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8.6. Trophic levels

Trophic levels in an ecosystem

The trophic levels are each sets of an ecosystem organisms obtained matter and energy in the same manner, thus occupying a place equivalent in trophic chains.

The energy that living beings need to carry out vital functions (nutrition, relationship and reproduction) comes from the Sun. Plants are in charge, through photosynthesis, of transforming inorganic matter into organic matter. This matter and energy will pass to the rest of the organisms in the ecosystem, such as herbivorous and carnivorous animals. Finally, the organisms decomposers will be responsible for breaking down the organic matter into inorganic, closing the cycle so that it can restart.

Therefore, we can distinguish three trophic levels:

Niveles tróficos de un ecosistema

By Roddelgado (Own work) [CC BY-SA 4.0], via Wikimedia Commons 

Interactive activity: Trophic levels.

Primary producers

Producing organisms are autotrophs (plants, algae, and some bacteria). They are capable of making their own organic matter from inorganic matter (water, mineral salts and carbon dioxide) and the energy of the sun, that is why they are called producers.
The producing organisms constitute the lowest trophic level, being the base on which the higher levels are sustained. They are the only ones that, through photosynthesis, are capable of capturing solar energy and transforming it into chemical energy.

Consumers

The consumer organisms are heterotrophic. They manufacture their organic matter from organic matter that comes from other living beings, that is why they are called consumers. They are also producers (they manufacture their own organic matter), but they are not primary producers. In turn, consumers can also be the source of organic matter for other consumers who feed on them. According to this, several types of consuming organisms are distinguished:

  • Primary consumers. They feed directly from primary producers. In general, this level in terrestrial ecosystems includes herbivores, which eat plants or algae, and in marine ecosystems, zooplankton, which feed on phytoplankton. Parasites, mutualists  and commensals  that feed on plants are also included at this level.

This level is the second in the food chain, receiving matter and energy from the first level (primary producers) and supplying them to the third level (secondary consumers).

  • Secondary consumers. They feed on primary consumers (herbivores), which is why they are called carnivores. They constitute the third trophic level.
  • Tertiary consumers. They feed off secondary consumers. They are large predators that feed on primary (herbivores) and secondary (carnivores) consumers. They are also called supercarnivores or super predators.
  • Quaternary consumers. In some ecosystems, there are quaternary consumers that feed on tertiary consumers.

The energy that passes from one level to another has been estimated to be only 10%, so the number of trophic levels is usually not more than five. This fact is known as the 10% rule.

Remember also that consumers are also other organisms heterotrophic, such as protozoa, the animal parasites or scavengers feeding on the organic matter and prepared by the producers.

Decomposers

Decomposing organisms (mainly bacteria and fungi) are heterotrophic organisms that obtain matter and energy from the organic remains of other lower-level living beings. They are in charge of transforming organic matter (feces, corpses) into inorganic matter that can be used again by producers. That is why they are essential for the recycling of matter, closing the cycle of matter in the ecosystem.

Interactive activity: Organisms and their diet.


 

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Biology and Geology teaching materials for Compulsory Secondary Education (ESO) and Baccalaureate students.