AQA A-Level Biology: Ecosystems and Sustainability

An area of bare rock is exposed after a volcanic eruption. Over several hundred years it develops into a stable woodland.

Describe and explain the process of primary succession that leads from the bare rock to the climax community.

The table shows energy data for the producers and the primary consumers in a grassland ecosystem, measured over one year.

Measurement	Energy / kJ m⁻² yr⁻¹
Net primary production (energy in producers available to primary consumers)	50000
Energy ingested by primary consumers	12000
Energy lost by primary consumers in faeces	4000
Energy lost by primary consumers in respiration	4500

(a) Using the data, calculate the percentage efficiency of energy transfer from the producers to the primary consumers. Use the equation below and show your working. (3 marks)

$\text{efficiency} = \frac{\text{energy in next trophic level}}{\text{energy in previous trophic level}} \times 100$efficiency=energy in previous trophic levelenergy in next trophic level​×100

(b) Explain why so much of the energy is not passed on to the primary consumers. (3 marks)

A student estimated the size of a population of woodlice in a leaf-litter habitat using the mark-release-recapture technique.

Stage of investigation	Number of woodlice
Caught, marked and released in the first sample	80
Caught in the second sample	100
Marked individuals found in the second sample	16

(a) Use the Lincoln index below to estimate the size of the woodlouse population. Show your working. (3 marks)

$N = \frac{n_1 \times n_2}{n_3}$N=n3​n1​×n2​​

where $n_1$n1​ = number marked in the first sample, $n_2$n2​ = total caught in the second sample and $n_3$n3​ = number of marked individuals in the second sample.

(b) State two assumptions that must be true for this estimate to be valid. (2 marks)

A farmer applies a large amount of nitrate-rich fertiliser to fields next to a freshwater lake. After heavy rain, some of the fertiliser is washed off the fields and drains into the lake. Over the following weeks, large numbers of fish in the lake die.

Explain the sequence of events that leads to the death of the fish.

A conservation trust manages part of a deciduous woodland by regularly coppicing (cutting some trees back to ground level on a rotation). An adjacent area of the same woodland is left unmanaged. The table shows the mean number of plant species recorded in each area.

Management of woodland area	Mean number of plant species per 100 m²
Regular coppicing	34
No management	11

Suggest and explain which management option better conserves biodiversity in this woodland.

Plants cannot use nitrogen gas from the air directly, yet they require nitrogen to make proteins and nucleic acids.

Explain how nitrogen-fixing bacteria and nitrifying bacteria together make nitrogen available to plants in a usable form.