Edexcel IGCSE Biology 复习笔记 4.2.1 Trophic Levels & Food Chains

Trophic levels

• Trophic levels are used to describe the feeding relationships between organisms
• Energy flows from the sun to the first trophic level (producers) in the form of light
  • Producers convert light energy into chemical energy and it flows in this form from one consumer to the next

   

• Eventually, all energy is transferred to the environment – energy is passed on from one level to the next with some being used and lost at each stage
• Energy flow is a non-cyclical process – once the energy gets to the top of the food chain or web, it is not recycled but ‘lost’ to the environment
• This is in direct contrast to the chemical elements that organisms are made out of, which are repeatedly recycled

Trophic Levels Table

• Animals (known as consumers) can be at different trophic levels within the same food web as they may eat both primary, secondary and/or tertiary consumers

Food chains

• A simple way to illustrate the feeding interactions between the organisms in a community is with a food chain
• You need to know the terms given to each step in a food chain:

1. Producer: food chains always begin with a producer
2. Primary consumer: producers are eaten by primary consumers (herbivores/omnivores)
3. Secondary consumer: primary consumers are eaten by secondary consumers (carnivores/omnivores)
4. Tertiary consumer: secondary consumers are eaten by tertiary consumers (carnivores/omnivores)

An example of a food chain

• A food chain shows the transfer of energy from one organism to the next
• The source of all energy in a food chain is light energy from the sun
• The arrows in a food chain show the transfer of energy from one trophic level of the food chain to the next

Trophic levels for a simple food chain

Transfer of energy

• In order for the energy to be passed on, it has to be consumed (eaten)
• However not all of the energy grass plants receive goes into making new cells that can be eaten
• The same goes for the energy the grasshopper gets from the grass, and the energy the frog gets from the grasshopper
• Only the energy that is made into new cells remains with the organism to be passed on
• Even then, some of this energy does not get consumed - for example few organisms eat an entire organism, including roots of plants or bones of animals - but energy is still stored in these parts and so it does not get passed on
• The majority of the energy an organism receives gets ‘lost’ (or ‘used’) through:
  • Making waste products eg (urine) that get removed from the organism
  • Movement
  • Heat (in mammals and birds that maintain a constant body t
    • • emperature)
      • Undigested waste (faeces) that is removed from the body and provides food for decomposers

       

    • This inefficient loss of energy at each trophic level explains why food chains are rarely more than 5 organisms long

     

    

     

    Energy is lost at each trophic level for several reasons

     

     

    Exam Tip

    This is a complicated concept but by learning the main ways in which energy is lost between trophic levels, you will be able to answer most questions on this topic.Make sure you read the question carefully and tailor your answer to the specific organism you are being asked about – e.g. plants do not produce urine or faeces so you could not give this as one of the ways in which they use energy that cannot be passed on!

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