Trophic levels of food chains

Please get in touch with your teacher or tutor in case you have a question related to this lesson

Slide 1

In this presentation, we’re going to talk some more about ecosystems and food chains. In particular, we’ll be interested in the feeding or trophic levels of food chains.
As we move up a food chain, organisms at higher feeding or trophic levels consume the organisms at lower feeding or trophic levels. Energy flows from the lower levels of the food chain to the higher levels of the food chain.


Slide 2

Food chains describe the feeding relationships between organisms in an ecosystem. For example, in the food chain on the right, the zooplankton at the bottom is eaten by the phytoplankton, which is eaten by crustaceans and small fish, which are eaten by the tuna, which are eaten by the dolphins. Finally, the dolphin is eaten by the shark.

Each of these organisms occupies a different feeding level in the food chain. We call these feeding levels trophic levels.


Slide 3

A trophic level or feeding level corresponds to a group of organisms which occupy the same feeding level in a food chain. For example, in the food chain on the right, the crustaceans and small fish occupy the same trophic level. Each arrow in the food chain corresponds to a transfer of energy from one trophic level to the next. Each trophic level has a different relationship with the primary energy source for the food chain. For most ecosystems, the primary energy source is the Sun. It provides the energy which the plants and phytoplankton turn into sugars to kick off the food chain. Deep sea food chains are an exception to this.


Slide 4

Most food chains include four main trophic levels. These are the producers, which are usually plants or phytoplankton. The producers are eaten by primary consumers. These are usually herbivorous animals.

The primary consumers are eaten by secondary consumers, which may be carnivorous or herbivorous animals. Next, the tertiary consumers eat the secondary consumers. Energy is transferred from the bottom of the food chain to the top. Some food chains include a fourth, or even more rarely, a fifth consumer level.


Slide 5

The first trophic level of most food chains consists of producers, which are plants or phytoplankton. These are organisms that are capable of producing their own food using the solar radiation from the Sun. This takes the form of sugars, which are produced in a chemical reaction called photosynthesis. Most terrestrial food chains have plants as their first trophic level. Most aquatic food chains begin with phytoplankton or seaweed.


Slide 6

The second trophic level of any food chain is made up of herbivorous animals. These organisms get their energy by eating the producers. We call them primary consumers. Some examples of primary consumers are grasshoppers, snails, giraffes, elephants and zooplankton.


Slide 7

Carnivores and omnivores make up the third trophic or feeding level. They eat the herbivores. Because they eat the primary consumers, we call them secondary consumers. Some examples of secondary consumers are rats and crustaceans.


Slide 8

Carnivores and omnivores also make up the fourth trophic level. Because they eat the secondary consumers, we call them tertiary consumers. Some examples of tertiary consumers might be hawks, large fish, owls, and dolphins, depending on the food chain.


Slide 9

At the fifth trophic level, we have the apex predators. These are animals that have no natural predators, and so they lie at the top of their food chains. Some examples are tigers, great white sharks, lions and humans.


Slide 10

The organisms at the same trophic level in a food chain feed on the organisms in the level below it. The apex predators at the fifth trophic level feed on the tertiary consumers. They, in turn, feed on the secondary consumers. The secondary consumers eat the primary consumers, and the primary consumers eat the producers. In a terrestrial food chain, we might have a tiger at the fifth trophic level, a hawk at the fourth trophic level, a rat at the third trophic level, a grasshopper at the second trophic level and plants at the first trophic level. In an aquatic food chain, sharks might be the apex predators, tuna the tertiary consumers, crustaceans the secondary consumers, zooplankton the primary consumers and phytoplankton the producers. Each of these food chains will have sunlight as its primary energy source.


Slide 11

No food chain could exist without the organisms at its first trophic level. These producers bring the energy of the Sun into the ecosystem so that it can be transferred to the other trophic levels. As the organisms of each level of the food chain are eaten by the organisms at the level above them, the Sun’s energy is transferred throughout the food chain.


Slide 12

As we travel up a food chain from producer to apex predator, the number of organisms at each level decreases. We can illustrate this using a trophic pyramid. Each level of the trophic pyramid represents the number of organisms, biomass of the organisms, or bioproductivity of the organisms at the corresponding trophic level. The base layer of the trophic pyramid represents the producers of the ecosystem as these have the largest biomass. Higher layers of the pyramid represent successively higher trophic levels until the highest trophic level (or the apex predators) is represented by the top layer of the pyramid.