Edexcel IGCSE Biology: Double Science 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 temperature)
  • 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