Trophic levels in an ecosystem

Energy and biomass pass along a food chain through a series of trophic levels, from producers up to top predators.

• •Producers (green plants and algae) make their own food using light energy from photosynthesis — they form trophic level 1.
• •Primary consumers (trophic level 2) eat producers; they are often herbivores.
• •Secondary consumers (trophic level 3) eat primary consumers, and tertiary consumers (trophic level 4) eat secondary consumers; these are often carnivores or omnivores.
• •Predators that have no natural predators of their own are called apex predators.
• •Decomposers (e.g. bacteria and fungi) are not part of the numbered trophic levels — they break down dead plant and animal matter and waste, releasing nutrients back into the ecosystem.

Biomass and energy decrease at each successive trophic level, which is why food chains rarely have more than four or five levels.

• •A pyramid of biomass is always a true pyramid shape (each bar shorter than the one below), unlike a pyramid of numbers, which can be an unusual shape — e.g. one large tree (producer) supporting many smaller insects (primary consumers).
• •Only around 10% of the biomass at each trophic level is transferred to the next — most is lost through respiration (as carbon dioxide and heat), egestion (undigested material lost as faeces), excretion (e.g. urea in urine), and because some body parts (e.g. bones, roots) are not eaten.
• •Efficiency of biomass transfer = (biomass transferred to the next level ÷ biomass available at the previous level) × 100.