energy transfer between trophic levels
We’ve already mentioned that the Sun is the source of energy for all ecosystems. But, as little as one per cent of this light energy can be absorbed and made available to food chains by green plants. The organisms in this food chain in turn pass on a tiny fraction of this remaining energy to the next stages in the chain.
But why is energy lost?
Food Chains:
Plants convert between one per cent and three per cent of the light energy into organic matter. Most of the Sun’s energy is lost due to:
What is gross productivity?
So we use net productivity to measure how much energy is stored:
Net productivity = gross production – respiratory losses
Even after this, consumers only transfer a small percentage of energy from producers or prey to their bodies.
This is down to:
The diagram below shows the series of energy losses:
But why is energy lost?
Food Chains:
Plants convert between one per cent and three per cent of the light energy into organic matter. Most of the Sun’s energy is lost due to:
- 90 per cent of the Sun’s energy is reflected back into space and the atmosphere itself absorbs some.
- Some wavelengths of light can’t be absorbed and used to photosynthesise.
- Any light that is absorbed might not hit the chlorophyll molecule directly.
- Other limiting factors such as carbon dioxide or water levels may prevent all the energy being converted.
What is gross productivity?
- This is the total quantity of energy that all the plants in a community convert into chemical energy that can be passed on.
- But: Plants uses 20-50 per cent of this chemical energy to respire themselves.
So we use net productivity to measure how much energy is stored:
Net productivity = gross production – respiratory losses
Even after this, consumers only transfer a small percentage of energy from producers or prey to their bodies.
This is down to:
- Not all of the organism is eaten.
- Some bits of the organism can’t be digested and is lost as faeces.
- Some energy leaves as excretory materials e.g. urine
- Heat from respiration is given out and energy is lost.
The diagram below shows the series of energy losses:
How do we calculate how efficiently energy is transferred?
Any energy available is generally measured in KJ m2 year-1 and we calculate the efficiency of this energy being transferred with this equation:
Energy transfer = energy available after the transfer
energy available before the transfer x 100
For example: energy available before transfer = 60 KJ m2 year-1
energy available after the transfer = 300 KJ m2 year-1
60 6000
300 x 100 = 300 = 20% efficiency
Energy transfer = energy available after the transfer
energy available before the transfer x 100
For example: energy available before transfer = 60 KJ m2 year-1
energy available after the transfer = 300 KJ m2 year-1
60 6000
300 x 100 = 300 = 20% efficiency