IB Biology SL D4.2 Stability and change Questions

Practise IB Biology SL D4.2 through mesocosm, keystone-species and pollution questions that connect ecosystem evidence with stability and change.

41 questions in this syllabus scope, with question parts counted together.

Syllabus
First assessment 2025
Course
Biology SL
Level
SL

Exam analysis

SL · 2010–2025
Papers with this topic37 / 141in the analysed sample
Latest appearanceNovember 2025
Tagged question parts71
Paper types5

Most tested objectives

  1. D4.2.9—Biomagnification14 papers
  2. D4.2.13 (HL)—Changes during primary succession13 papers
  3. D4.2.5—Keystone species10 papers
  4. D4.2.4—Mesocosm model8 papers
  5. D4.2.10—Plastic pollution of oceans4 papers

Common question formats

  • Comparison
  • Data analysis
  • Definition or recall
  • Evaluation
  • Extended response
  • Process explanation
  • Structured response

Recent exam appearances

November 2025Paper 1A SL · TZ1Q23[ 1 ]
May 2025Paper 2 SL · TZ2Q4(a), 4(b), 4(c)[ 6 ]
May 2025Paper 2 SL · TZ1Q2(c)[ 2 ]

141 papers analysed · Tagging incomplete

Exam points

  • Evaluate mesocosm controls and limitations, including sealed-system matter exchange and light input.
  • Use biodiversity and food-web evidence to explain ecosystem stability and resilience.
  • Define a keystone species by its disproportionate effect on community structure.
  • Predict how keystone loss changes populations, trophic cascades and biodiversity.
  • Trace nutrient enrichment through algal blooms, decomposition, rising BOD and oxygen loss.
  • Trace pollutant magnification through food chains and explain DDT-related harm to top predators.
  • Explain how plastic ingestion or entanglement causes gut damage, starvation or choking in marine animals.
  • Identify rewilding actions that restore habitats, connectivity and natural trophic processes.

Covered syllabus points

D4.2.1—Stability of natural ecosystemsD4.2.2—Requirements for stabilityD4.2.3—Amazon rainforest deforestationD4.2.4—Mesocosm modelD4.2.5—Keystone speciesD4.2.6—Sustainable resource harvestingD4.2.7—Agriculture sustainability factorsD4.2.8—EutrophicationD4.2.9—BiomagnificationD4.2.10—Plastic pollution of oceansD4.2.11—Rewilding

Question 1

[Maximum number: 3]

Primary plant succession has been observed in sand dunes adjacent to the northern end of Lake Michigan, one of the Great Lakes in North America. The youngest sand dunes have beach grass (Ammophila breviligulata) and prairie bunch grass (Schizachyrium scoparium). The oldest dunes have coniferous trees (Pinus strobus and Pinus resinosa).

Outline how the type of stable ecosystem that will develop in an area can be predicted based on climate.

Exam points

  • Explain how biodiversity can support ecosystem stability and resistance to change through complex food webs and alternative food sources.
  • Identify species adapted to the same abiotic conditions.

Question 1

[Maximum number: 2]

Beavers (Castor fiber) have been reintroduced into many European countries. They cut down trees and use them as a source of food as well as for building their homes and dams.

Figure for Question 1 — IB Biology SL

Studies show that species richness of large invertebrates increases in the presence of beavers. Suggest reasons why biodiversity is important in an ecosystem.

No public sample questions are available for this syllabus point yet.

Exam points

  • Explain mesocosm advantages for ecological studies, including controlled variables and easier measurement, and compare the model with natural conditions.
  • Describe how to set up and control a mesocosm, including starting conditions, suitable organisms and light.
  • Distinguish matter from energy exchange in a closed mesocosm and interpret which variables can change in a terrestrial model.
  • Use continuous waste recycling as evidence that a sealed mesocosm remains sustainable over time.

Question 1

[Maximum number: 1]

A simple sealed terrestrial mesocosm was set up to demonstrate that natural ecosystems can be sustainable over long periods of time.

What indicates that the mesocosm is still sustainable after several months?

A

Wastes are constantly recycled.

B

The diversity of living organisms increases.

C

Energy is conserved.

D

The mass of carbon increases.

Exam points

  • Define a keystone species by its disproportionate effect on community structure or ecosystem functioning.
  • Explain how removing a keystone species can alter other populations, trigger trophic cascades and destabilize a community.

Question 1

[Maximum number: 2]

Feeding relationships between organisms on rocky shores along the Pacific Coast of North America were studied. The food web shows the proportions of the total energy consumed by the two carnivorous species, a starfish (Pisaster ochraceus) and a small gastropod (Thais emarginata).

Figure for Question 1 — IB Biology SL

Pisaster is considered a keystone species. Explain the effects of Pisaster on this community.

No public sample questions are available for this syllabus point yet.

No public sample questions are available for this syllabus point yet.

Exam points

  • Identify bacterial decomposition of organic matter as a direct cause of oxygen decline during eutrophication.
  • Recognize increased biochemical oxygen demand as an effect of nitrogen-fertilizer leaching into aquatic ecosystems.

Question 1

[Maximum number: 1]

What directly causes oxygen levels to drop during eutrophication in aquatic ecosystems?

A

Increased photosynthesis by aquatic plants

B

Enhanced respiration by fish

C

More decomposition of organic matter by bacteria

D

Lower levels of nitrogen in the water

Exam points

  • Define biomagnification and explain how persistent toxins become more concentrated at successive trophic levels.
  • Use DDT or mercury examples to explain toxic effects on top predators and people higher in food chains.
  • Use predator data to identify the lowest biomagnification factor and explain it through diet or metabolism.

Question 1

[Maximum number: 4]

Certain mercury compounds are toxic to living organisms. Health warnings have been issued against consuming large amounts of certain fish because they contain high levels of mercury. Explain how methyl mercury has reached potentially harmful concentrations in the human food supply.

Exam points

  • Explain how plastic ingestion and entanglement harm marine organisms through starvation, digestive damage, choking or reduced predator escape.
  • Explain how persistent macroplastics fragment into microplastics that can carry toxins into marine food webs and accumulate in organisms.

Question 1

[Maximum number: 4]

The impact of microplastics on the ecology of coastal biota was assessed using beach hoppers (Platorchestia platensis), small marine crustaceans which feed on marine sediments. These crustaceans have the ability to jump, which allows them to escape from predators.

Figure for Question 1 — IB Biology SL

Scientists investigated the possible effects of microplastics on their behaviour and survival, by observing the height and frequency of jumps in two groups of beach hoppers:
- experimental group: fed on marine sediments contaminated with microplastics
- control group: fed on uncontaminated marine sediments.

The graphs show the results.

Figure for Question 1 — IB Biology SL
Figure for Question 1 — IB Biology SL

for beachhoppers. Marine Biology 163, 199 (2016). Reproduced with permission from Springer Nature.
https://doi.org/10.1007/s00227-016-2973-0.]

Question (a)

(a)

Using the data, discuss the effect that ingestion of microplastic will have on the success of beach hoppers in escaping from predators in their natural environment.

[ 2 ]

Question (b)

(b)

Macroplastics also affect the survival of other marine organisms. Outline how this may happen.

[ 2 ]

Exam points

  • Outline methods for restoring natural processes: reintroduce native or apex species, remove invasive species, restore habitat connectivity or reduce human management.
  • Recognize rewilding degraded ecosystems as a way to minimize biodiversity loss.

Question 1

[Maximum number: 1]

What could minimize the loss of biodiversity?

A

Increase the rate of deforestation

B

Introduce species into new environments to control predation

C

Reduce the use of renewable energy

D

Rewilding of degraded ecosystems

Question 1

[Maximum number: 6]

Beavers (Castor fiber) have been reintroduced into many European countries. They cut down trees and use them as a source of food as well as for building their homes and dams.

Figure for Question 1 — IB Biology SL

Question (a)

(a)

Beavers are a keystone species. Outline the importance of keystone species.

[ 2 ]

Question (b)

(b)

Studies show that species richness of large invertebrates increases in the presence of beavers. Suggest reasons why biodiversity is important in an ecosystem.

[ 2 ]

Question (c)

(c)

Outline two methods of restoration of natural processes in ecosystems by rewilding, other than reintroducing a keystone species.
1.
2.

[ 2 ]
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