Food chains and food webs
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
Food Chains
A food chain shows the transfer of energy from one organism to the next, beginning with a producer.
Worked Example: Constructing and Interpreting a Food Chain
- Question: a hawk eats a snake, which had eaten a mouse, which had eaten grass seeds. Construct a food chain for this information, and state which organism is the producer.
- Answer: grass → mouse → snake → hawk. The arrows point in the direction of energy transfer, from the organism being eaten to the organism eating it. Grass is the producer, as it is the only organism in the chain that makes its own food
Food Webs
A food web is a network of interconnected food chains, showing the feeding relationships between all the organisms in a community.
A food web links together several food chains that share the same organisms
Producers, Consumers, Herbivores and Carnivores
- A producer is an organism that makes its own organic nutrients, usually using energy from sunlight, through photosynthesis
- A consumer is an organism that gets its energy by feeding on other organisms
- A herbivore is an animal that gets its energy by eating plants
- A carnivore is an animal that gets its energy by eating other animals
- A decomposer is an organism that gets its energy from dead or waste organic material
Trophic Levels
A trophic level is the position of an organism in a food chain, food web or ecological pyramid. Consumers are classed according to their position in a food chain:
| Trophic level | Position in the food chain |
|---|---|
| Producer | Makes its own food - the first trophic level |
| Primary consumer | Eats the producer |
| Secondary consumer | Eats the primary consumer |
| Tertiary consumer | Eats the secondary consumer |
Human Impact on Food Chains and Food Webs
Food chains and food webs can be used to describe the impact humans have on a habitat:
- Overharvesting of a food species - e.g. overfishing of cod removes very large numbers of cod from a marine food web. This reduces food for cod's predators, whose numbers may then fall, while the small fish that cod normally prey on may increase, disrupting the rest of the web
- Introducing a foreign (non-native) species - e.g. introducing the cane toad to Australia. With no natural predators in its new habitat, the population grew rapidly; it also preyed on and poisoned native animals that had no evolved defence against its toxin, disrupting existing food chains
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