19.3 - Trophic Levels

Trophic levels and producers

• A trophic level is the position (feeding level) an organism occupies in a food chain.
• The levels are numbered from level 1 upwards, so a food chain reads as a sequence of trophic levels from the producer at the bottom to the top predator.
• Each step up the chain is the next number.

Level 1 - Producers

• Level 1 is always a producer.
• Producers are plants and algae. They're called producers because they make their own food by photosynthesis, using light energy from the Sun.
• This makes them the base of the whole chain:
  • They capture the energy that everything else depends on.
  • They produce the biomass (the mass of living material) that is passed along the chain to organisms above them.
• All the energy in a food chain originally enters through the producers.

The consumer levels

The levels above the producer are the consumers - animals that get their energy by eating other organisms.

• A fourth consumer can also be called a quaternary consumer.
• The pattern then keeps repeating up the chain.
• A carnivore at the very top of a chain, with no predators of its own, is an apex predator.

Organisms that don't fit the ladder

• Omnivores, such as bears, eat both producers and other animals, so they don't sit cleanly at one level. They feed at more than one trophic level: level 2 when they eat producers and level 3 when they eat primary consumers.
• Food chains rarely have more than four or five levels, because energy is lost at each transfer (only about a tenth passes on each time), so there's too little left to support organisms much higher up.

Decomposers as a trophic level

• Decomposers are a further feeding group in a food chain.
• They get their energy from dead and waste organic matter, not from living prey - the remains of dead plants and animals, plus waste such as droppings.
• The main decomposers are bacteria and fungi.
• Detritivores - Small animals such as earthworms that feed on dead material and break it into smaller pieces - are also involved.

How decomposers feed

1. Decomposers don't take food into a gut. Instead they secrete enzymes out into the dead matter around them.
2. These enzymes break large insoluble molecules down into small soluble ones.
3. The soluble food molecules then diffuse back into the decomposer, where they're used for energy and growth.
4. This is how a fungus or soil bacterium "eats" a dead leaf without ever swallowing it.

Recycling nutrients

• By breaking down dead matter, decomposers return its nutrients to the soil and environment.
• Producers can then take these nutrients up and use them again.
• This makes decomposers the link that closes the loop of a food chain, recycling the materials locked up in dead organisms back to the producers at the start.