5.3 Energy & Ecosystems (A Level only)

Only around 1% of the light energy that reaches the Earth’s atmosphere is captured by plants and made available to other organisms in the food chain. Suggest reasons for this low value.  

Fields on two separate farms, X and Y, were used to grow the same crop. The fields were divided into sections, with different masses of fertiliser containing nitrate ions added to these sections.  After two months, samples of the crop plants from each section were taken from the fields and their mass determined. The results are shown in Table 1.

Table 1

Use Table 1 to explain the differences in the amounts of crop harvested from both fields at nitrate levels of 0 to 40 kg ha^-1.

Recommend a suitable application mass per hectare of fertiliser to the farmer responsible for Field X. Give reasons for your recommendation.  

Outline an experimental method by which scientists can estimate the total amount of biomass in a crop field such as X or Y.

Artificial fertilisers are used on farms to improve yields of crops such as barley. Fertilisers replace mineral ions removed from the soil when crops are harvested. Nitrate is one of these ions.

Give three nitrogen-containing molecules that a crop synthesises using nitrate ions. 

Scientists investigated the use of artificial fertiliser on barley crops in a European country between 1960 and 1995. They calculated a Fertiliser Response Ratio (FRR) which is the number of kg of barley produced per kg of fertiliser used.  Figure 1 shows their results. (A hectare is a unit of area commonly used in agriculture)

Figure 1

Use Figure 1 to calculate the percentage change in the mass of barley produced per hectare in 1960 compared with 1995.

Use the data in Figure 1 to evaluate the use of artificial fertilisers on barley crops.

Barley is the fourth most common crop plant in the world. Its common uses are in animal fodder and for making malt, a major raw material in beer and whisky production. Much of its chemical energy (stored in its biomass) is not passed into these processes. 

Suggest two reasons why. 

Figure 1 shows a bomb calorimeter, used for measuring the energy content of biomass. A sample of dry biomass is burned in pure oxygen, and the temperature change of the surrounding water is measured.  

Figure 1

Suggest why pure oxygen is used, rather than air. 

A typical oak tree has a mass of 15,000 kg and can be assumed to be 50% dry mass.

Explain why the energy content of its biomass, measured in a calorimeter, can only be an estimate.

Table 1 shows a mathematical model used to estimate the transfer of energy through consumers in a natural grassland ecosystem.  

Table 1

Write an equation to show how Net Production (NP) is calculated from the energy in ingested food. 

NP = 

Using the data in Table 1, calculate the ratio RL:AD for mammal primary consumers and for insect primary consumers and explain the difference between the two calculated values.  

Farmers and scientists have selectively bred sheep to have a low Residual Food Intake (RFI). This is calculated as the difference between the amount of food an animal actually eats and its expected food intake based on its size and growth rate.

Explain the advantage to farmers of having sheep with a low RFI.

Bacteria found in the digestive systems of sheep break down food and produce Methane. Scientists investigated the relationship between RFI and methane production. They measured the rate of methane production of 140 sheep over a ten-day period. Some of their results are shown in Table 1.

Table 1

Use the data to identify and explain two benefits that will arise from the use of low RFI feeds. 

Other scientists investigated the release of methane from rice fields. This second group of scientists investigated the effect of adding organic material (straw) and mineral substances on the release of methane from rice fields. 

The results are shown in Table 2.

Table 2

Evaluate the claim that rice farmers should treat their fields with sulfate and refrain from using straw.  

Methane is produced by microorganisms in the soil. The scientists found that rice fields that are flooded produce larger volumes of methane than rice dry fields.

Suggest why.

Woodlice are detritivorous insects (they feed on the dead remains of plants). A biologist estimated the numbers of woodlice at two different sites in a woodland ecosystem. They also estimated the net primary production of the sites to see if this influenced the numbers of woodlice present. Their results are shown in Table 1 below.

Table 1

Although net primary production is a measure of plant biomass formed per year, it does not represent the total amount of plant biomass formed per year by photosynthesis. Explain why.

Using the information in Table 1, explain why there is a greater number of woodlice in site X than in site Y. 

One way to estimate plant yield is to measure the dry mass of the plant sample. Suggest how the biologist in part a) could determine the dry mass of plant material samples from sites X/Y and explain why the biologist shouldn’t use the fresh mass of these plant material samples to compare the plant yields of these two sites.

The grass in an ecosystem has a gross primary production of 20,000 kJ m^-2 yr^-1. The net primary production of this grass is calculated to be 12,000 kJ m^-2 yr^-1. How much energy does one square metre of this grass lose as heat from respiration in one year?

Figure 1 shows the net production of different trophic levels in a food chain.

Figure 1

State the equation for the net production of consumers.

The deer in Figure 1 ingests 17,874 kJ m^-2 yr^-1 of food. Calculate the total energy loss of the deer.

In Figure 1, the net production of the deer is less than the net primary production of the grass. Explain why.

The food chain in Figure 1 has three trophic levels. Explain why it is rare to find natural food web with more than five trophic levels.

The total dry biomass of a 1 m² patch of gorse plants growing in an area of moorland in Devon was measured each year for 20 years. The graph in Figure 1 shows the results of this study.

Figure 1

Using the construction lines on the graph in Figure 1, calculate the mean yearly increase in biomass of the gorse plants in g yr^-1.

The graph in Figure 1 suggests that the gorse did not increase in biomass by the same amount each year. Giving examples, suggest what factors might cause this.

A group of students estimated that the total light energy falling on the 1 m² patch of gorse in Figure 1 was 2,563,000 kJ yr^-1. Using a calorimeter, they also found the chemical potential energy in 1 g of the gorse to be 14.8 kJ. 

Use this information to calculate the efficiency of photosynthesis in the gorse plants in this 1 m² patch.

Suggest why the efficiency of photosynthesis in the gorse plants is relatively low.

The graph in Figure 1 shows the percentage of wheat crop lost to two insect pests months before and 6 months after a chemical pesticide was sprayed on the crop.

Figure 1

How might the Rootworm beetle and the Weevil reduce the net primary production of the wheat crop in Figure 1?

What can you conclude about the effectiveness of the chemical pesticide sprayed on the wheat crop in Figure 1?

Suggest how the owner of the wheat crop in Figure 1 could further reduce the percentage losses caused to their crop by insect pests.

The owner of the wheat crop in Figure 1 decides to use biological control against the insect pest. They introduce a species of toad. By eating the insect pests, the toads ingest 10,000 kJ m^-2yr^-1 of energy but lose 2,000 kJ m^-2yr^-1 of this energy in faeces and urine. They lose a further 7,000 kJ m^-2y^r-1 using energy for respiration.

Using the information above, calculate the net production of the toads and the percentage efficiency of energy transfer from the insects to the toads. Include the correct units in your answers.