Science. Class 7. A food chain shows the different organisms that live in a habitat, and what eats what.

Transcription

1 Food chains Science 7 A food chain shows the different organisms that live in a habitat, and what eats what. Producers and consumers A food chain always starts with a producer, which is an organism that makes food. This is usually a green plant, because plants can make their own food by photosynthesis. A food chain ends with a consumer, which is an animal that eats a plant or another animal. Here is an example of a simple food chain: grass cow human The arrow points to the organism that is doing the eating: If you get the arrows the wrong way round, instead of showing that humans eat cows, you are showing that cows eat humans, and that grass eats cows. Plants use the energy from sunlight to make food in their leaves by a process called photosynthesis. They are called producers because they produce their own food. Most animals cannot make their own food. They must eat something else in order to get the energy to live and grow. They are called consumers. Some animals eat plants. They are called herbivores. They need to eat a lot of plants, because plants contain relatively little energy. Animals that eat meat are called carnivores. They get their meat by eating other animals, usually smaller than themselves. Omnivores (like humans) eat both plants and animals. Some animals, like earthworms, eat bits of plants and animals that are dead, rotten or are other animal s droppings! They are called decomposers. They are important for recycling nutrients. Fungi and micro organisms are also decomposers. The nutrients go back into the soil where they can be used again by plants to help them grow. tcspafclass7.wordpress.com Page 1 of 10

2 Other words in a food chain There are several words used to describe the organisms in a food chain. Study this food chain: Grass Grasshopper Frog Hawk Producer Primary consumer Secondary consumer Tertiary consumer The plant is the producer and the animals are consumers. Notice that the first consumer in the chain is also called the primary consumer, the next one is the secondary consumer and the one after that is the tertiary consumer. A consumer that eats plants is called a herbivore, and a consumer that eats other animals is called a carnivore. An omnivore is an animal that eats both plants and animals. Predators and prey A predator is an animal that eats other animals, and the prey is the animal that gets eaten by the predator. In the food chain above: the frog is a predator and the grasshopper is its prey. the hawk is a predator and the frog is its prey. tcspafclass7.wordpress.com Page 2 of 10

3 Pyramids of numbers The population of each organism in a food chain can be shown in a sort of bar chart called a pyramid of numbers. The more organisms there are, the wider the bar. The producer in the food chain always goes at the bottom of the pyramid of numbers. Think about this food chain: clover snail thrush hawk Clover is a plant and it is the producer in this food chain. Its bar goes at the bottom of the pyramid: Energy is lost to the surroundings as we go from one level to the next, so there are fewer organisms at each level in this food chain. A lot of clover is needed to support the snail population. A thrush eats lots of snails, and a hawk eats lots of thrushes, so the population of hawks is very small. tcspafclass7.wordpress.com Page 3 of 10

4 Other pyramid shapes Sometimes the pyramid of numbers doesn't look like a pyramid at all. This could happen if the producer is a large plant such as a tree, or if one of the animals is very small. Remember, though, that whatever the situation, the producer still goes at the bottom of the pyramid.here are two examples like this: An oak tree is very large so many insects can feed on it Fleas are very small and lots of them can feed on a rabbit. tcspafclass7.wordpress.com Page 4 of 10

5 Food webs When all the food chains in a habitat are joined up together they form a food web. Here is an example of a food web: Although it looks complex, it is just several food chains joined together. Here are some of the food chains in this food web: grass insect vole hawk grass insect frog fox grass insect vole fox Notice that the frogs, voles and insects have more than one predator, but the rabbits and slugs have just one predator. This leads to some interesting effects if the population of a particular organism in the food web decreases. Some animals can just eat more of another organism if food is in short supply, while others may starve and die. This in turn can affect the populations of other organisms in the food web. tcspafclass7.wordpress.com Page 5 of 10

6 Changes to food webs Let's look at the food web again and ask some questions about its predator-prey relationships, with some possible answers. What would happen if the grass died? The grass is the producer, so if it died the consumers that feed on it - rabbits, insects and slugs - would have no food. They would starve and die unless they could move to another habitat. All the other animals in the food web would die too, because their food supplies would have died out. The populations of the consumers would fall as the population of the producer fell. What would happen if the population of slugs decreased? Slugs, rabbits and insects all eat grass. If there were fewer slugs there would be more grass for the rabbits and insects. With more food the populations of rabbits and insects would increase. However, the thrushes would have to eat more insects to maintain their population, so it is also possible that the population of insects could decrease. This is turn may reduce the populations of voles and frogs. What would happen if the population of insects decreased? There would be more food for the rabbits and slugs, so their populations would increase. However, there would be less food for the frogs and voles, so their populations would decrease. This means less food for the foxes and hawks. However, there are likely to be more rabbits and thrushes for them to eat, so their populations are likely to stay the same. tcspafclass7.wordpress.com Page 6 of 10

7 SECOND TERM - CLASS.7 SCIENCE WORKSHEET NO.3 HABITATS AND FOOD WEBS Q.1-Choose the best option. 1. A green plant is usually a: producer consumer director 2. What does a food chain always start with? a top predator a herbivore a producer 3. Which is the secondary consumer in this food chain: grass -> grasshopper -> frog -> hawk grass grasshopper frogs 4.What does an omnivore eat? animals only plants only animals and plants 5. What is wrong with this pyramid of numbers? tcspafclass7.wordpress.com Page 7 of 10

8 hawks don't eat thrushes the clover should be at the bottom snails don't eat clover 6. What can we conclude from this pyramid of numbers? the producer must be very large the producer must be very small the primary consumer must be a carnivore tcspafclass7.wordpress.com Page 8 of 10

9 7. How many predators are there in this food web? What is likely to happen to the population of slugs if the population of thrushes decreases? it will increase it will decrease it will stay the same tcspafclass7.wordpress.com Page 9 of 10

10 9. What is likely to happen to the population of foxes if the population of rabbits decreases? it will increase it will decrease it will stay the same 10. What is likely to happen to the population of thrushes if the population of voles increases? it will increase it will decrease it will stay the same tcspafclass7.wordpress.com Page 10 of 10