Food chains, Food webs & Pyramids (Edexcel IGCSE Biology: Double Science)

• A simple way to illustrate the feeding interactions between the organisms in a community is with a food chain
• You need to know the terms given to each step in a food chain:

1. Producer: food chains always begin with a producer
2. Primary consumer: producers are eaten by primary consumers (herbivores/omnivores)
3. Secondary consumer: primary consumers are eaten by secondary consumers (carnivores/omnivores)
4. Tertiary consumer: secondary consumers are eaten by tertiary consumers (carnivores/omnivores)

An example of a food chain

• A food chain shows the transfer of energy from one organism to the next
• The source of all energy in a food chain is light energy from the sun
• The arrows in a food chain show the transfer of energy from one trophic level of the food chain to the next

Trophic levels for a simple food chain

Transfer of energy

• In order for the energy to be passed on, it has to be consumed (eaten)
• However not all of the energy grass plants receive goes into making new cells that can be eaten
• The same goes for the energy the grasshopper gets from the grass, and the energy the frog gets from the grasshopper
• Only the energy that is made into new cells remains with the organism to be passed on
• Even then, some of this energy does not get consumed - for example few organisms eat an entire organism, including roots of plants or bones of animals - but energy is still stored in these parts and so it does not get passed on
• The majority of the energy an organism receives gets ‘lost’ (or ‘used’) through:
  • Making waste products eg (urine) that get removed from the organism
  • Movement
  • Heat (in mammals and birds that maintain a constant body temperature)
  • Undigested waste (faeces) that is removed from the body and provides food for decomposers

• This inefficient loss of energy at each trophic level explains why food chains are rarely more than 5 organisms long

Energy is lost at each trophic level for several reasons

• A food web is a network of interconnected food chains
• Food webs are more realistic ways of showing connections between organisms within an ecosystem as animals rarely exist on just one type of food source

Food Web Definitions Table

A food web shows the interdependence of organisms

• Food webs give us a lot more information about the transfer of energy in an ecosystem
• They also show interdependence - how the change in one population can affect others within the food web
• For example, in the food web above, if the population of earthworms decreased:
  • The population of grass plants would increase as there are now fewer species feeding off them
  • The populations of frogs and mice would decrease significantly as earthworms are their only food source
  • The population of sparrows would decrease slightly as they eat earthworms but also have another food source to rely on (caterpillars)
• Most of the changes in populations of animals and plants happen as a result of human impact - either by overharvesting of food species or by the introduction of foreign species to a habitat
• Due to interdependence, these can have long-lasting knock-on effects on organisms throughout a food chain or web

• A pyramid of numbers shows how many organisms we are talking about at each level of a food chain.
• The width of the box indicates the number of organisms at that trophic level
• For example, consider the following food chain:

Sun → Grass → Vole → Owl

• Ask yourself the following questions:
  • Is it likely that there would be more voles in an area than grass plants?
  • How many voles might one barn owl need to eat per day? If it’s more than one, is it likely that there are more barn owls in an area than voles?
• So, a pyramid of numbers for this food chain would look like this:

A pyramid of numbers

• Despite the name (and the example above), a pyramid of numbers doesn’t always have to be pyramid-shaped, for example:

Pyramids of numbers are not always pyramid-shaped

• This is because the size of the organism is also important - one large organism, like the oak tree in the pyramid above, contains enough energy to support many smaller organisms (the insects)

Rules to remember when drawing a pyramid of numbers

• You cannot change the trophic level of the organisms - they must stay in the same order as in the food chain with producers on the bottom, followed by primary consumers, then secondary consumers, then tertiary consumers
• Generally, the larger an individual organism is, the fewer of them there are

Pyramids of biomass

• A pyramid of biomass shows how much mass the creatures at each level would have without including all the water that is in the organisms (their ‘dry mass’)
• Pyramids of biomass are ALWAYS pyramid-shaped, regardless of what the pyramid of numbers for that food chain looks like
• This is because the mass of organisms has to decrease as you go up a food chain – if we take our first food chain as an example, it’s impossible to have 10kg of grass-feeding 50kg of voles feeding 100kg of barn owls

A pyramid of biomass

• Pyramids of biomass provide a much better idea of the quantity of the plant or animal material at each level of a food chain and therefore are a better way of representing interdependence within the food chain

Pyramids of energy

• Pyramids of energy illustrate the amount of energy contained within the biomass of individuals within different trophic levels
• The area of each box represents the quantity of energy present
• These pyramids always have a wide base (due to the large amount of energy contained within the biomass of producers)
• As you move up the pyramid to higher trophic levels the quantity of energy decreases as not all energy is transferred to the biomass of the next trophic level (roughly only 10 % of the energy is passed on)

A pyramid of energy transfer