Unit 5: The structure of the ecosystems                      1   2   3   4   5   6

4. Thopic structure of the ecosystem

All living beings need energy and matter to perform their vital functions. They obtain this energy and matter from the nutrients in food.

Organisms can be classified according to the way they obtain food. The way they do it also determines their trophic level, that is, their place in the food chain.

a) Trophic levels

A trophic level is a group of living beings in an ecosystem that obtain matter and energy in a similar way and so occupy the same place in the food chain.

We can distinguish three trophic levels in an ecosystem:

- Producers

      These are autotrophic organisms: plants, algae and some bacteria.

 They take in solar energy during photosynthesis and use it to transform

 inorganic matter (CO₂, H₂O and mineral salts) from their surroundings

 into organic matter (glucose).

- Consumers

These are heterotrophic organisms: animals, fungi and some bacteria.

They obtain matter and energy by feeding off other living things or decomposing organic matter. 

They are divided into:

- Primary consumers

They are herbivores, which feed on the producers. 

- Secondary consumers

They are carnivores, which feed on the primary consumers. 

- Tertiary consumers

They are also carnivores, which feed on the secondary consumers.

Carnivors can be:

- Predators. They hunt other animals (e.g. lions, eagles, etc)

- Scavengers. They feed on carcasses (e.g. hyenas, vultures, etc)

Some secondary and tertiary consumers are omnivores and also feed on

producers (e.g. bears, wild boars, humans, etc)

Other consumers are detritivores, such as earthworms or many insects, that feed on plant or animal organic remains that are in the soil.

- Decomposers

They are heterotrophic organisms. They are bacteria and fungi which decompose the organic remains (carcasses, excrements, etc.) of other living beings and transform them into inorganic matter useful for producers.

b) Food chains and food webs

The food chain or trophic chain is the linear graphic representation of the feeding relationships between organisms at different trophic levels in an ecosystem.

Each organism feeds on the one below it in the food chain and in turns is eaten by the organism above it. It is represented by arrows. The tip of the arrow points to the eater and the other end points to the organism that is eaten.

There are food chains in both terrestrial and aquatic ecosystems.

Usually trophic relationships are extremely complex. Normally a consumer feed on more than one species from the previous level in a food chain, and then in turns is eaten by a variety of species from a higher level. So food chains are interrelated in different ways.

A food web or  trophic web is a graphical representation of the feeding relationships between species in an ecosystem, which combines several interrelated food chains.

4.1. Which graphic representation gives us more information about

        the ecosystem,the food web or the food chain? Why?

4.2. Put the following living beings in order to represent a food chain,

        and identify the producers and the different types of consumers.

        What types of organisms are not represented?

4.3. Why cannot be considered scavengers or detritivores as decomposers?

4.4. Listen and answer. Which trophic level  is each sentence refered to?

a. Producers               b. Consumers               c. Decomposers