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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Trophic Levels
Every organism in an ecosystem needs energy to survive. This energy flows through an ecosystem in one direction, starting with the sun and moving through feeding relationships. Trophic levels help us understand how this energy moves through an ecosystem and how organisms are connected through what they eat.
Key Definitions:
Producers form the first trophic level. These are organisms that make their own food using energy from the sun through photosynthesis. They convert light energy into chemical energy stored in glucose. Examples include green plants, algae and some bacteria.
Primary consumers form the second trophic level. These are herbivores that eat producers. They get their energy by consuming plant material. Examples include rabbits, caterpillars, cows and many insects.
Secondary consumers form the third trophic level. These are carnivores that eat primary consumers. They get their energy by eating herbivores. Examples include foxes, frogs and small birds.
Tertiary consumers form the fourth trophic level. These are carnivores that eat secondary consumers. They get their energy by eating other carnivores. Examples include eagles, sharks and wolves.
The top predators in a food chain are called apex predators. They have no natural predators themselves. Some food chains may have quaternary consumers (fifth trophic level), but this is less common. Examples include polar bears, killer whales and large sharks.
Humans can occupy different trophic levels depending on their diet! A vegetarian would be a primary consumer (trophic level 2), while someone eating chicken would be a secondary consumer (trophic level 3) and someone eating a predatory fish like tuna could be a tertiary consumer (trophic level 4).
As energy moves from one trophic level to the next, a lot of it is lost. In fact, only about 10% of the energy from one trophic level is transferred to the next. This is known as the 10% rule.
Energy is lost between trophic levels for several reasons:
If we start with 10,000 kJ of energy at the producer level:
This is why food chains rarely have more than 4-5 trophic levels - there simply isn't enough energy left to support another level!
Ecological pyramids are visual representations of the relationships between different trophic levels. There are three main types:
Shows the number of individual organisms at each trophic level. Not always pyramid-shaped! For example, one tree (producer) can support many insects (primary consumers).
Shows the total dry mass of organisms at each trophic level. Usually pyramid-shaped on land, but can be inverted in aquatic ecosystems where producers (phytoplankton) reproduce and are consumed rapidly.
Shows the amount of energy at each trophic level. Always pyramid-shaped because energy is always lost between levels according to the 10% rule.
Let's look at a simple grassland ecosystem:
In a pyramid of numbers, we might have thousands of grass plants, hundreds of grasshoppers, dozens of frogs and just a few snakes.
In a pyramid of biomass, the total dry mass of all the grass would be greater than the total dry mass of all grasshoppers, which would be greater than the total dry mass of all frogs and so on.
In a pyramid of energy, if the grass captured 10,000 kJ of energy from the sun, the grasshoppers would receive about 1,000 kJ, the frogs about 100 kJ and the snakes about 10 kJ.
In marine ecosystems, the pyramid of biomass can sometimes appear inverted. This is because the producers (phytoplankton) are tiny but reproduce extremely quickly. They're eaten almost as fast as they grow by primary consumers like zooplankton. The zooplankton live longer and thus accumulate more biomass than the phytoplankton at any given time. However, the pyramid of energy is still upright because energy is always lost between trophic levels.
The efficiency of energy transfer between trophic levels affects how many organisms can be supported at each level. This has important implications for human food production and sustainability.
Because of the 10% rule, it's more efficient for humans to eat at lower trophic levels. For example:
Understanding trophic levels helps us understand:
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