Edexcel IGCSE Biology: Double Science 复习笔记 4.2.3 Food Pyramids & Transfer of Energy

Food Pyramids

• 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

Exam Tip

Remember that pyramids of biomass are ALWAYS pyramid-shaped, so they are simple to draw, but pyramids of number can be any shape – so make sure you learn the rules for drawing a pyramid of numbers.

Transfer of Energy

• 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 (eat) 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 (eat) 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

Losses of energy

• Not all energy is transferred from one trophic level to the next
• Approximatelyonly 10% of the energy 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 energy available eventually becomes too small to support another trophic level

Energy/biomass is lost at each trophic level for several reasons

• Losses of energy 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
• Energy is used for movement
• Energy is used to generate heat
• Energy is used for metabolic processes
• Some absorbed material is lost as waste:
• Carbon dioxide and water are waste products of respiration
• Water and urea are the waste products in the urine, which is produced when proteins are broken down

Calculating the efficiency of energy and biomass transfers

• You may be asked to calculate the efficiency of energy and biomass transfers between trophic levels using percentages

Worked Example

Figure 1 shows:

• A food chain with four trophic levels
• The total biomass of the organisms at each trophic level

Calculate the efficiency of biomass transfer from the first to the