Question 1
A grassland is strongly affected by a seasonal fire, but its plant community and productivity return close to their previous state within a few months. What does this show about the grassland?
A.
Low resistance and low resilience
B.
High resistance and high resilience
C.
High resistance and low resilience
D.
Low resistance and high resilience
Question 2
Why are sealed glass vessels useful as mesocosms for investigating ecosystem stability?
A.
They remove natural processes so that only artificial processes occur
B.
They prevent energy transfer while allowing matter exchange with the air
C.
They eliminate decomposers so that nutrients cannot be recycled
D.
They prevent matter exchange while allowing energy transfer as light and heat
Question 3
The area of a forest decreased from to . What is the percentage change from the original area?
A.
B.
C.
D.
Question 4
What positive feedback could cause large areas of Amazon rainforest to shift towards a drier savanna-like ecosystem after deforestation?
A.
Less forest reduces transpiration, lowering rainfall and increasing further forest loss
B.
Less forest reduces photosynthesis, raising rainfall and increasing further forest loss
C.
Less forest increases transpiration, raising rainfall and increasing further forest loss
D.
Less forest increases nutrient recycling, lowering rainfall and increasing further forest loss
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Question 5
A predator species with low biomass is removed from a rocky shore. A mussel species then spreads over most of the rock surface and many algae and invertebrate species disappear. What role did the predator have in the ecosystem?
A.
A decomposer recycling mineral nutrients
B.
A keystone species maintaining community structure
C.
An invasive species reducing genetic diversity
D.
A producer maintaining primary production
Question 6
A new burrowing mammal becomes established in a grassland and changes soil structure, seedling survival and plant competition. What type of trigger for succession is described?
A.
A primary trigger because succession always begins without developed soil
B.
A climatic trigger because all succession is caused by rainfall change
C.
A biotic trigger because a living organism altered the community and environment
D.
An abiotic trigger because soil minerals are not living components
Question 7
What sequence is most typical of terrestrial primary succession on bare rock after glacier retreat?
A.
Herbs and grasses, then lichens and mosses, then trees, then shrubs
B.
Trees, then shrubs, then herbs and grasses, then lichens and mosses
C.
Lichens and mosses, then herbs and grasses, then shrubs, then trees
D.
Shrubs, then trees, then lichens and mosses, then herbs and grasses
Question 8
What is meant by cyclical succession in an ecosystem?
A.
A change that can occur only after lava forms new bare rock
B.
A random change in species composition with no repeated pattern
C.
A repeated sequence of community states rather than one permanent climax community
D.
A stable community in which no species composition ever changes
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Question 9
A grassland and an old-growth forest are both described as stable ecosystems, but they respond differently to disturbance.
Distinguish between resistance and resilience in ecosystems.
[2]
Suggest why a grassland that burns frequently may still be described as stable.
[1]
Question 10
Fertilizer runoff enters a lake and causes an algal bloom. What change is most likely after large numbers of algae die?
A.
Biochemical oxygen demand increases and dissolved oxygen decreases
B.
Biochemical oxygen demand decreases and dissolved oxygen decreases
C.
Biochemical oxygen demand decreases and dissolved oxygen increases
D.
Biochemical oxygen demand increases and dissolved oxygen increases
Question 11
Sites on a glacier foreland are compared from recently exposed ground to ground exposed for several centuries. What trend is expected during primary succession?
A.
Soil depth decreases as organic matter accumulates in the community
B.
Plant size, primary production and nutrient cycling generally increase
C.
Plant size increases but primary production and nutrient cycling generally decrease
D.
Species diversity decreases as food webs become more complex
Question 12
An area with a cool wet climate and suitable soil would be expected to develop into woodland, but it remains grassland while sheep grazing continues. What explains this?
A.
Cyclical succession because grazing always creates a recurring natural cycle
B.
Climax succession because grazing proves grassland is the expected endpoint
C.
Primary succession because grazing creates a surface with no soil
D.
Arrested succession because grazing removes tree and shrub seedlings
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Question 13
A wetland is drained for agriculture. The water table falls, oxygen enters the soil, peat decomposition increases and plants adapted to drier soils become established. What is the effect on succession?
A.
Primary succession begins because drainage increases existing soil depth
B.
Succession is deflected away from the wetland climax community
C.
Succession returns immediately to the wetland climax community
D.
Cyclical succession begins because oxygen prevents all decomposition
Question 14
A small island forest has persisted for many centuries. Recently, severe storms have removed topsoil from exposed slopes and a disease has infected many trees of one species.
State two requirements for long-term stability in ecosystems.
[2]
Outline how the loss of topsoil could reduce the stability of the forest.
[2]
Question 15
Remote-sensing data were used to estimate the area of Amazon rainforest remaining in a region at two times.
Time | Estimated rainforest area / km |
|---|---|
Initial | 4.10 |
Later | 3.28 |
Calculate the percentage change in rainforest area from the initial area.
[2]
Explain why deforestation could act as a positive feedback in Amazon rainforest stability.
[1]
Question 16
A student sets up sealed glass aquatic mesocosms containing pond water, algae, small invertebrates and sediment. The mesocosms are kept near a window but not in direct sunlight.
Explain why a sealed glass vessel is useful for modelling ecosystem stability.
[2]
Suggest one abiotic variable that could be changed to investigate its effect on stability in these mesocosms.
[1]
State one ethical consideration when maintaining the mesocosms.
[1]
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Question 17
A glacier retreats and exposes bare mineral ground. Over time, pioneer organisms colonise the area and the physical environment changes.
Define ecological succession.
[1]
Explain how an abiotic change can trigger succession in this example.
[2]
State one biotic factor that could also trigger succession in an ecosystem.
[1]
Question 18
A grassland and an old forest were monitored for two years after separate disturbances. Biomass is shown as a percentage of the biomass measured before the disturbance.
Identify which ecosystem showed greater resistance to disturbance. Give a reason using the data.
[1]
Calculate the mean monthly recovery rate of the grassland biomass between its minimum value and 24 months after disturbance.
[1]
Distinguish resistance from resilience using evidence from the graph.
[2]
Question 19
Students set up sealed aquatic mesocosms using pond water, algae, pond snails and sediment. The vessels were exposed to different light intensities for 30 days. Dissolved oxygen, pH and snail survival were recorded.
Mesocosm | DO day 0 / mg dm | DO day 30 / mg dm | pH day 0 | pH day 30 | Snails alive / 10 |
|---|---|---|---|---|---|
Low light | 8.0 | 2.8 | 7.3 | 6.5 | 4 |
Medium light | 8.0 | 7.7 | 7.3 | 7.2 | 9 |
High light | 8.0 | 10.1 | 7.3 | 8.0 | 8 |
Identify which mesocosm was most stable over the 30 days. Give one reason.
[1]
Explain why a sealed glass vessel can be used as a model of nutrient cycling in an ecosystem.
[2]
Suggest one ethical reason for not adding more snails to each sealed mesocosm.
[1]
Question 20
On a rocky shore, a predatory sea star feeds on mussels. When the sea star is removed experimentally, mussels spread over much of the rock surface and several other invertebrate and algal species decline.
Define a keystone species.
[1]
Explain why removal of the sea star reduces species diversity on the rocky shore.
[2]
State the ecological risk if a keystone species is permanently removed from an ecosystem.
[1]
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Question 21
After heavy rain, water draining from fertilized fields enters a lake. Two weeks later, the lake has a dense algal bloom and dead fish are found near the shore.
State the term for nutrient enrichment of a water body that stimulates excessive algal growth.
[1]
Explain how leaching of fertilizers can lead to death of fish in the lake.
[3]
Question 22
Primary succession was studied on land exposed after glacier retreat. Sites closer to the glacier had been exposed for less time than sites farther from the glacier.
Site | Years since exposure / years | Soil depth / cm | Dominant vegetation | Species richness / species | Mean plant height / cm | Above-ground biomass / g m | Surface litter / g m |
|---|---|---|---|---|---|---|---|
1 | 0 | 0 | Bare rock; no plants | 0 | 0 | 0 | 0 |
2 | 10 | 0.2 | Lichens and mosses | 4 | 1 | 18 | 2 |
3 | 20 | 1.0 | Mosses and herbs | 8 | 8 | 90 | 8 |
4 | 35 | 2.5 | Herbs and small shrubs | 13 | 30 | 280 | 25 |
5 | 55 | 6.0 | Shrubs | 19 | 95 | 900 | 70 |
6 | 80 | 12.0 | Shrubs and young trees | 26 | 260 | 2200 | 160 |
State why succession on newly exposed ground after glacier retreat is primary succession.
[1]
Describe two changes expected in plant communities during primary succession.
[2]
Explain why nutrient cycling usually increases during primary succession.
[1]
Question 23
On some rocky shores in New Zealand, communities pass through repeated stages. Bare rock is colonised by barnacles and algae, mussels later form dense mats, and wave action may remove mussels to expose bare rock again.
Outline what is meant by cyclical succession.
[2]
Identify the event in the example that restarts the cycle.
[1]
Question 24
A cool, wet hillside with suitable soil would be expected to develop woodland through succession. It has remained open grassland for many decades because sheep graze there throughout the year.
Define climax community.
[1]
Explain how grazing by livestock can cause arrested succession in this hillside ecosystem.
[2]
State one environmental factor, other than grazing, that can influence the type of climax community that develops.
[1]
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Question 25
Satellite data were used to estimate Amazon rainforest area and mean dry-season rainfall in a region of the Amazon basin. The model also estimates the area below which rainfall-generating processes may not be maintained, but this threshold is uncertain.
Year / description | Forest area / million km | Mean dry-season rainfall / mm day |
|---|---|---|
1970 | 4.10 | 7.8 |
1980 | 3.98 | 7.5 |
1990 | 3.85 | 7.1 |
2000 | 3.71 | 6.8 |
2010 | 3.53 | 6.2 |
2020 | 3.34 | 5.8 |
Uncertain model threshold | 3.3-3.5 | n/a |
Calculate the percentage change in forest area from 1970 to 2020.
[1]
Describe the relationship between forest area and dry-season rainfall shown by the data.
[1]
Explain how deforestation could lead to a tipping point in Amazon ecosystem stability.
[3]
Question 26
In a rocky-shore investigation, the sea star Pisaster ochraceus was removed from some experimental plots but left in control plots. Mussel cover and species richness were measured after 12 months.
Plot | Mussel cover / % | Species richness / species |
|---|---|---|
Control | 35 | 15 |
Sea-star removal | 80 | 6 |
Describe the effect of sea-star removal on mussel cover and species richness.
[2]
Explain why the sea star can be described as a keystone species in this community.
[2]
Question 27
A community harvests seeds from a long-lived rainforest tree, the Brazil nut tree (Bertholletia excelsa). Researchers counted adult trees, saplings and seedlings in forest areas with different harvesting intensities.
Identify the harvesting intensity that is least likely to be sustainable. Give a reason using the data.
[1]
Explain why counting only adult trees could give a misleading assessment of sustainability.
[2]
Suggest one method that could be used to assess whether harvesting of a marine fish species is sustainable.
[1]
Question 28
A chronosequence was studied on land exposed by a retreating glacier. Sites of different ages since ice retreat were used to infer changes during primary succession.
Age since retreat / years | Soil depth / cm | Plant species richness / species |
|---|---|---|
0 | 0.0 | 0 |
10 | 0.2 | 1 |
25 | 0.8 | 4 |
50 | 3.2 | 8 |
75 | 7.9 | 14 |
100 | 15.4 | 21 |
State why this is an example of primary succession.
[1]
Describe two trends shown in the chronosequence.
[2]
Explain how pioneer organisms can make the abiotic environment more suitable for later successional stages.
[2]
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Question 29
Researchers monitored fixed quadrats on a rocky shore where wave action periodically removed mussel mats. The dominant community state in each quadrat was recorded each season.
State the community state most likely to colonise bare rock first in this cycle.
[1]
Explain why the data are evidence of cyclical succession rather than succession towards one unchanging climax community.
[2]
Predict the effect of a long period with reduced wave disturbance on the cycle.
[1]
Question 30
An upland area has a cool wet climate and suitable soil for woodland. Some plots are grazed by sheep throughout the year, while fenced plots exclude sheep. Tree seedling density and vegetation height were measured for eight years.
Year | Grazed tree seedlings / | Fenced tree seedlings / | Grazed vegetation height / cm | Fenced vegetation height / cm |
|---|---|---|---|---|
1 | 1 | 3 | 8 | 12 |
2 | 1 | 6 | 7 | 18 |
3 | 0 | 10 | 7 | 28 |
4 | 1 | 15 | 6 | 40 |
5 | 0 | 20 | 7 | 53 |
6 | 1 | 24 | 6 | 67 |
7 | 0 | 30 | 7 | 81 |
8 | 1 | 36 | 6 | 96 |
Identify the climax community expected under these environmental conditions if grazing pressure is removed.
[1]
Explain how grazing by sheep can cause arrested succession.
[2]
Use the data to support the conclusion that succession was arrested in the grazed plots.
[1]
Question 31
A drained wetland has a network of ditches that lowers the water table. After drainage, the surface peat becomes exposed to air and plants adapted to drier soil begin to colonise.
Explain why drainage can deflect succession away from a wetland climax community.
[3]
Suggest why restoration after blocking the ditches may be slow.
[1]
Question 32
In an abandoned field, tall shrubs become established after several years. Measurements show that the soil surface beneath the shrubs is cooler and more humid than in nearby open areas, and fewer seedlings of sun-loving herbs survive beneath the shrub canopy.
Site | Relative light intensity / % | Soil surface temperature / | Relative humidity / % | Sun-loving herb seedling survival / % |
|---|---|---|---|---|
Open field | 100 | 31 | 40 | 76 |
Beneath shrubs | 24 | 25 | 67 | 18 |
Identify one abiotic variable altered by the shrubs.
[1]
Analyse how the shrubs could cause a change in the plant community over time.
[3]
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Question 33
Water samples were taken downstream from farmland after a period of heavy rain. Fertilizer application, nitrate concentration, biochemical oxygen demand and dissolved oxygen were measured at sites along the stream.
State the source of the nitrate entering the stream.
[1]
Describe the relationship between biochemical oxygen demand and dissolved oxygen shown by the data.
[2]
Explain how fertilizer leaching can lead to the death of fish in the stream.
[2]
Question 34
In the same deglaciated valley, some plots contained nitrogen-fixing shrubs while nearby plots of the same age did not. Soil nitrate concentration and tree seedling density were measured.
Nitrogen-fixing shrubs present | Soil nitrate concentration / arbitrary units | Tree seedling density / seedlings m |
|---|---|---|
No | 0.18 | 12 |
Yes | 0.42 | 28 |
Calculate the percentage increase in soil nitrate concentration in plots with nitrogen-fixing shrubs compared with plots without them.
[1]
Suggest why tree seedling density was higher in plots containing nitrogen-fixing shrubs.
[2]
Identify whether the presence of nitrogen-fixing shrubs is a biotic or abiotic trigger of successional change.
[1]
Question 35
Two abandoned fields were monitored for early successional change. Field A was covered by mineral sediment after a flood. Field B was colonised by an invasive burrowing mammal that disturbed the soil and reduced grass cover.
Year after disturbance | A trigger | A grass cover / % | A shrub cover / % | A light at soil / % | A soil temp / | B trigger | B grass cover / % | B shrub cover / % | B light at soil / % | B soil temp / |
|---|---|---|---|---|---|---|---|---|---|---|
0 | mineral sediment after flood | 0 | 0 | 100 | 24 | invasive burrowing mammal | 40 | 0 | 92 | 24 |
1 | — | 5 | 1 | 95 | 23 | — | 31 | 1 | 89 | 23 |
2 | — | 12 | 4 | 86 | 22 | — | 23 | 4 | 82 | 22 |
3 | — | 18 | 11 | 73 | 21 | — | 15 | 9 | 71 | 21 |
4 | — | 14 | 22 | 59 | 20 | — | 10 | 20 | 58 | 20 |
5 | — | 8 | 35 | 44 | 19 | — | 7 | 32 | 45 | 19 |
Distinguish the trigger of successional change in Field A from that in Field B.
[2]
Explain how the establishment of shrubs could create feedback that changes later succession in either field.
[2]
Question 36
A long-established desert ecosystem receives water mainly from fog. A proposed road would increase dust deposition and fragment the habitats of several plant and invertebrate populations.
Explain why resistance and resilience are different properties of ecosystem stability.
[2]
I.
State one requirement for the long-term stability of this ecosystem.
[1]
II.
Explain how reduced genetic diversity could make one population in this ecosystem less stable.
[1]
Discuss whether the proposed road is more likely to reduce resistance, resilience, or both in the desert ecosystem.
[2]
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Question 37
Students set up sealed aquatic mesocosms to investigate whether nutrient concentration affects ecosystem stability over four weeks.
I.
Explain why a sealed glass vessel is a useful model for investigating nutrient recycling.
[1]
II.
Explain why energy transfer must still be possible in the sealed vessel.
[1]
Suggest two variables that should be monitored to assess stability in the mesocosms.
[2]
Evaluate sealed aquatic mesocosms as models of natural ecosystems.
[3]
Question 38
On a rocky shore, bare rock is colonised by crustose algae and barnacles. Mussels then settle and form dense patches. Storm waves later remove some mussel patches, exposing bare rock again.
I.
Define cyclical succession.
[1]
II.
State the disturbance that restarts the cycle in this example.
[1]
Compare and contrast cyclical succession with primary succession.
[2]
Discuss why an ecosystem showing cyclical succession can still be considered stable.
[2]
Question 39
A drained wetland was monitored before and after drains were blocked as part of a restoration project. Water-table depth, soil oxygen concentration, peat depth and plant community composition were recorded.
Stage | Water-table depth / cm below surface | Soil oxygen concentration / % | Peat depth / cm | Wetland plant cover / % | Dry grassland plant cover / % |
|---|---|---|---|---|---|
Before drainage | 5 | 2 | 80 | 92 | 8 |
Drained period | 35 | 14 | 54 | 15 | 85 |
After drains blocked | 10 | 5 | 56 | 68 | 32 |
Calculate the percentage decrease in peat depth during the drained period.
[1]
Explain how drainage can deflect succession away from a wetland climax community.
[2]
Evaluate whether blocking the drains fully restored the wetland climax community.
[2]
Question 40
Remote-sensing estimates suggest that an area of Amazon rainforest declined from to over several decades. Climate models predict lower rainfall in some deforested regions, but differ in the minimum forest area needed to maintain regional rainfall.
I.
Calculate the percentage change in rainforest area. Show your working.
[2]
Explain how deforestation of a large area of Amazon rainforest could act as a tipping point in ecosystem stability.
[3]
Evaluate the use of remote-sensing data and models for deciding a safe limit to deforestation.
[2]
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Question 41
On a rocky shore, a predatory sea star is present at low biomass but feeds on mussels. In an experimental area, the sea star is removed and mussel cover increases rapidly.
Time / months | Shore | Sea star biomass / g m | Mussel cover / % | Species richness / no. of species |
|---|---|---|---|---|
0 | Control | 0.3 | 22 | 8 |
0 | Removal | 0.0 | 22 | 8 |
3 | Control | 0.3 | 23 | 8 |
3 | Removal | 0.0 | 57 | 6 |
6 | Control | 0.3 | 24 | 8 |
6 | Removal | 0.0 | 84 | 4 |
12 | Control | 0.3 | 24 | 8 |
12 | Removal | 0.0 | 98 | 2 |
I.
Identify the term used for a species with a disproportionately large effect on community structure relative to its abundance.
[1]
II.
Explain why the sea star in this example fits this term.
[1]
Explain how removal of the sea star could lead to ecosystem collapse on the rocky shore.
[2]
Discuss why maintaining keystone species can increase both resistance and resilience in ecosystems.
[3]
Question 42
A dairy farm applies nitrogen and phosphate fertilizers to pasture next to a lake. After heavy rain, the lake becomes green and fish are found dead near the shore.
I.
State what is meant by eutrophication.
[1]
II.
State what is meant by biochemical oxygen demand, BOD.
[1]
Explain the sequence of events leading from fertilizer leaching to death of fish in the lake.
[3]
Evaluate three farm-management changes that could improve the sustainability of the pasture system.
[3]
Question 43
A chronosequence was studied on land exposed by a retreating glacier. Sites differed in the number of years since exposure of bare mineral substrate.
I.
State why this is an example of primary succession.
[1]
II.
Identify one likely pioneer group in this succession.
[1]
Explain how pioneer organisms can change abiotic conditions so that later species can establish.
[2]
Analyse why primary production, species diversity, food-web complexity and nutrient cycling usually increase during primary succession.
[3]
Question 44
A temperate valley has a cool wet climate and deep soils. Historical records suggest it was formerly woodland, but it has remained open grassland for many decades under sheep grazing. Nearby, a drained wetland has been converted to pasture.
I.
State what is meant by a climax community.
[1]
II.
Predict the likely climax community in the valley if grazing stopped, giving a reason.
[1]
Explain how grazing by farm livestock can cause arrested succession.
[2]
Explain how drainage can deflect succession away from a wetland climax community.
[2]
Evaluate whether removing grazing and blocking drains would be sufficient to restore the former ecosystems.
[2]
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Question 45
A lake margin is gradually drying as sediment accumulates. At the same time, an introduced herbivore has begun grazing young reeds and tree seedlings around the lake.
I.
Define ecological succession.
[1]
II.
Identify one abiotic trigger and one biotic trigger of succession in this example.
[1]
Explain how changes in the biotic community can alter abiotic conditions during succession at the lake margin.
[2]
Discuss how the introduced herbivore could change the pathway and rate of succession.
[3]
Question 46
A coastal community is considering measures to reduce plastic pollution and restore a degraded bay. Surveys have found macroplastic fishing gear on beaches, microplastics in mussels and elevated mercury concentrations in large predatory fish.
I.
Distinguish between macroplastic and microplastic pollution.
[1]
II.
Explain why plastics are persistent in marine ecosystems.
[1]
Compare the effects of macroplastic and microplastic pollution on marine life.
[2]
Explain why mercury concentrations may be highest in large predatory fish.
[2]
Evaluate how scientists and managers could use evidence from the bay to change citizen behaviour and restore natural processes.
[2]
Question 47
A heathland reserve contains patches dominated by young heather, mature heather, old degenerating heather, lichens and bare ground. Managers are deciding whether to allow all patches to develop naturally or to use occasional grazing and small controlled burns.
I.
Identify the type of succession represented by a repeated sequence of heathland patch states.
[1]
II.
State one reason old heather patches may not remain unchanged indefinitely.
[1]
Explain why this heathland does not fit a simple model of succession towards one permanent climax community.
[2]
Evaluate the use of occasional grazing and small controlled burns in maintaining the stability of the heathland reserve.
[3]
Question 48
A rewilding project aims to restore a drained floodplain. Drainage channels will be blocked, grazing by cattle will be reduced and woodland patches will be connected to wetland areas.
I.
State one human influence that can cause arrested succession in this floodplain.
[1]
II.
Outline how habitat connectivity could affect succession in the restored floodplain.
[1]
Explain how blocking drainage channels may change the likely climax community.
[2]
Compare the effects of reducing cattle grazing and re-establishing wetland hydrology on succession.
[2]
Evaluate whether the project is an example of rewilding or simply artificial habitat creation.
[2]
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