Measuring biodiversity at HL: Measuring biodiversity — richness, evenness and diversity indices — works exactly the same for SL and HL. You'll reuse these measures at HL when you study biodiversity hotspots and judge how well conservation is working.
No HL-only material to add — just learn how diversity is measured.
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📊 Measuring biodiversity
Big idea: Biodiversity can be measured, not just described. Measuring biodiversity helps scientists compare ecosystems and track change over time.
🌿 What does species diversity mean?
Species diversity looks at how many species live in an area and how evenly individuals are shared between them.
- Richness
- Evenness
- High diversity needs BOTH richness and evenness
Lots of species ❌ if one species dominates → diversity is still low.
🌻 Richness vs evenness (simple example)
Imagine a field with many lavender plants and just one sunflower.
- There is more than one species → richness is greater than 1
- But almost all individuals are lavender → low evenness
- The sunflower is unlikely to reproduce → low long-term diversity
Rich but uneven = still low biodiversity.
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🧮 Simpson's Reciprocal Index (D)
Big Idea: Simpson's Reciprocal Index turns biodiversity into a single number.
It tells you how diverse an ecosystem is by looking at how many species there are AND how evenly they're spread out.
Think of it like a classroom: if 30 students are in a room but 28 speak English and only 2 speak Spanish, there's low language diversity. But if 15 speak English and 15 speak Spanish, diversity is higher—even though the total is the same!
📐 The Formula
Don't panic! Let's break this down into plain English.
- D = the diversity score (higher = more diverse)
- N = total number of ALL individuals (count everything!)
- n = number of individuals of ONE species
- Σ (sigma) = "add up" – you do this for each species
Higher D = more biodiversity. The minimum D is 1 (only one species). There's no maximum!
🌻 Worked Example 1: Uneven Field (Low Diversity)
Imagine a field with 9 daisies and 1 sunflower.
- Step 1: Count the total (N) → 9 + 1 = 10
- Step 2: Calculate N(N-1) → 10 × 9 = 90
- Step 3: For each species, calculate n(n-1):
- • Daisies: 9 × 8 = 72
- • Sunflowers: 1 × 0 = 0
- Step 4: Add them up → 72 + 0 = 72
- Step 5: Divide → D = 90 ÷ 72 = 1.25
Result: D = 1.25 → This is LOW diversity. Almost all plants are daisies, so the ecosystem isn't very diverse.
🌸 Worked Example 2: Even Field (High Diversity)
Now imagine the same field with 5 daisies and 5 sunflowers.
- Step 1: Count the total (N) → 5 + 5 = 10
- Step 2: Calculate N(N-1) → 10 × 9 = 90
- Step 3: For each species, calculate n(n-1):
- • Daisies: 5 × 4 = 20
- • Sunflowers: 5 × 4 = 20
- Step 4: Add them up → 20 + 20 = 40
- Step 5: Divide → D = 90 ÷ 40 = 2.25
Result: D = 2.25 → This is HIGHER diversity! Same total plants, but they're spread more evenly between species.
🔍 Comparing the Two Examples
Uneven Field
- 9 daisies, 1 sunflower
- Total: 10 plants
- D = 1.25
- Low diversity
Even Field
- 5 daisies, 5 sunflowers
- Total: 10 plants
- D = 2.25
- Higher diversity
Same number of plants, but different D values! Evenness matters just as much as the total.
📐 The Formula
🌲 Worked Example 3: Three Species
A pond has 10 frogs, 10 fish, and 10 snails.
- Step 1: N = 10 + 10 + 10 = 30
- Step 2: N(N-1) = 30 × 29 = 870
- Step 3: Each species: n(n-1) = 10 × 9 = 90 (×3)
- Step 4: Sum = 90 + 90 + 90 = 270
- Step 5: D = 870 ÷ 270 = 3.22
Result: D = 3.22 → Even higher! More species AND perfect evenness = high biodiversity.
💡 What D Values Mean
- D = 1 → Only one species (no diversity at all)
- D = 1–2 → Low diversity (one species dominates)
- D = 2–5 → Moderate diversity
- D > 5 → High diversity (many species, evenly spread)
In exams, you might need to calculate D or explain why one ecosystem has a higher D than another. Always mention evenness!
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🧪 Why measuring biodiversity matters
Measuring biodiversity helps scientists and conservationists make decisions.
- Compare different habitats objectively
- Monitor changes over time (is diversity increasing or decreasing?)
- Identify ecosystems under threat
- Evaluate if conservation efforts are working
IB-style question — Calculate Simpson's Reciprocal Index
A student surveyed invertebrates in two 1 m² quadrats in a coastal sand-dune system. In quadrat A, she counted: sand-hopper (n = 30), velvet mite (n = 10), ground beetle (n = 20), rove beetle (n = 15), springtail (n = 25). Total N = 100. Using the formula D = N(N−1) / Σn(n−1), calculate Simpson's Reciprocal Diversity Index for quadrat A. Show your working. [2]
How to answer it, step by step
- Calculate Σn(n−1)
• 30×29=870, 10×9=90, 20×19=380, 15×14=210, 25×24=600
• Σn(n−1) = 870+90+380+210+600 = 2150 - Apply the formula
• D = 100×99 / 2150 = 9900 / 2150 ≈ 4.60
Final answer
Always show the substitution line — a wrong answer with no working scores 0; correct working with an arithmetic slip still earns 1 mark.
IB-style question — Outline how to measure species diversity in an ecosystem
Outline the procedure a field ecologist would use to measure species diversity in a shrubland ecosystem, naming an appropriate index and explaining how data would be collected. [4]
How to answer it, step by step
- Collect data with random quadrats
• Place quadrats (e.g. 1 m²) at randomly selected positions to avoid bias.
• In each quadrat, count the number of individuals of each distinguishable species. - Calculate an index
• Use Simpson's Reciprocal Index: D = N(N−1) / Σn(n−1), where N = total individuals, n = individuals per species.
• Higher D = greater diversity.
Final answer
Full marks need: a named index with symbols defined, AND a valid random/systematic sampling method — 'count species in quadrats' alone scores ≤ 2.