📊 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.
🧮 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.
Get feedback like a real examiner
Submit your answers and get instant feedback — what you did well, what's missing, and exactly what to write to score full marks.
📐 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!
🧪 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
Learn what examiners really want
See exactly what to write to score full marks. Our AI shows you model answers and the key phrases examiners look for.
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 each n(n−1)
• Sand-hopper: 30 × 29 = 870
• Velvet mite: 10 × 9 = 90
• Ground beetle: 20 × 19 = 380
• Rove beetle: 15 × 14 = 210
• Springtail: 25 × 24 = 600
• Σn(n−1) = 870 + 90 + 380 + 210 + 600 = 2150 - Apply the formula
• D = N(N−1) / Σn(n−1)
• D = 100 × 99 / 2150
• D = 9900 / 2150
• D ≈ 4.60
Final answer
Award [1] for correct final answer (≈ 4.60); award [1 max] for correct working even if arithmetic error. Always show the substitution line — if you write only the answer with no working and it is wrong, you score 0.
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
- Define species diversity
• Species diversity combines species richness (number of species present) AND evenness (relative abundance of each species) — both components must be mentioned. - Sampling method
• Place quadrats (e.g. 1 m² frames) at randomly or systematically selected positions across the study area to avoid sampling bias.
• Take a sufficient number of quadrats (e.g. ≥ 10) so results are statistically reliable.
• Within each quadrat, count the number of individuals of each distinguishable species (species do not need to be formally named, only distinguished). - Choose an index and calculate
• Use Simpson's Reciprocal Diversity Index: D = N(N−1) / Σn(n−1), where N = total individuals and n = individuals of each species.
• Pool data from all quadrats, then substitute into the formula.
• Higher D indicates greater diversity.
Final answer
Full marks require: naming the index, defining all formula symbols, AND describing a valid random/systematic sampling method. Writing only 'count species in quadrats' without addressing evenness or the index scores ≤ 2.