Trophic levels

A simple food chain is:

algae → mosquito larvae → dragon fly larvae → perch

All other food chains in an ecosystem can be added together to make a food web. These stages in a food chain or web are called trophic levels.

The arrows show the transfer of biomass from one trophic level to another.

At the bottom of all food chains is a producer. This is almost always a plant or alga which can photosynthesise to convert carbon dioxide and water into glucose. This provides all the biomass for the food chain. Algae are the producers in the food chain above.

All following trophic levels relate to consumers, which cannot make their own food.

The second trophic level in all food chains is a herbivore or omnivore called a primary consumer. Mosquito larvae are the primary consumers in the above food chain.

The third stage is a carnivore or omnivore which eats the primary consumer. This is called the secondary consumer and is dragonfly larvae in the above food chain.

There may be additional carnivorous consumers which would be called tertiary and quaternary. The final level is a carnivore often called the top or apex predator - in this example, a perch. Organisms at the top of a food chain have no predators.

Feeding relationships in ecosystems, demonstrated by a food pyramid with 4 tiers, the widest at the bottom and the narrowest at the top.  Tier 1 at bottom, is the producer, represented by plants.  Tier 2 is the primary consumer, represented by a snail. Tier 3 is the secondary consumer represented by a frog and tier 4 is the tertiary consumer represented by a fox.

Decomposers are bacteria and fungi that break down dead plant and animal matter. They secrete enzymes on the surface of the dead organisms to break the organism down and then absorb the digested, smaller food molecules.

Common words used with food chains and their meaning

ProducersGreen plants - they make glucose during photosynthesis.
Primary consumersUsually eat plant material - they are herbivores (or omnivores). For example, rabbits, caterpillars, cows and sheep.
Secondary consumersEat animal material - they are carnivores (or omnivores). For example, cats, dogs and lions.
PredatorsKill for food. They are either secondary or tertiary consumers.
PreyThe animals that predators feed on.
ScavengersFeed on dead animals. For example, crows, vultures and hyenas are scavengers.
DecomposersFeed on dead and decaying organisms, and on the undigested parts of plant and animal matter in faeces.