Producers come first: Producers make their own carbon compounds by photosynthesis: grass, trees, pondweed, seaweed, phytoplankton. That is why they are at the start of every food chain.
The points to remember
- Producers make their own carbon compounds from carbon dioxide and water, by photosynthesis.
- So they are always first in a food chain.
- Every consumer depends on them, directly (eating them) or indirectly (eating something that did).
- Without producers, no carbon compounds and no energy would enter the food chain at all.
Remember it as: No producer, no chain.
Real example: in the Southern Ocean around Antarctica, tiny phytoplankton start the chain that feeds krill, penguins, seals and whales.
Where this fits: What photosynthesis is (2.2.4) and producers as the first trophic level (2.2.5) have their own pages. Here: why producers start the chain and what comes after them.
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Producers do far more than feed herbivores. Each job below is a separate point in a 'role of producers' answer.
What producers do for an ecosystem
- They turn light energy into chemical energy by photosynthesis.
- They turn carbon dioxide and water into glucose and oxygen.
- Glucose becomes biomass: the basis of food chains, food and energy for consumers.
- They release the oxygen that most living things need.
- They take in carbon dioxide, helping keep its level in the air in balance.
- They give habitats, hold soil in place and help cycle matter.
Remember it as: Food, oxygen, a carbon store and a home.
Real example: ocean phytoplankton make roughly half of the oxygen released by photosynthesis on Earth, and they take up carbon dioxide from the air that dissolves in the sea.
A few producers do not use light: Some bacteria make carbon compounds using energy from chemical reactions instead of light: chemosynthesis. They are producers too.
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Consumers get their carbon compounds from producers or from other consumers. Each step away from the producer is a new trophic level.
The points to remember
- Primary consumers eat producers.
- Secondary consumers eat primary consumers.
- Tertiary consumers eat secondary consumers, and so on up the chain.
- So organic matter flows producer, primary, secondary, tertiary consumer: each a new trophic level.
Remember it as: Make it, eat it, eat the eater.
Real example: in Wytham Woods, Oxford, winter moth caterpillars eat young oak leaves in spring, blue tits feed the caterpillars to their chicks, and sparrowhawks hunt the blue tits.
Numbering the levels: How to number trophic levels and state the level of any organism is on the next page (2.2.11).
A common task: the paper gives a food web and asks you to construct, sketch or draw a food chain with four trophic levels. Five checks make it right.
Building a food chain from given data
- Start with a producer named in the figure.
- Each next organism must eat the one before it, as the figure shows.
- Use the species names as written: never just 'plants', 'trees' or 'fish'.
- Draw one set of arrows, each pointing to the eater: the direction energy flows.
- Use only links shown in the figure: a link you know from elsewhere does not count.
Remember it as: Producer first, arrows to the eater, names from the figure.
Two separate marks: One mark for four organisms that really form a chain in the figure; one for arrows that all point to the eater. A second set of arrows, for example for waste or heat, loses the arrow mark.
Try it: grasses, rabbits, tawny owls is only three levels. Acorns, wood mice, weasels, tawny owls is four.
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How this comes up: Paper 1 or 2: a food web from the resource booklet. Sketch a food chain with four trophic levels [2].
Figure 1 shows a simplified food web in Wytham Woods, an oak woodland near Oxford, England.
With reference to Figure 1, sketch a food chain with four trophic levels.
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