3.5.3 Energy and ecosystems

Question 1

What is biomass?

Answer 1

The mass of living material.

Question 2

How is energy transferred through an ecosystem?

Answer 2

Energy is transferred through an ecosystem when organisms eat other organisms (a food chain).

Question 3

State the formula for net primary production.

Answer 3

net primary production = gross primary production - respiratory loss
(NPP = GPP - R)

Question 4

Briefly explain why not all the energy from one trophic level gets transferred to the next trophic level.

Answer 4

Not all of the food is eaten, so the energy it contains is not taken in.
Of the parts that are ingested:
- Some are indigestible, so are egested as faeces.
- Some energy is also lost through respiration or excretion of urine

Question 5

A farmer grows cabbages in one of his fields.
Suggest how he could estimate the chemical energy stored in the dry mass of one of his cabbages.
[3 marks]

Answer 5

• He could dry out one of his cabbages, e.g. in an oven.
• He could then burn a known mass of dry tissue in a calorimeter…
• …and use the change in water temperature to calculate the chemical energy stored in the dry biomass of the cabbage.

Question 6

Using the estimate of the chemical energy stored in the dry mass, the energy of the cabbage field was calculated as 15 600 kJ m⁻² yr⁻¹.
Does this represent the gross or net primary production? Give a reason for your answer.
[2 marks]

Answer 6

• Net primary production, because some of the chemical energy converted by the plant through photosynthesis is immediately used for respiration,…
• …so it does not get stored as the biomass of the cabbages.

Question 7

What is a food web?

Answer 7

A system showing lots of food chains in an ecosystem and how they overlap.

Question 8

What is the role of decomposers in a food web?

Answer 8

Decomposers break down dead or undigested material, allowing nutrients to be recycled.

Question 9

How does simplifying a food web involving a crop increase the net primary production of the crop?

Answer 9

By getting rid of certain food chains, energy losses will be reduced and the NPP of the crop will increase.

Question 10

How do insect pests reduce the NPP of crops?

[1 mark]

Answer 10

Insect pests eat the crop, reducing the amount of energy available for crop growth.

Question 11

Suggest two ways in which the farmer growing the crops could further reduce the percentage crop loss to insect pests.
[2 marks]

Answer 11

Any two from:

• Use an insecticide that kills multiple pests.
• Use another pesticide in conjunction with the first one.
• Use biological controls as well as chemical insecticides.

Question 12

Explain two ways in which livestock farmers can increase the net production of their animals.
[2 marks]

Answer 12

• Keep them in pens, so respiratory losses through movements are reduced.
• Keep them warm, so less energy is wasted in generating body heat.