D4.2.4—Mesocosm model
Mesocosms model ecosystem stability by enclosing simplified communities where matter cycling, energy input, variables, and population changes can be observed.
- Syllabus
- First assessment 2025
- Objective
- D4.2.4
- Level
- HL
Exam analysis
Chance of appearing4%of analysed past papers
Latest appearanceMay 2024
Most common paperPaper3
Typical marks1–3
Common command terms
- Discuss
- Outline
- State
- Suggest
- Explain
Scoring notes
- Common mistake
- Assuming a sealed mesocosm exchanges no energy, when light or heat can still pass between the system and surroundings.
Recent exam appearances
May 2024Paper2 ["HL"] · TZ25(b)[ 2 ]D4.2.4—Mesocosm model
May 2023Paper3 ["HL"] · TZ23(b)[ 1 ]D4.2.4—Mesocosm model
November 2018Paper3 ["HL"] · TZ014(d)[ 2 ]D4.2.4—Mesocosm model
May 2017Paper3 ["HL"] · TZ13(b)[ 3 ]D4.2.4—Mesocosm model
Practice this objective
Coverage 2017–2024 · Updated 16 Jul 2026
Use Mesocosms as Controlled Ecosystem Models
A mesocosm is a contained ecosystem model used to test how a controlled variable affects stability.
| Design choice | Purpose |
|---|---|
| Sealed glass vessel preferred to an open tank | Prevents matter entering or leaving while allowing energy transfer such as light and heat |
| Aquatic or microbial community | More likely than a terrestrial system to function at small contained scale |
| Replicated control and treatment vessels | Separates the manipulated variable from background variation |
| Repeated abiotic and biotic measurements | Tracks stability, disturbance and recovery through time |
Replicated sealed aquatic mesocosms can receive different light treatments while temperature, starting organisms and nutrient quantities are held constant.
A mesocosm supports causal inference about its model conditions but does not reproduce every migration, weather event or interaction in a natural ecosystem; it also requires ethical care and maintenance.
Mesocosm model
Assessment in practice
1–2 marksHow it is assessed
This objective is assessed through essay response, commonly using Discuss / Outline / State.
Command terms
Discuss / Outline / State / Suggest / Explain
What earns marks
Build the answer around this relationship: A sealed mesocosm restricts matter exchange but can still exchange energy with its surroundings.
Watch for
Assuming a sealed mesocosm exchanges no energy, when light or heat can still pass between the system and surroundings.
Representative question
Question 1
Mesocosm experiments using water from Narragansett Bay were completed in the laboratory during a six month period. Discuss advantages and limitations of carrying out mesocosm investigations. be marked.
Core Stability and Change
Core D4.2 is secure when students can judge whether a system is being stabilized or pushed toward change. The route is: identify the stability support or disturbance, explain the mechanism, and state the ecosystem consequence using evidence.
- energy, nutrient cycling, diversity, and tolerance ranges maintain persistence
- Amazon deforestation and keystone removal can push systems toward instability
- harvest and agriculture require recovery, soil, nutrients, biodiversity, and monitoring
- eutrophication, biomagnification, and plastics harm ecosystems through specific mechanisms
Ecosystem Stability and Human Impact
Core transfer questions ask students to explain why an ecosystem remains stable or why a disturbance pushes it toward change. Strong answers do not list threats; they explain mechanisms such as lost rainfall recycling, trophic cascade, overharvest, nutrient enrichment, toxin biomagnification, plastic movement, or restoration through rewilding.
- Use stability requirements: energy input, nutrient cycling, biodiversity, genetic diversity, and abiotic tolerance ranges.
- Explain disturbance mechanisms such as Amazon tipping points, trophic cascades, overharvesting, agricultural damage, eutrophication, biomagnification, plastics, or rewilding.
- Support claims with evidence from controlled models, monitoring, food webs, or pollution pathways.
Concept essentials
- A sealed mesocosm restricts matter exchange but can still exchange energy with its surroundings.
- Mesocosms support replication and variable control while preserving some ecological interactions.
- Model limitations arise because natural environments vary more than small enclosed systems.