7.4 Trophic Levels in an Ecosystem

A petfood company is developing a new brand of cat food that does not contain meat from fish, chicken or traditional 'livestock' animals like sheep, pigs and cows.

Instead, the cat food is made from insects. These insects are bred in the cat food factory and fed on waste vegetable matter. 

Sketch a pyramid of biomass for the food chain that produces cat food in this manner. 

Use your knowledge of trophic levels and pyramids of biomass to suggest two reasons why a company would wish to make pet food from insects. 

Other sources of food for the insects described in Question 2a) include:

• Spent grains eg. from beer brewing process
• By-products from the manufacture of food for human consumption eg.
  • Palm kernels from palm oil production
  • Offal and animal products

State two environmental benefits of using these food sources for farming insects for pet food manufacture. 

A pyramid of biomass for a natural food chain would differ in appearance from a pyramid of biomass for a man-made process such as pet food manufacture. The two pyramids might look like those sketched A and B in Figure 1 below. 

Figure 1

Deduce whether sketch A or B is the pyramid for a natural food chain.

Explain your choice. 

Figure 2 shows a food chain for organisms in an area of grassland. Also included is the biomass of the organisms at each trophic level.

Figure 2

Draw a pyramid of biomass for the food chain in Figure 2 on Figure 3.

You should:

• use a suitable scale
• label the x-axis
• label each trophic level

Figure 3

Biomass is a measure of the mass of a number of organisms, so a scientist might be tempted to measure biomass in the units of mass, kilograms (kg). However, the unit used in the food chain shown in Figure 2 is kg/m².

Explain why. 

Some units of biomass are expressed with a consideration of time.

For example, one such unit is kg m^-2 yr^-1, which stands for kilograms per square metre per year. 

Suggest a reason why a time consideration is given to the measurement of biomass. 

Calculate the percentage efficiency of the food chain shown in Figure 2.

Give your answer to 2 significant figures. 

Explain why a pyramid of biomass often only has a narrow line to depict the apex predator, rather than rectangular box.

Figure 4 below shows a food chain from the Weddell Sea, an area of the Southern Ocean off the coast of Antarctica. 

Figure 4

Human fishing activity has removed large amounts of sea bass, which is a popular food in many countries. 

There is some debate about the effect this would have on the numbers of Antarctic squid and icefish in the Weddell Sea. Some scientists believe that the numbers of squid and icefish will decrease, whilst others claim that squid and icefish numbers will not be affected. 

Explain both sides of this debate.

(i) Why numbers may decrease

[1 mark]

(ii) Why numbers might stay the same.

[2 marks]

Part of the Weddell Sea food chain from Figure 4 in Question 1a) is shown below in Figure 5, which includes data on the biomass of the organisms that form part of that food chain. 

Figure 5

Draw and label a pyramid of biomass for this food chain. 

Using the data from Figure 5, complete Table 2 below to show the percentage efficiency of each biomass transfer of the Weddell Sea.

Table 2

Biomass Transfer	Efficiency (%)
Phytoplankton to krill
Krill to sea bass
Sea bass to Weddell seal

Explain, as fully as you can, why the conversion of shrimp biomass into sea bass biomass is more efficient than that of sea bass biomass into Weddell seal biomass in the cold Weddell Sea. 

The flow of biomass through part of a food chain is shown in Figure 6 below. The diagram is drawn to scale. 

Figure 6

Use the information in Figure 6 to calculate:

(i) The amount of biomass transferred to the primary consumers;

[1 mark]

(ii) The percentage of the biomass originally gained during photosynthesis that was finally transferred to secondary consumers. 

[2 marks]

For the food chain such as the one shown in Figure 6 in Question 5a), describe the role of decomposers in the transfer of biomass.

Use your knowledge to give an example of decomposition and biomass transfer in a named ecosystem. 

Figure 1 shows a food chain in an ecosystem that has five different organisms occupying five trophic levels, A, B, C, D and E.

Figure 1

Name the type of organism not shown in the food chain in Figure 1 and explain the role it plays in an ecosystem.

Table 1 below shows the energy transferred to the next trophic level by each organism in one year.

Table 1

Organism	Energy transferred in kJ per year
A	950 000
B	136 000
C	25 000
D	2 125

Explain why organism D transfers less energy to the next trophic level than organism A.

Explain why food chains rarely have more than five trophic levels.

Figure 1 shows the flow of energy through a food chain in a habitat over the course of one year. The figures are in kJ m^–2.

Figure 1

Calculate the percentage of the energy in sunlight that was transferred into energy in the mayfly larvae

Show your working.

Give your answer in standard form.

The percentage of the sun’s energy that is transferred to photosynthetic plants is usually very low.

Suggest three reasons why.

Compare the amount of energy transferred to the trout with the amount of energy transferred to humans.

Suggest explanations for the difference in the amount of energy transferred to the two different consumers.

Two food chains are shown below:

Food chain 1: Soya bean plants → Salmon  → Human

Food chain 2: Soya bean plants → Human

Table 1 shows some data about the energy transferred between organisms in the food chains.

Table 1

Food	Consumer	Percentage of available energy transferred as useful energy
Soya bean plant	Human	5%
Soya bean plant	Salmon	15%
Salmon	Human	10%

Using the information provided, compare the efficiency of these two food chains.

Figure 1 below shows a pyramid of numbers and a pyramid of biomass for an area of woodland.

Figure 1

Explain why the level labelled X is a different width in the two pyramids

Figure 2 shows a food chain in a woodland area.

Figure 2

Energy from the sun is captured by the plants in the wood.

Describe and explain the route of this energy through the food chain after it has been captured by the plants in the wood.

Table 1 contains information about two farms. Both farms are of similar size and have similar conditions.

Table 1

The data in Table 1 show that farms like Haverstock Farm can be more than 12 times as efficient at meeting the food energy requirements for our growing human population than farms which grow feed for livestock.

Explain why Haverstock Farm is much more efficient at meeting human food energy requirements

Decomposers, including fungi, bacteria and arthropods are of fundamental importance in agriculture.

Explain why decomposers are so important to famers.

Describe the mechanism by which fungi obtain nutrients from dead organic matter.