OCR A Level Biology: Mastering Biodiversity and Simpson's Index (Section 4.2.1 a-d)
Prior Knowledge to Recap
Before diving into biodiversity questions, ensure you understand these foundational concepts:
The three levels of biodiversity: genetic diversity (variation within species), species diversity (number and abundance of species), and habitat/ecosystem diversity (range of different habitats)
Species richness vs species evenness: richness is the total number of different species present, whilst evenness refers to how similar the abundance of each species is
Simpson's Index formula: D = 1 - Σ(n/N)² where n is the number of individuals of each species and N is the total number of all individuals
Sampling techniques: random sampling (using coordinates), systematic sampling (transects), and stratified sampling (proportional sampling across different areas)
The purpose of sampling: it's often impractical to count every organism, so representative samples provide estimates of biodiversity
Links to GCSE Content
Ecosystems and communities: understanding how organisms interact within habitats (GCSE Biology)
Variation and classification: recognising that organisms show variation both within and between species (GCSE Biology)
Mathematical skills: calculating percentages, means, and working with formulas (GCSE Maths)
Common Question Types and How to Answer Them
Let me walk you through five frequently asked questions from past OCR papers, showing you exactly what examiners are looking for.
Question 1: Defining Biodiversity
How to Answer:
The correct answer is C: the variety of genes, species and habitats.
This is a straightforward recall question testing whether you know the complete definition. Many students incorrectly choose A because they only think of species diversity, but biodiversity encompasses all three levels: genetic, species, and habitat diversity.
Mark scheme guidance: Award 1 mark for C only.
Common mistake: Option D refers only to genetic diversity within a single species, whilst option A refers only to species diversity. The complete definition of biodiversity must include all three levels.
Question 2: Calculating Simpson's Index
How to Answer:
This question tests your mathematical skills. Follow these steps systematically:
Step 1: Calculate N (total) N = 6 + 7 + 3 + 8 = 24
Step 2: Create a working table
Species n n/N (n/N)² Meadow buttercup 6 0.250 0.063 Common daisy 7 0.292 0.085 Red clover 3 0.125 0.016 Ribwort plantain 8 0.333 0.111
Step 3: Sum the (n/N)² column Σ(n/N)² = 0.063 + 0.085 + 0.016 + 0.111 = 0.275
Step 4: Subtract from 1 D = 1 - 0.275 = 0.725
Step 5: Round to 2 s.f. D = 0.73
Mark scheme guidance:
Correct answer of 0.73 = 3 marks (even without working)
Σ(n/N)² = 0.275 and 1 - Σ = 0.725 = 2 marks
Some correct values for n/N and (n/N)² = 1 mark
Error carried forward allowed if method correct
Top tip: Always add extra columns to tables for your working. This makes your calculations clearer and helps you spot errors. Keep at least 3 decimal places in your working, only round at the final answer.
Question 3: Species Richness vs Species Evenness
Find this question in the PDF: Question 4(b)(i) and (ii) (pages 4-5)
Copy and paste Question 4(b) parts (i) and (ii) from your PDF to see Table 2.1 with the butterfly data.
How to Answer:
(i) Species richness:
Answer: Area 2
Justification: Area 2 has 6 species present (including silver-studded blue), whereas Area 1 has only 5 species (silver-studded blue is absent).
Mark scheme guidance: Award 1 mark for identifying Area 2 with correct justification (more/6 species).
(ii) Species evenness:
Answer: Area 2
Justification: The range of individual numbers is smaller in Area 2 (2-11, range = 9) compared to Area 1 (0-16, range = 16), showing more even distribution of individuals across species.
Mark scheme guidance: Award 1 mark for identifying Area 2 with justification that the range of n is smaller.
Key definitions to remember:
Species richness = the number of different species present
Species evenness = how similar the population sizes are across all species (the relative abundance of each species)
Common mistake: Students often state "Area 2 has more species" without being specific. Always give the actual numbers (6 species vs 5 species) for a strong justification.
Question 4: Sampling Strategy
How to Answer:
Your answer should include three key elements:
1. Sampling strategy (1 mark): Use stratified AND random sampling
2. Explanation of proportional sampling (1 mark): The number of samples within each area should be proportional to their size
3. Specific calculation (1 mark):
Total area = 800 + 2400 + 3200 = 6400 m²
Conifer: (800 ÷ 6400) × 100 = 12.5% → 8 samples (if taking 64 total)
Marshy: (2400 ÷ 6400) × 100 = 37.5% → 24 samples
Grazed: (3200 ÷ 6400) × 100 = 50% → 32 samples
Example full answer: "The scientists should use stratified random sampling. They should divide the ecosystem into the three distinct habitat areas and take samples randomly within each one to avoid bias. The number of samples in each area should be proportional to its size. For example, if taking 64 samples in total: 8 samples in the conifer area (12.5%), 24 in the marshy area (37.5%), and 32 in the heavily grazed area (50%)."
Mark scheme guidance:
Stratified AND random (within each area) = 1 mark
Idea that number of samples should be proportional to area size = 1 mark
Correct suggestion for number of samples (e.g., 8, 24, 32) = 1 mark
Why stratified sampling? When a habitat has distinct different zones or types, stratified sampling ensures all areas are represented fairly in proportion to their size.
Question 5: Interpreting Simpson's Index Values
Copy and paste Question 35 from your PDF to see the multiple choice question about ancient woodland.
How to Answer:
The correct answer is A: Biodiversity is high.
Understanding Simpson's Index values:
The index ranges from 0 to 1
Values close to 1 = high biodiversity (many species, evenly distributed)
Values close to 0 = low biodiversity (few species or one dominant species)
A value of 0.85 is close to 1, indicating high biodiversity
Why the other options are wrong:
Option B is incorrect because 0.85 is high, not low. Values below 0.3 would typically indicate low biodiversity.
Option C is incorrect because Simpson's Index doesn't measure interspecific variation (differences between species as groups). It measures species diversity (richness and evenness).
Option D is incorrect because Simpson's Index doesn't measure intraspecific variation (genetic differences within a single species). That would be genetic diversity, measured differently.
Mark scheme guidance: Award 1 mark for A only.
Critical concept: Don't confuse the types of variation and biodiversity:
Genetic biodiversity = variety of alleles within and between populations (intraspecific)
Species biodiversity = variety and abundance of species (what Simpson's Index measures)
Habitat biodiversity = variety of different habitats in an area
Simpson's Index specifically measures species biodiversity by combining species richness (how many species) and species evenness (how evenly distributed).
Additional Exam Technique Tips
For multiple choice questions: Eliminate obviously wrong answers first. Often you can narrow it down to two options, then think carefully about the precise definitions.
For calculations:
Always show your working in a clear table format
Don't round intermediate values too early (keep 3+ decimal places)
Only round your final answer to the specified number of significant figures
Even if you get the wrong answer, clear working can earn method marks
For "justify" questions: Simply restating the question isn't enough. You must provide specific evidence from the data (e.g., actual numbers, ranges, or calculations).
For sampling questions: Always specify:
The type of sampling (random/systematic/stratified)
How you'd implement it (coordinates, transects, etc.)
The number of samples and why
Time management: These questions appear throughout the papers. Don't spend too long on 1-mark multiple choice questions. If unsure, make an educated guess and move on.
By practising with actual past paper questions in their original format, you'll become familiar with the exam style and what examiners expect