Feeding RelationshipsEdexcel IGCSE Biology: Revision notes

A trophic level describes an organism's feeding position in a food chain.

• Producers — usually green plants or algae; make their own food by photosynthesis, forming the base of every food chain
• Primary consumers — herbivores that eat producers directly
• Secondary consumers — carnivores or omnivores that eat primary consumers
• Tertiary consumers — carnivores that eat secondary consumers, often top predators
• Decomposers — bacteria and fungi that break down dead organisms and waste at every level, releasing nutrients back into the ecosystem

A food chain shows the transfer of energy from one organism to the next, always starting with a producer. Arrows point in the direction food (and energy) is transferred — for example: grass → rabbit → fox.

Real ecosystems are more complex than a single chain: most organisms eat more than one food source and are eaten by more than one predator. A food web shows all the interconnected food chains in a habitat, giving a more realistic picture of feeding relationships.

Ecological pyramids represent quantities at each trophic level, drawn as horizontal bars stacked with producers at the bottom.

• Pyramid of numbers — shows the number of individual organisms at each level; can be an unusual shape (e.g. one large tree supporting many insects gives a narrow base)
• Pyramid of biomass — shows the total mass of living material at each level; almost always a true pyramid shape, since biomass decreases up the chain
• Pyramid of energy transfer — shows the amount of energy available at each level over a given time; always a true pyramid, as energy is lost at every stage

When one organism eats another, matter (such as carbon and nitrogen compounds) and energy stored in food are transferred to the consumer. Some of this material is built into the consumer's own tissue (growth), some is used for respiration to release energy, and some is lost as waste (faeces) or is indigestible and not transferred at all.

Only around 10% of the energy available at one trophic level is transferred to the next. The rest is lost because:

• A large amount of energy is used by the organism in respiration and ultimately lost as heat to the surroundings
• Energy is used for movement and other life processes
• Not all of the organism is eaten (e.g. bones, roots left behind)
• Some material is not digested and is lost in faeces

This energy loss explains why food chains rarely have more than four or five trophic levels — there is not enough energy left to support another level.