Edexcel A-Level Biology: Ecosystems

Only a small fraction of the energy entering an ecosystem at the producer level reaches the top carnivores, and food chains rarely have more than four or five trophic levels.

Describe and explain why energy transfer between trophic levels is inefficient, and explain how this limits the length of food chains. In your answer you should refer to gross and net primary productivity, the losses that occur at each transfer, and the consequence for the number of trophic levels.

(6 marks)

Ecologists studied energy flow through a grassland food chain. They estimated the energy available at each trophic level over one year, in kJ m⁻² yr⁻¹. The results are shown below.

Trophic level	Energy available / kJ m⁻² yr⁻¹
Producers (grasses)	18 000
Primary consumers (rabbits)	1 980
Secondary consumers (foxes)	178

(a) Calculate the percentage efficiency of energy transfer from producers to primary consumers. Show your working. (2 marks)

(b) The efficiency of transfer from primary consumers to secondary consumers is 9.0 per cent. Suggest and explain why this value differs from the producer-to-primary-consumer efficiency you calculated in (a). (4 marks)

A student estimated the size of a population of woodlice in a leaf-litter habitat using the mark-release-recapture method. The data collected are shown below.

Quantity	Value
Number caught and marked on the first occasion ($n_1$n1​)	60
Number caught on the second occasion ($n_2$n2​)	75
Number of marked individuals recaptured ($m$m)	9

The population was estimated using the Lincoln index:

$N = \frac{n_1 \times n_2}{m}$N=mn1​×n2​​

(a) Calculate the estimated size of the woodlouse population, $N$N. Show your working. (2 marks)

(b) State three assumptions that must be true for this estimate to be valid. (3 marks)

A volcanic eruption produced a new island of bare, nutrient-poor lava rock with no soil. Over the following decades ecologists recorded the organisms colonising it. The first colonisers were lichens growing directly on the rock. Much later, mosses, then small flowering plants, and eventually shrubs became established, and the variety of animal species also rose.

Suggest and explain how this community developed from bare lava rock into one able to support shrubs and a range of animals. (5 marks)

A farmer grows wheat in the same field year after year and finds that the yield falls and more nitrogen fertiliser is needed each season. An agronomist advises the farmer to grow a clover crop in the field every third year and to plough the clover back into the soil rather than harvest it. Clover is a legume whose roots contain nodules housing nitrogen-fixing bacteria.

Explain how this advice could reduce the farmer's need for nitrogen fertiliser. (4 marks)

Carbon is continually cycled between the atmosphere and living organisms.

Describe how carbon dioxide is removed from the atmosphere and how it is returned to the atmosphere by living organisms. In your answer, name the processes involved. (3 marks)