If Daintree has 180 bird species and Kosciuszko has 60, does that automatically make Daintree more biodiverse? Not necessarily. This lesson shows why biodiversity measurement needs both richness and evenness, and how Simpson's Index lets us compare communities quantitatively instead of relying on raw species counts alone.
IQ4~50 minLesson 15 of 185 MC + 3 short answer
📊
Choose how you work — type your answers below or write in your book.
Use digital mode if you want to work through the Simpson's Index example directly on-screen. Switch to book mode if you want to set up the table and formula by hand first, then return here to compare your working and conclusion.
Printable worksheet
Download this lesson's worksheet
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Commit to your first judgement before the numbers complicate it.
1. If two habitats have the same number of species, are they automatically equally biodiverse?
2. If one species dominates a community and the rest are rare, what do you predict happens to its biodiversity score?
Write your starting answer now. We will revisit it after the richness, evenness and Simpson's Index comparison.
Write your initial answer in your book, then return later to compare it with your final explanation.
Write this in your book, then revisit it later.
Saved locally
📚 Know
Key facts and definitions for Measuring Biodiversity
Relevant terminology and conventions
🔗 Understand
The concepts and principles underlying Measuring Biodiversity
How to explain the reasoning behind key ideas
✅ Can Do
Apply concepts from Measuring Biodiversity to exam-style questions
Justify answers using appropriate biological reasoning
Key Terms
community and the restrare, what do you predict happens to its biodiversity score?
How evenly individualsdistributed across the species present
Why quantitative indicesuseful but incomplete
Homeostasisthe body stays exactly the same all the time
Understanding how systems interactessential for HSC success
how balanced the communityacross those species
Know
The difference between species richness and species evenness.
The Simpson's Index formula and what higher values mean.
The main limitations of biodiversity indices.
Understand
Why richness alone can give a misleading picture of biodiversity.
Why evenness changes the ecological meaning of a community.
Why quantitative indices are useful but incomplete.
Can Do
Compare two habitats using richness and evenness.
Work through a Simpson's Index calculation logically.
Evaluate the strengths and limits of index-based biodiversity measures.
Misconceptions to Fix
✗
Wrong: Homeostasis means the body stays exactly the same all the time.
✗
Right: Homeostasis involves dynamic equilibrium — constant small adjustments around a set point.
Core Content
Key Point
Connect this concept to the broader biology framework. Understanding how systems interact is essential for HSC success.
01
Richness and Evenness
Why counting species is only the start
Methods for measuring biodiversity: richness and evenness
Species richness tells you how many species are present. Species evenness tells you how balanced the community is across those species. You need both to measure biodiversity well.
Two habitats can have the same richness but differ strongly in evenness. If one habitat has 5 species with similar population sizes, while another has 5 species but one of them dominates almost every sample, the first habitat is more even and usually judged more diverse. Richness alone misses that ecological difference.
RichnessCounts how many different species are present.
EvennessTracks how balanced the abundances are.
Main IdeaHigh richness with low evenness can still mean lower effective diversity.
Habitat
Species Richness
Pattern of Abundance
Interpretation
Habitat A
5 species
20, 19, 21, 18, 22 individuals
High evenness and strong overall diversity
Habitat B
5 species
90, 3, 2, 3, 2 individuals
Same richness, but low evenness because one species dominates
Common misconception: “more species” does not always mean “more biodiversity” if almost the whole community is still dominated by one species.
02
Simpson's Index of Diversity
A quantitative way to combine richness and evenness
Simpson's Index of Diversity turns species abundance data into a single value between 0 and 1, where values closer to 1 indicate higher diversity.
D = 1 - Σ [n(n-1) / N(N-1)]
n = number of individuals of each species
N = total number of individuals
Higher D = greater diversity
This formula works because communities dominated by one species generate larger contributions to the summed fraction, which lowers the final diversity score. More balanced communities generate a smaller sum and therefore a higher D value.
Exam tip: you do not just calculate D. You must interpret it. A higher Simpson's D means a more diverse community because individuals are less concentrated in one or a few species.
03
Worked Example and Practical Limits
From quadrat data to interpretation, then to what the index still misses
Use a worked example to see why evenness can change the biodiversity conclusion even when species counts look similar.
Species
Habitat A Count
n(n-1)
Habitat B Count
n(n-1)
Species 1
12
132
24
552
Species 2
10
90
3
6
Species 3
9
72
2
2
Species 4
11
110
1
0
Species 5
8
56
0
0
Total
50
460
30
560
Habitat A:
N = 50
N(N-1) = 50 x 49 = 2450
Σ[n(n-1)] = 460
D = 1 - (460 / 2450)
D = 1 - 0.188
D = 0.812
Habitat B:
N = 30
N(N-1) = 30 x 29 = 870
Σ[n(n-1)] = 560
D = 1 - (560 / 870)
D = 1 - 0.644
D = 0.356
Habitat A has the higher Simpson's Index, so it is more diverse. The reason is not just that it has more species represented in this sample, but that its individuals are spread more evenly across those species. Habitat B is strongly dominated by one species, which lowers its diversity score sharply.
Practical method: this kind of data often comes from a quadrat survey, where the same-sized area is sampled repeatedly in each habitat and the organisms are counted for comparison.
Why It Matters
Column B
That is why biodiversity measurement increasingly combines field surveys with broader datasets. Citizen-science platforms such as iNaturalist and records such as NSW BioNet Atlas help expand biodiversity monitoring across time and space, though their data still need careful interpretation.
Richness and Evenness
Richness = number of species.
Evenness = how evenly individuals are distributed among those species.
Simpson's Index
D = 1 - Σ[n(n-1) / N(N-1)]
Higher D means greater biodiversity.
Interpretation
Communities dominated by one species usually have lower evenness and lower D.
You must interpret what the number means, not just calculate it.
Limitations
Indices do not capture all functional or genetic diversity.
Sampling quality affects the result.
Activities
ApplyBand 3-4
Activity 01
Richness or Evenness Problem?
Pattern A - Diagnose and justify
Two grassland sites both contain 7 plant species. Site X has almost equal numbers of each species, while Site Y is dominated by one grass species. Identify which site is likely to have the higher Simpson's Index and explain why.
A strong answer should mention evenness, not just richness.
Write your reasoning in your book first, then record the concise answer here.
Write the explanation in your book, then condense it here.
EvaluateBand 4-5
Activity 02
Index Does Not Mean Everything
Pattern B - Evaluate limitations
Explain why Simpson's Index is useful for comparing habitats, but still cannot capture the full biodiversity of an ecosystem on its own.
A strong answer should mention at least one strength and two limitations.
Draft the longer response in your book, then write the distilled version here.
Write the extended evaluation in your book, then summarise it here.
Revisit Your Thinking
The key correction in biodiversity measurement is this: more species is not the whole story. A biodiversity judgment becomes stronger when richness, evenness and sampling quality are all considered together.
If your first answer relied only on species count, the upgrade is to ask how individuals are distributed and what the index does or does not capture.
Assessment
MC
Check Your Understanding
Answer first, then read the explanation
1. What does species richness measure?
What is NOT does species richness measure?
2. Two habitats have the same richness, but one has a much lower evenness. What is the best inference?
Two habitats have the same richness, but one has a much lower evenness. Identify the best inference?
3. In Simpson's Index of Diversity, what does a higher value of D indicate?
4. What is one important limitation of biodiversity indices such as Simpson's D?
What is NOT one important limitation of biodiversity indices such as Simpson's D?
5. What is the role of citizen science in biodiversity measurement?
What is NOT the role of citizen science in biodiversity measurement?
Short Answer - 10 marks
1. Distinguish between species richness and species evenness. (3 marks)
1 mark: richness defined | 1 mark: evenness defined | 1 mark: clear distinction
2. Explain why two habitats can have the same richness but different Simpson's Index values. (3 marks)
1 mark: same richness possible | 1 mark: evenness difference | 1 mark: effect on D explained
3. Assess one strength and one limitation of using Simpson's Index to measure biodiversity. (4 marks)
Q1 - A: Species richness is the number of different species present.
Q2 - B: Same richness can still produce different biodiversity if evenness differs.
Q3 - B: Higher Simpson's D means greater biodiversity in this formula.
Q4 - B: Simpson's Index does not capture all functional or genetic diversity.
Q5 - B: Citizen science contributes biodiversity records that support monitoring.
SA1: Species richness is the number of different species in a defined area. Species evenness is how evenly individuals are distributed among those species. The distinction is that richness counts how many species there are, while evenness describes the balance of their abundances.
SA2: Two habitats can have the same richness because they may contain the same number of species. However, if one habitat is dominated by one species while the other has individuals spread more evenly across all species, their evenness differs. Simpson's Index includes abundance patterns, so the more even habitat will usually have a higher D value.
SA3: One strength of Simpson's Index is that it combines richness and evenness into one quantitative measure, which makes habitat comparison more rigorous than using species counts alone. One limitation is that it does not capture all aspects of biodiversity, such as genetic diversity within species or functional diversity across ecological roles. Therefore, Simpson's Index is useful, but it should be interpreted alongside other ecological evidence rather than treated as the whole story.
AR
Rapid Recall
Say each answer aloud before moving to the next prompt
What is the difference between richness and evenness?
Why can the same richness still produce different biodiversity scores?
What does a higher Simpson's D value mean?
Why is a quadrat useful in biodiversity measurement?
What is one limitation of Simpson's Index?
How can citizen science support biodiversity monitoring?
