Food chains and trophic levels at Higher Level: The same ideas as SL, with a longer example: the five-level Arctic Ocean chain from ice algae to polar bears, and whale falls on the deep sea floor.
Practise this as you read
- State any organism's trophic level by name and number.
- Explain where decomposers fit in a food web.
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What moves along a food chain: In a food chain, carbon compounds, and the chemical energy they contain, pass from one organism to the next when it is eaten.
The points to remember
- When one organism eats another, its carbon compounds pass to the eater.
- The chemical energy stored in those compounds passes on with them.
- Each arrow in a food chain shows this transfer, from the food to the eater.
- So carbon compounds and energy move step by step along the chain, from the producer upwards.
- Energy flows one way only: it never goes back down the chain.
Remember it as: Eat the food, take its carbon and its energy.
Real example: in the Arctic Ocean, ice algae feed zooplankton, and the carbon compounds pass on through Arctic cod and ringed seals to polar bears.
Which way the arrow points: An arrow means 'is eaten by': it points from the food to the eater, the way energy flows. Drawn from the eater to the food, it is backwards. Moving from prey to predator is a transfer: the compounds change place, not form (1.2.4).
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The stages in a food chain are called trophic levels. Every organism in a chain sits at one of them.
The points to remember
- Each stage in a food chain is a trophic level: the position an organism occupies, counted from the producer.
- Trophic level 1: producers (plants, algae). Level 2: primary consumers, the herbivores.
- Level 3: secondary consumers. Level 4: tertiary consumers. Level 5: quaternary. The last is the top predator.
- To state a level, give the name and add the number: 'secondary consumer (trophic level 3)'.
- An omnivore can feed at more than one trophic level.
Remember it as: Count from the producer: 1, 2, 3, 4.
Real example: in ice algae, zooplankton, Arctic cod, ringed seals, polar bears, the polar bear is a quaternary consumer at trophic level 5.
Say the name: 'Trophic level 1' alone, 'third consumer', 'prey' or 'animal' may not be credited when a question says 'State'. Write 'producer' or 'secondary consumer', then add the number.
Papers often give a food web and ask for the trophic level of one organism, or for a producer.
Reading levels from a food web
- Find the producers first: no arrows point into them.
- Follow the arrows to the eater, counting each step from the producer.
- An organism's level depends on the chain it is in; in a web it can sit in several chains.
- Use the names in the figure: 'Antarctic krill', not 'an animal'.
Try it: amphipods are primary consumers, Arctic cod secondary, ringed seals and beluga whales tertiary, and polar bears quaternary consumers.
Name a producer: Asked for a producer, give one organism from the figure, such as 'phytoplankton' or 'ice algae'.
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Dead organisms and waste from every trophic level end up with the decomposers.
The points to remember
- Decomposers feed on dead matter and waste from every level, so they have no single place in a chain.
- That is why they are usually left out of food chains.
- They still take in carbon compounds and the chemical energy in them.
- Their respiration turns that chemical energy into heat, which is lost from the food web.
- In a food web, they link to every level through dead matter.
Remember it as: Every level ends up with the decomposers.
Real example: when a whale dies and sinks to the deep sea floor, a whale fall feeds scavengers and bacteria for decades; their respiration releases its energy as heat.
Their feeding strategies: Which organisms are decomposers, detritivores and scavengers is on 2.2.9.
How this comes up: Paper 2, Section A: a food chain figure and short parts on what it shows [2].
Figure 1 shows a food chain in the Arctic Ocean.
Outline what is passed from one trophic level to the next in the food chain shown in Figure 1.
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