4.2 Feeding Relationships

REVISION NOTES

IGCSE Edexcel Biology

4.2.1 Understand the names given to different trophic levels, including producers, primary, secondary and tertiary consumers and decomposers

Trophic levels:

Describes the feeding relationships between different organisms
Energy flows from one trophic level to another
Eventually all energy is transferred to the environment
Some energy is lost to the environment during transfer between trophic levels
Starts with energy flowing from the sun to the producers (first trophic level) in the form of light
Organisms can be classified at different trophic levels in the same food web
They may eat primary, secondary or tertiary consumers

4.2.2 Understand the concepts of food chains, food webs, pyramids of number, pyramids of biomass and pyramids of energy transfer

Food chain:

Food web:

Network of connected food chains in an ecosystem
Show interdependence on different species
Interdependence is when species depend on each other for resources
- Changes in the population of one species can affect the population of others
Animals generally rely on multiple food sources
E.g. if the earthworm population gets wiped out, the population of grass would increase significantly
- The population of frogs and mice would significantly decrease as they don’t have other food sources to rely on
- The population of sparrows would not be affect as much due to the presence of caterpillar
- This ensures that some of their population survives

Pyramid of numbers:

The width of the box represents the number of organisms at each trophic level
The size of the box depends on the species population and size of the organism
- One large producer can contain enough energy to support a large population of smaller primary consumers such as insects
All pyramids should follow the order of trophic levels in the food chain
- Producers at the bottom and tertiary consumers at the top
- It does not have to look like the traditional pyramid shape

Pyramid of biomass:

The width of the box represents the dry biomass (without water) of each organism
The size of the box will always be decreasing
- Biomass of organisms will always be decreasing as you go up the food chain
- Some of the food consumed at the next trophic level is excreted as waste
Better way to represent interdependence in the food chain

Pyramids of energy:

Represents the amount of energy in the biomass of organisms at each trophic level
Will always have a very wide base as producers contain the most amount of energy
As you go up the food chain, the amount of energy at each trophic level decreases
- Only around 10% of it is passed on

4.2.3 Understand the transfer of substances and energy along a food chain

Transfer of energy:

Energy from the sun is absorbed by producers converted into chemical energy through photosynthesis
The glucose produced is used for the biomass of producers
Primary consumers eat producers and break down the biomass through digestion
- Chemical energy released is used for the maintenance or increase of their own biomass
Secondary consumers eat primary consumers where the biomass is broken down through digestion
- Chemical energy released is used for the maintenance or increase of their own biomass
Chemical energy is transferred throughout the trophic levels by to consumption

4.2.4 Understand why only about 10% of energy is transferred from one trophic level to the next

Loss of energy:

Energy is passed on to different trophic levels through consumption
Not all energy absorbed or consumed is used for making biomass
Some energy is lost to the environment through:
- Metabolic waste excreted by the organism e.g. urine
- Movement requiring energy
- Heat produced to maintain body temperature
- Undigested waste products that are removed from the body e.g. faeces
Only energy converted into biomass can be passed onto the next trophic level
- Parts of an organism that is consumed will contribute to the transfer of energy
- Energy in parts of organisms not consumed will degrade
- The energy will be lost to the surroundings