Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.11.2Additional lessons

Ecosystem Stability: Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 93 of 101

📚 IB Biology HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
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.
0%read

Part 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.
FeatureMesocosmNatural ecosystem
Control of variablesHighLow
SizeSmallLarge
Species numberFewMany
RealismLimitedComplete

Part 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.

Part 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.

Part 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.

  1. N = 10 + 6 + 4 = 20, so N(N − 1) = 20 × 19 = 380.
  2. Σn(n − 1) = (10 × 9) + (6 × 5) + (4 × 3) = 90 + 30 + 12 = 132.
  3. D = 380 ÷ 132 = 2.88 (to 3 s.f.).

Answer D ≈ 2.88

Part 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