7.4 Trophic levels

Question 1

Tropic levels

Answer 1

Trophic levels describe the position of an organism in a food chain, web or pyramid.
Trophic levels can be represented by numbers, starting at level 1 with plants and algae. Further trophic levels are numbered subsequently according to how far the organism is along the food chain.

Question 2

Pyramid of biomass

Answer 2

Pyramids of biomass can be constructed to represent the relative amount of biomass in each level of a food chain. Trophic level 1 is at the bottom of the pyramid.
1 - producers
2 - primary consumers
3 - secondary consumers
4 - tertiary consumers
5 - quaternary consumers

Question 3

Biomass transfer

Answer 3

Energy flows from the Sun to the first trophic level (producers) in the form of light.
Producers convert light energy into chemical energy.
This occurs during photosynthesis, when producers convert carbon dioxide and water into glucose and oxygen.
Producers use this glucose (during respiration) to produce their own biomass.
Biomass is a store of chemical energy.
When primary consumers consume producers, they break down the biomass of the producer (digestion) and use the chemical energy to increase or sustain their own biomass.

When secondary consumers consume primary consumers, they break down the biomass of the primary consumer (digestion) and use the chemical energy to increase or sustain their own biomass, and so on.
In this way, as chemical energy is transferred from one trophic level to the. next, biomass is also transferred.

Question 4

Loss of biomass

Answer 4

Not all biomass is transferred from one trophic level to the next.
Approximately, only 10% of the biomass of each trophic level is passed on to the next.
This is why food chains are rarely made up of more than six trophic levels - the total amount of biomass available eventually becomes too small to support another trophic level.

Question 5

Cause for loss of biomass

Answer 5

Losses of biomass are due to:
Organisms rarely eat every part of the organism they are consuming - some of the biological material of plants and animals may be inedible (eg. many predators do not consume the bones of their prey).
Not all the ingested material is digested and absorbed, some is egested as faeces
Some absorbed material is lost as waste:
Carbon dioxide and water are waste products of respiration (when glucose is used by an organism to provide energy for moving and keeping warm, rather than to produce more biomass).
Water and urea are the waste product in urine which is produced when proteins are broken down.

Question 6

Efficiency of biomass

Answer 6

Percentage efficiency transfer = (biomass in higher tropic level / biomass in Lower tropic level) x 100