EcosystemsEdexcel A-Level Biology B: Topic test

20 questions, 54 marks

Edexcel A-Level Biology B

Ecosystems topic test

Total 54 marks

Name

Class

Date

1
Marine ecologists sampled the English Channel in spring. At one moment, the dry biomass of phytoplankton was 4 g m⁻², whereas the dry biomass of the zooplankton that feed on them was 21 g m⁻².
(a)
Which statement best explains why the pyramid of biomass for this sample is inverted?
[1 mark]
AZooplankton photosynthesise, so they can accumulate more biomass than the phytoplankton
BPhytoplankton reproduce rapidly and are eaten as fast as they grow, so a small biomass at one moment supports a larger biomass of zooplankton
CEnergy transfer from phytoplankton to zooplankton is more than 100% efficient
DZooplankton have a lower respiration rate than phytoplankton, so they store more energy
(b)
Why do ecologists measure dry mass rather than fresh mass when constructing a pyramid of biomass?
[1 mark]
AWater has a high energy content, so fresh mass exaggerates the energy available
BDry mass can be measured without killing any organisms
CFresh mass cannot be measured accurately for small organisms
DThe water content varies between organisms and stores no chemical energy, so fresh mass misrepresents the biomass available
(c)
A pyramid of energy for the same community can never be inverted. Explain why.
[2 marks]
Total for question 1: 4 marks
2
A student investigates whether the abundance of dandelions differs between a regularly mown lawn and an unmown meadow of similar size. She places 0.25 m² quadrats at 20 random coordinates in each area and counts the dandelions in each quadrat.
(a)
Why does she use random coordinates to place the quadrats?
[1 mark]
ATo avoid bias in where the quadrats are placed
BTo make sure that every quadrat contains at least one dandelion
CTo show how abundance changes along an environmental gradient
DTo ensure that the distribution of dandelions is uniform
(b)
Which statistical test should she use to find out whether the mean number of dandelions per quadrat differs significantly between the two areas?
[1 mark]
ASpearman rank correlation coefficient
BA mark-release-recapture calculation
CA t-test
DA percentage cover calculation
(c)
Explain why she takes 20 quadrats in each area rather than 3.
[2 marks]
Total for question 2: 4 marks
3
In a salt marsh, the gross primary productivity (GPP) of the vegetation is 12 500 kJ m⁻² year⁻¹ and the plants lose 4 750 kJ m⁻² year⁻¹ in respiration. The energy stored in the biomass of the primary consumers (insects) is 840 kJ m⁻² year⁻¹ and in the secondary consumers (wading birds) is 63 kJ m⁻² year⁻¹.
(a)
Calculate the percentage efficiency of energy transfer from the plants to the insects. Give your answer to three significant figures.
[3 marks]
(b)
Calculate the percentage efficiency of energy transfer from the insects to the wading birds, and explain how energy is lost between these two trophic levels.
[4 marks]
Total for question 3: 7 marks
4
Tropical rainforest soils are low in nutrients, yet the forest has a very high net primary productivity. Almost all of the nutrients are held in the living plants and the leaf litter. When an area is cleared and burned to grow crops, yields are good for about three years and then fall sharply.
(a)
Explain the role of microorganisms in recycling nutrients in an ecosystem such as the rainforest.
[6 marks]
(b)
Explain why crop yields fall sharply after about three years on the cleared rainforest soil.
[6 marks]
Total for question 4: 12 marks
5
In a laboratory, two species of Paramecium, P. aurelia and P. caudatum, are grown separately and together in cultures with a constant daily supply of bacteria as food. Grown separately, each species reaches a stable population size. When grown together, the population of P. aurelia rises while that of P. caudatum falls, and P. caudatum has disappeared after about 16 days.
(a)
Which term describes the interaction between the two species in the mixed culture?
[1 mark]
APredation
BMutualism
CInterspecific competition
DIntraspecific competition
(b)
Which factor limits the population size of each species when it is grown separately?
[1 mark]
AIntraspecific competition for the limited supply of food
BInterspecific competition from the other Paramecium species
CSeasonal changes in temperature
DPredation by P. aurelia
(c)
Explain why P. caudatum disappears from the mixed culture.
[2 marks]
Total for question 5: 4 marks
6
A research team studying plankton in the North Atlantic reports that surface water warmed over 30 years and that, over the same period, the abundance of cold-water plankton declined. They present their findings at an international conference and then submit them to a scientific journal.
(a)
What is the main purpose of peer review before the paper is published?
[1 mark]
ATo prove that the conclusions are certain
BTo make the findings available to the general public
CTo allow the team to repeat the experiments
DTo have independent experts check the validity of the methods, data and conclusions
(b)
What is the main benefit of presenting the results at a scientific conference?
[1 mark]
AThe results are accepted as fact by other scientists
BOther scientists can question the evidence and suggest improvements, which helps to evaluate the findings
CThe journal is then obliged to publish the paper
DGovernments are legally required to act on the results
(c)
The team found a correlation between the rise in water temperature and the decline in cold-water plankton. Explain why this does not prove that warming caused the decline.
[2 marks]
Total for question 6: 4 marks
7
Atlantic herring stocks are managed by a fisheries agency. It sets a total allowable catch (a quota) each year based on scientific estimates of stock size, sets a minimum mesh size for nets, and closes some areas during the spawning season. Fishing communities say that the limits reduce their income.
(a)
Explain how a minimum mesh size and a closed spawning season help to conserve herring stocks.
[3 marks]
(b)
Explain how a quota can help to resolve the conflict between human needs and conservation, and suggest one problem with using quotas.
[4 marks]
Total for question 7: 7 marks
8
Ecologists studied primary succession on bare rock exposed as a glacier retreated in Alaska. They sampled sites that had been ice-free for 5, 20, 45, 80, 120 and 200 years. Pioneer mosses and lichens were found at the youngest sites, alder shrubs at the 45-year sites and spruce forest at the oldest sites. Soil depth and soil nitrate concentration both increased with time since the ice retreated.
(a)
Explain how and why the community changes during this succession.
[6 marks]
(b)
The ecologists want to test for a correlation between the age of a site and the number of plant species found. Evaluate their study design and describe how they should analyse the data.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).