Food Webs and Ecological Pyramids

8SCIENCE Tues., Nov. 9

Review: what is a food web?

• Food chains and food webs show how organisms get energy from their food

• Food webs show interconnected or overlapping food chains

Ecological pyramids

• Shows how energy is transferred from one organism to another

• Pyramid bottom shows the producers

• Pyramid top showsthe top carnivores or top predators

How much energy is passed from one organism to another?

• Remember: most food chains only have 3 – 4 links or levels– Why?

• At each level energy is lost because organisms need to use their energy from food to perform their daily activities:– look for more food– find mates– create a shelter

• When an organism eatsanother organism for energy, it is only receiving about:– 10% of the available energy

Bioaccumulation

• When an organism eats another organism, it gets energy from what that organism ate as well– Example: if a wolf is eating a rabbit for energy, it is also

eating the grass that the rabbit ate for energy• If the food the organism eats is contaminated with

chemicals (like a pesticide), this can affect the top carnivores or predators

• As one organism eats another the toxic chemicals also move UP the food chain– Example: Mercury poisoning from fish, humans can

become sick

Deadly Links Game

• The purpose of this game is to understand the relationships in the food chain, and how these can lead to bioaccumulation of substances

• Instructions:– You will be assigned to be a hawk, shrew or grasshopper• Blue = grasshopper • Black = shrew • Light color = hawk

– If you are a grasshopper you will be given a stomach (clear plastic bag)

– When I say go, the grasshoppers may walk around and collect as much food as you can – you will have 30 seconds

Deadly Links Instructions continued…

• After 30 seconds the shrews will be released into the area and they may catch the grasshoppers for food– If you are caught: You MUST give the shrew your energy

(food) by giving them your ‘stomach’ (clear plastic bag)– The shrews will continue to collect food for another 30

seconds• Lastly, the hawks will be released and try to catch the shrews

for energy while the shrews continue to try and catch the grasshoppers

• After 30 seconds, I will ask you to return to your seat with the food that you have been able to ‘catch’ and sit quietly, wait for the next instructions

Deadly Links instructions continued….

• Count the amount of food/energy you have caught

• Does anyone have any white beans? – If you have over 10 white beans you have the

lethal amount of pesticide called DDT– This means you have died from DDT poisoning!

• How many organisms are still alive? • What type of organisms are still alive?

Deadly Links questions:• Please answer these questions on a separate piece of paper and hand them

in for the end of class: EACH person must answer their own questions for a grade out of

1) Draw the food chain that we acted out in the deadly links game. Use the names of the animals and don’t forget about your arrows! (4)

2) Why do you think there were more grasshoppers than shrews at the beginning? (1)

3) Why did the shrew take the grasshopper’s bag when he/she was tagged? (1)

4) Why was the number of hawks the same at the beginning and at the end? (1)

5) Describe what happened when the white/yellow beans actually turned out to be poison for the animals. (3)