4.11.2Additional lessons
Ecosystem Stability: Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 93 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Mesocosms
Mesocosms help to test ideas about stability safely and cheaply.
Definition
- Definition A mesocosm is a small, enclosed, controlled model of an ecosystem, such as a sealed aquarium or a garden bottle.
Purpose
- Purpose It lets investigators change one variable, such as light or nutrients, while holding others constant.
Measurements
- Measurements Gas levels, species counts, biomass and water clarity can be recorded over weeks or months.
Limitations
- Limitations The small size and simple community mean results may not match what happens in a real ecosystem.
| Feature | Mesocosm | Natural ecosystem |
|---|---|---|
| Control of variables | High | Low |
| Size | Small | Large |
| Species number | Few | Many |
| Realism | Limited | Complete |
Section 2 of 5Calculating Simpson's Reciprocal Index
1
Count the number of individuals, n, of each species in the sample and the total, N.
2
For each species work out n(n − 1), then add the values to get Σn(n − 1).
3
Use D = N(N − 1) ÷ Σn(n − 1) to find the index.
4
A higher D shows a richer and more even community, which is usually linked with greater stability.
5
Repeat after a disturbance to see whether diversity recovers.
Section 3 of 5Interpreting Stability Data
Interpreting Stability Data
Trend over time
- Trend over time Describe fluctuations in population or oxygen concentration, quoting the highest and lowest values.
Recovery
- Recovery Check whether values return to the starting level after a disturbance, and how long this takes.
Comparison
- Comparison Compare treated and control mesocosms to link a change to the variable that was altered.
Evaluation
- Evaluation Comment on replicates, duration and how well the model represents a real ecosystem.
Section 4 of 5Worked Example
Question
- A sample has 10 individuals of species A, 6 of B and 4 of C.
- Calculate Simpson's reciprocal index.
- N = 10 + 6 + 4 = 20, so N(N − 1) = 20 × 19 = 380.
- Σn(n − 1) = (10 × 9) + (6 × 5) + (4 × 3) = 90 + 30 + 12 = 132.
- D = 380 ÷ 132 = 2.88 (to 3 s.f.).
Answer D ≈ 2.88
Section 5 of 5Exam Checklist & Tips
4.11.2 – Key Points for the Exam
- Mesocosm A contained and controlled model ecosystem used for experiments.
- Simpson's reciprocal index A measure of biodiversity that accounts for both richness and evenness.
- Species richness The number of different species in a community.
- Evenness How similar the abundances of the species are.
- Replicate A repeat of an experimental set-up to improve reliability.
Common Mistakes
- Using Simpson's index without squaring correctly; the formula uses n(n − 1), not n².
- Believing a mesocosm can model a real ecosystem exactly.
- Forgetting that a low index in one sample does not prove low diversity in the whole area.
- Mixing up species richness and diversity.
Last updated:
\n