Trophic levels and food chains (spec 4.5-4.6)
Producers β primary β secondary β tertiary consumers. Decomposers recycle.
A trophic level is a position in a food chain, defined by how the organism gets its energy.
| Trophic level | Name | Description | Example |
|---|---|---|---|
| 1 | Producer | Autotrophs β make their own food by photosynthesis | Grass, oak tree, phytoplankton |
| 2 | Primary consumer | Herbivore β eats producers | Rabbit, cow, caterpillar, zooplankton |
| 3 | Secondary consumer | Carnivore β eats primary consumers | Frog (eats insects), small fish |
| 4 | Tertiary consumer | Carnivore β eats secondary consumers | Hawk (eats frog), shark |
| Side chain | Decomposer | Bacteria + fungi β break down dead matter | Mushrooms, soil bacteria |
Producers form the BASE of every food chain. Energy enters the chain only through producers, who CAPTURE light energy from the Sun and FIX it into glucose during photosynthesis.
Food chains use ARROWS to show ENERGY FLOW:
grass β grasshopper β frog β snake β hawk
The arrows point FROM the prey TO the predator (because energy/atoms flow this way).
Food webs. Most real ecosystems are not simple chains β each organism has multiple prey and multiple predators. A food web is a network of overlapping food chains.
Example:
- grass β rabbit β fox
- grass β mouse β fox
- grass β mouse β owl
If one species is removed (e.g. rabbits die from disease), the effects ripple through the web β foxes lose food and may decline; mice may benefit from less competition; grass may grow more abundantly. Food-web questions test students' ability to TRACE these effects.
Decomposers. Bacteria and fungi that break down DEAD organisms and waste using extracellular enzymes. They recycle nutrients (carbon, nitrogen) back into the ecosystem. Some decomposers (e.g. earthworms, woodlice) are eaten by larger animals β forming a parallel detritus food chain.
- Trophic level 1 = producer (always).
- Arrows show ENERGY FLOW (prey β predator).
- Decomposers form a parallel detritus chain.
- Food WEBS show real-life complexity β many prey + many predators.