D4.2 Stability and change
Practice exam-style IB Biology questions for Stability and change, aligned with the syllabus and grouped by topic.
Question 1
What is meant by ecosystem stability?
A.
The replacement of all species by better-adapted species over time
B.
The ability of community structure and ecosystem processes to persist through time
C.
The complete prevention of all disturbances from affecting an ecosystem
D.
The absence of births, deaths and seasonal changes in a community
Question 2
A grassland burns in a dry season but regrows rapidly after rain. Which property is shown most clearly by the regrowth?
A.
Resilience
B.
Biomagnification
C.
Resistance
D.
Eutrophication
Question 3
What requirement for ecosystem stability is represented by decomposers releasing mineral nutrients from dead organic matter?
A.
Continuing supply of energy
B.
Reduction of tolerance limits
C.
Recycling of nutrients
D.
Genetic drift in populations
Question 4
What can pass between a sealed glass mesocosm and its surroundings?
A.
Energy only
B.
Matter and energy
C.
Neither matter nor energy
D.
Matter only
Question 5
What is ecological succession?
A.
The immediate return of a community to its original state after disturbance
B.
A sequence of changes in community composition and abiotic conditions over time
C.
The random movement of alleles between isolated populations
D.
The complete recycling of energy through a food web
Question 6
What distinguishes primary succession from secondary succession?
A.
Primary succession always ends within one year
B.
Primary succession begins where developed soil is absent or extremely undeveloped
C.
Primary succession occurs only in marine ecosystems
D.
Primary succession is caused only by human grazing
Question 7
Define resistance in an ecosystem.
[1]
Define resilience in an ecosystem.
[1]
Give one example of a disturbance after which an ecosystem may show resilience.
[1]
Question 8
Why can loss of a large area of Amazon rainforest increase the risk of further forest loss?
A.
Reduced herbivory allows tree seedlings to be eaten by decomposers
B.
Reduced photosynthesis immediately increases soil mineral nutrients
C.
Reduced rainfall increases genetic diversity in all tree populations
D.
Reduced transpiration can reduce rainfall, increasing drought and fire risk
Question 9
What is a keystone species?
A.
A species with a disproportionately large effect on community structure compared with its abundance
B.
The first species to colonize a newly formed substrate
C.
The most abundant producer in a community at any time
D.
A species that can survive outside its tolerance limits
Question 10
What condition is required for sustainable harvesting of a renewable fish stock?
A.
Catch per unit effort is ignored if total catch is high
B.
The population is kept as small as possible
C.
The harvesting rate is lower than the replacement rate
D.
All adult fish are removed before reproduction
Question 11
Which event is an abiotic trigger of succession?
A.
Arrival of a new herbivore in a grassland
B.
Introduction of a pathogen killing a dominant plant
C.
Retreat of a glacier exposing bare mineral substrate
D.
Competition between two established shrub species
Question 12
What is usually true during primary succession on newly exposed bare rock?
A.
Food webs become simpler as more species arrive
B.
Primary production falls continuously after pioneer colonization
C.
Plant size decreases as shrubs replace lichens
D.
Soil depth and nutrient cycling increase over time
Question 13
What is cyclical succession?
A.
Succession prevented by a continuous human influence
B.
A permanent unchanging climax community after primary succession
C.
Repeated succession through a recurring sequence of community states
D.
A sudden increase in pollutant concentration at higher trophic levels
Question 14
What is arrested succession?
A.
Succession held at an earlier or alternative stage by a continuing factor
B.
Succession caused only by natural seed dispersal
C.
Succession that starts on rock without any soil
D.
Succession that always reaches forest regardless of climate
Question 15
Outline four requirements for long-term stability in an ecosystem. [4]
Question 16
A sealed aquatic mesocosm contains pond water, algae, small invertebrates and sediment.
State why it can be considered a closed system for matter.
[1]
Explain why algae are needed.
[1]
Explain why sediment may improve long-term stability.
[1]
Question 17
A predatory sea star is removed from a rocky shore. Mussels then spread over most of the rock surface and many other species decline.
State the term used for a species such as the predatory sea star.
[1]
Explain why removal of this species can reduce diversity.
[1]
Question 18
Outline four factors, other than crop yield, that should be considered when assessing the sustainability of an agricultural system. [4]
Question 19
Define ecological succession.
[1]
Distinguish between an abiotic trigger and a biotic trigger of succession, giving one example of each.
[1]
Question 20
Outline four general changes that usually occur during primary succession in a terrestrial ecosystem. [4]
Question 21
Describe an example of cyclical succession on a rocky shore. [3]
Question 22
What directly causes dissolved oxygen concentration to fall during eutrophication?
A.
Phosphate ions prevent water from dissolving oxygen at any temperature
B.
Fertilizer molecules chemically bind all oxygen in water
C.
Aerobic bacteria respire while decomposing dead algal biomass
D.
Fish release nitrogen gas during photosynthesis
Question 23
In a rocky shore cycle, mussel beds detach after wave action exposes rock again. What does this illustrate?
A.
Primary succession because no organisms can ever recolonize
B.
Arrested succession because grazing livestock prevent trees growing
C.
Cyclical succession because earlier colonization states can recur
D.
A climax community because the mussel bed is permanently unchanged
Question 24
How can grazing by farm livestock maintain grassland where woodland would otherwise develop?
A.
Livestock remove tree seedlings while many grasses regrow from low growth points
B.
Livestock make the soil completely free of decomposers
C.
Livestock eliminate all competition between grass species
D.
Livestock increase rainfall enough to prevent tree germination
Question 25
Explain how deforestation of the Amazon rainforest could lead to a tipping point in ecosystem stability. [3]
Question 26
A community harvests fruit from a long-lived rainforest tree.
State the condition for the harvest to be sustainable.
[1]
Suggest two measurements of the tree population that could be used to assess sustainability.
[1]
Explain why harvesting nearly all seeds may be unsustainable even if many adult trees remain.
[1]
Question 27
Explain how mercury can reach high concentrations in large predatory fish. [4]
Question 28
Explain how pioneer organisms contribute to soil formation during primary succession on bare rock. [4]
Question 29
Young trees establish in open ground during succession.
State one abiotic condition at soil level that may be altered by the young trees.
[1]
Explain how this alteration can change the plant community.
[1]
Question 30
Explain why cyclical succession challenges the idea of a single unchanging climax community. [3]
Question 31
Explain how drainage of a wetland can deflect succession away from a wetland climax community. [4]
Question 32
Satellite estimates of forest cover were made for a region of the Amazon rainforest.
| Year | Forest area / million ha |
|---|---|
| 2000 | 550 |
| 2005 | 538 |
| 2010 | 523 |
| 2015 | 507 |
| 2020 | 493 |
Calculate the percentage change in forest area from the first to the final year shown.
[1]
Describe the trend in forest area.
[1]
Suggest why there is uncertainty in using such data to predict an Amazon rainforest tipping point.
[1]
Question 33
Students set up sealed aquatic mesocosms at different light intensities and recorded dissolved oxygen concentration after seven days.
Identify the independent variable.
[1]
Describe the relationship shown between light intensity and dissolved oxygen.
[1]
Explain why very low light intensity may reduce mesocosm stability.
[1]
Question 34
A lake receiving runoff from farmland was sampled during summer.
State the variable that best indicates oxygen demand by decomposers.
[1]
Describe the relationship between nitrate concentration and algal density.
[1]
Explain the change in dissolved oxygen after the algal bloom.
[1]
Suggest one agricultural practice that could reduce eutrophication.
[1]
Question 35
A chronosequence was sampled at increasing distances from a retreating glacier. Distance from the glacier represents time since exposure.
| Distance from glacier / km | Dominant vegetation | Soil organic matter / % | Plant richness / species per 10 m² | NPP / g C m⁻² yr⁻¹ |
|---|---|---|---|---|
| 0.2 | Lichens and mosses | 0.4 | 3 | 25 |
| 0.8 | Mosses and herbs | 1.2 | 8 | 85 |
| 1.7 | Grasses and herbs | 2.9 | 14 | 190 |
| 3.5 | Dwarf shrubs | 5.6 | 21 | 330 |
| 6.0 | Shrubs and young trees | 9.8 | 27 | 470 |
Identify the earliest successional stage shown.
[1]
Describe two trends in the data with increasing time since exposure.
[1]
Explain one mechanism causing the change in plant community.
[1]
Question 36
Compare a climax community with arrested succession. [4]
Question 37
A marine fishery was monitored for catch per unit effort (CPUE), stock biomass and percentage of mature adults.
| Year | Catch / t | CPUE / kg h^-1 | Biomass / kt | Mature adults / % |
|---|---|---|---|---|
| 2015 | 4200 | 18.5 | 62 | 68 |
| 2016 | 4400 | 16.2 | 55 | 61 |
| 2017 | 4300 | 13.4 | 46 | 54 |
| 2018 | 3600 | 10.1 | 36 | 45 |
| 2019 | 2900 | 7.8 | 28 | 36 |
| 2020 | 2200 | 5.6 | 21 | 29 |
Identify one variable in the data that can indicate decreasing stock size.
[1]
Describe the change in age structure during the monitoring period.
[1]
Suggest two management measures that could improve sustainability.
[1]
Explain why high total catch may not indicate sustainable harvesting.
[1]
Question 38
Researchers measured plastic fragments in seawater and the proportion of sampled seabirds containing plastic in their stomachs at several coastal sites.
| Coastal site | Microplastic fragments / m^-3 | Seabirds sampled / n | Seabirds containing plastic / % | Seabird mortality / category |
|---|---|---|---|---|
| Alder Bay | 70 | 34 | 12 | Low |
| Coral Point | 150 | 29 | 19 | Low |
| Dune Harbour | 260 | 31 | 34 | Moderate |
| Estuary Head | 410 | 27 | 46 | Moderate |
| Firth Cove | 620 | 25 | 68 | High |
| Gannet Island | 790 | 22 | 61 | Moderate |
Identify the site with the greatest seabird ingestion of plastic.
[1]
Describe the relationship between seawater microplastic abundance and seabird ingestion.
[1]
Suggest one effect of ingested plastic on marine animals.
[1]
Evaluate one limitation of using these data to conclude that microplastics caused seabird deaths.
[1]
Question 39
After an invasive shrub arrived on grassland, researchers measured light at ground level and the cover of native herbs.
Describe the change in ground-level light after shrub arrival.
[1]
Describe the change in native herb cover.
[1]
Suggest how the invasive shrub could trigger succession.
[1]
State whether this is primarily an abiotic or biotic trigger.
[1]
Question 40
Permanent quadrats on a rocky shore were surveyed after storm waves removed mussel mats from some areas.
| Year / yr | Quadrat A state | Quadrat B state | Quadrat C state | Quadrat D state |
|---|---|---|---|---|
| 2018 | Mussel mat | Mussel mat | Mussel mat | Mussel mat |
| 2019 | Bare rock | Mussel mat | Bare rock | Mussel mat |
| 2020 | Barnacles + algae | Bare rock | Barnacles + algae | Mussel mat |
| 2021 | Mussel mat | Barnacles + algae | Mussel mat | Bare rock |
| 2022 | Mussel mat | Mussel mat | Mussel mat | Barnacles + algae |
| 2023 | Bare rock | Mussel mat | Mussel mat | Mussel mat |
| 2024 | Barnacles + algae | Bare rock | Bare rock | Mussel mat |
| 2025 | Mussel mat | Barnacles + algae | Barnacles + algae | Bare rock |
| 2026 | Mussel mat | Mussel mat | Mussel mat | Barnacles + algae |
Identify the community state that occurs immediately after mussel removal.
[1]
Describe the sequence of community states shown in the quadrats.
[1]
Explain why the data support cyclical succession rather than progression to one permanent climax community.
[1]
Question 41
Outline two reasons why sealed aquatic mesocosms are useful models for investigating ecosystem stability.
[1]
Evaluate the use of sealed glass mesocosms as models of natural ecosystems.
[1]
Question 42
Outline how nitrogen and phosphate fertilizers can enter aquatic ecosystems.
[1]
Explain the sequence of events leading from fertilizer leaching to death of fish during eutrophication.
[1]
Question 43
Define primary succession and name one terrestrial situation in which it occurs.
[1]
Explain the changes that occur during primary succession after glacier retreat.
[1]
Question 44
Two adjacent fields with similar soil and climate were monitored for 20 years. One field was grazed by sheep throughout; grazing was excluded from the other.
| Year | Shrub cover grazed / % | Shrub cover ungrazed / % | Seedlings grazed / m^-2 | Seedlings ungrazed / m^-2 |
|---|---|---|---|---|
| 0 | 4 | 5 | 0.3 | 0.4 |
| 5 | 5 | 12 | 0.4 | 1.2 |
| 10 | 5 | 25 | 0.2 | 3.5 |
| 15 | 4 | 43 | 0.3 | 7.0 |
| 20 | 6 | 62 | 0.4 | 10.5 |
Compare tree seedling density in the two fields at the end of the study.
[1]
Describe the trend in shrub cover in the ungrazed field.
[1]
Explain how grazing can cause arrested succession.
[1]
Evaluate one limitation of the conclusion that grazing alone caused the difference between fields.
[1]
Question 45
A drained peat wetland was rewetted after drainage channels were blocked. Measurements were taken before and after rewetting.
| Variable | Before drainage | During drainage | After blocking |
|---|---|---|---|
| Water table below surface / cm | 5 | 47 | 12 |
| Soil redox potential / mV | +80 | +410 | +140 |
| Cellulose mass loss / % month⁻¹ | 3.2 | 10.8 | 4.5 |
| Wetland plant cover / % | 84 | 17 | 46 |
Identify the variable that indicates the wetland became more waterlogged after restoration.
[1]
Describe the change in aerobic decomposition indicator after rewetting.
[1]
Explain how drainage can deflect succession away from a wetland climax.
[1]
Suggest why recovery after rewetting may be slow.
[1]
Question 46
State two types of evidence that can be used to assess whether harvesting a marine fish species is sustainable.
[1]
Discuss how sustainability of harvesting renewable resources from natural ecosystems can be assessed and managed, using one marine fish and one terrestrial plant example.
[1]
Question 47
State two methods used in rewilding.
[1]
Discuss how rewilding can restore natural processes in ecosystems, including the example of Hinewai Reserve in New Zealand.
[1]
Question 48
Outline one example of cyclical succession.
[1]
Compare cyclical succession with succession leading to a climax community.
[1]
Question 49
Define climax community and arrested succession.
[1]
Discuss how environmental conditions and human influences affect the endpoint of succession, using grazing and wetland drainage as examples.
[1]
Question 50
Distinguish between abiotic and biotic triggers of succession, giving one example of each.
[1]
Evaluate the statement: “succession is caused by organisms changing abiotic conditions, and abiotic conditions changing which organisms can survive.”
[1]
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