Trophic Levels (DP IB Environmental Systems & Societies (ESS))

Trophic Levels & Feeding Relationships

What are trophic levels?

• The trophic level is the position that an organism occupies in a food chain, or a group of organisms in a community that occupy the same positions in a food chain

Trophic Levels

• Producers (autotrophs) are typically plants or algae and produce their own food using photosynthesis and form the first trophic level in a food chain

  • Exceptions include chemosynthetic organisms that produce food without sunlight

  • For example, bacteria living inside the giant tube worm (Riftia pachyptila), found near hydrothermal vents on the ocean floor (where there in no light), use hydrogen sulfide from the vents to produce organic compounds through chemosynthesis, providing a source of energy for the tube worm

• The chemical energy stored in producers is then transferred to primary consumers as they consume (eat) producers

• The chemical energy is then transferred from one consumer to the next as they eat one another

• Apex predators are at the very top of the food chain – they are carnivores with no predators

  • The chemical energy stored within apex predators can be passed on to decomposers when apex predators die and are decomposed

Feeding Relationships

• Feeding relationships involve producers, consumers and decomposers

• These can be modelled using food chains, food webs and ecological pyramids

Trophic levels for a simple food chain - the blue arrows show how the chemical energy originally produced by the primary producer (grass) is transferred to other organisms in the community

Trophic levels for a simple food web – note that some organisms can belong to more than one trophic level (such as the squirrel, fox and eagle in this food web)

Decomposers

• The net primary production (i.e. the chemical energy) of producers is also available to another group of organisms known as decomposers

• The two main groups of decomposers are bacteria and fungi

• Decomposers carry out a very important function in ecosystems - they break down dead plant and animal material (in the process gaining the chemical energy still stored in the dead matter)

• They do this by:

  • Secreting digestive enzymes onto the surface of the dead organism

  • These enzymes break down the dead matter into small soluble food molecules

  • These molecules are then absorbed by the decomposers

• This process of decomposition also helps to release organic nutrients back into the environment (e.g. the soil), which are essential for the growth of plants and other producers