A food web is made up of a number of food chains. It represents the different feeding relationships in an ecosystem or a biome. It is usually morecomplicated than a food chain because organisms can get their energy or food from more than one source. The presence of a number of food sources makes thesystem more stable. If one organism is removed, the whole system will not collapse, unlike in a single food chain.

(image source: http://per8eocreview.wikispaces.com/ )

Activity 3: human food web

1. Divide into teams of at least five students each and stand in a circle.
2. One person in each group takes a ball of wool or string. This person is the sun and starts the food web.
3. The first person (sun) holds onto the end of the wool and throws the ball of string to another person in the group.
4. The person who catches the ball has to name themselves something that uses energy from the sun (primary producer).
5. The person holding the ball (primary producer) has to hold the string with one hand and toss the ball to another student in the circle with his/her otherhand.
6. The person who catches the ball has to name something that eats or is eaten by the previous organism named.
7. Carry on until everyone in the circle is holding the ball. You can throw the ball to someone who has already named themselves, as long as they eat you orare eaten by you
8. Look at the web you have created and the ones the other groups have created.
9. Are some webs more complex than others? Why?

(source: http://www.oercommons.org/courses/got-energy-spinning-a-food-web/view)

Trophic levels and the food pyramid:

The trophic level of an organism is the position it holds in a food chain and depends on how much energy it consumes or produces. The trophic level of eachorganism can be drawn as a pyramid starting with the producers at the bottom and moving up through the food chain.

The organism at the bottom gives the most energy and needs the least and the organism at the top needs the most energy and releases the least. Energy islost from activities at every level - through heat, egestion, urination andreproduction (pregnancy and egg-laying). This is why there is less and less energy as you move up the pyramid.

Producers, eg. Plants are on the first level, or bottom of the pyramid, because they produce their own nutrients using energy from the sun and therefore have alot of energy to pass on.

Primary Consumers, eg. Herbivores are on the second level because they feed off plants

Secondary Consumers, eg. Carnivores feed on herbivores so they get their energy from plants indirectly and are on the third level.

Tertiary Consumers, eg. Carnivores feed on organisms below them in the pyramid

(image source: http://per8eocreview.wikispaces.com/)

Figure 3. Food pyramid

Trophic levels can be drawn as a pyramid of numbers, where each level shows the number of organisms (look at figure 3). They can also contain the biomass of apopulation. The biomass is the mass of living organisms in an ecosystem.

Activity 4:

Look at the food web and the diagram showing the different trophic levels.

1. Identify a food chain that has three trophic levels.
2. Identify a food chain that has four trophic levels.
3. Name 2:
   1. Producers
   2. Primary consumers
   3. Secondary consumers
   4. Tertiary consumers
4. There are very few tertiary consumers compared to the primary consumers. Why?
5. What will happen if the impala is removed from the food web?

(source: http://www.learner.org/courses/envsci/interactives/ecology/ )

VIDEO: http://www.youtube.com/watch?v=TE6wqG4nb3M

This a catchy song about food chains to help you remember.

VIDEO: http://www.youtube.com/watch?v=-YwW-iWxLr4&NR=1

Bill Nye the Science Guy talks about the Food Web