Classification, biodiversity and sampling
Classify marine organisms, explain biodiversity, measure populations and use sampling and statistics to investigate distribution.
Section 4 · Classification, biodiversity and sampling
Exam scope and assessment notes
This lesson maps Cambridge International AS & A Level Marine Science 9693 sections 4.1, 4.2, 4.3, 4.4 for examinations in 2025–2030. AS candidates study Topics 1–5; A Level candidates study all nine topics and are expected to know the AS content. These are original StudyForA explanations and practice, not Cambridge questions, mark schemes or grades. The syllabus requires core practical activities and scientific-method experience. Online explanation and simulated data questions can rehearse methods but do not count as completing practical work or demonstrate safe, competent use of equipment. Learners need suitable teacher-supervised laboratory and field opportunities.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before you start: What does a species-richness count measure?
What you will learn
- Use classification and identification features to compare key marine groups
- Explain genetic, species and ecological diversity and its benefits
- Select ethical random or systematic sampling methods for a question
- Calculate and interpret Simpson’s index and Spearman’s rank correlation
Classification and marine groups
Classification groups organisms by shared characteristics and evolutionary relationships. Use observable features and a dichotomous key to identify organisms; a label should follow the evidence, not habitat alone. Compare representative groups such as algae, plankton, corals, molluscs, crustaceans, echinoderms, fish, marine reptiles, birds and mammals. For each named group, identify defining structures, feeding or locomotion and the environmental conditions it occupies.
Biodiversity
Biodiversity can be genetic (variation within a species), species diversity (species number and relative abundance) and ecological (variety of ecosystems). Marine biodiversity supports ecosystem stability, coastal protection, climate regulation, food and medicines. Species richness counts types; diversity also reflects how individuals are distributed among them. Simpson’s index D uses the equation supplied in the examination; interpret higher or lower values according to the formula printed in the question, because conventions differ.
Populations, sampling and analysis
Choose random quadrats to estimate abundance in an area and systematic transects to examine a gradient. Frame quadrats, line/belt transects and mark-release-recapture suit different organisms and settings. Record sample size, location, tide and method; use ethical, safe protocols. Simpson’s index compares species diversity. Spearman’s rank tests the association between ranked biotic/abiotic variables; values near +1 or −1 indicate stronger positive/negative rank association, and near 0 little association. Correlation alone does not establish cause.
Sources for this lesson
Pause and explain
What does a Spearman rank coefficient near −1 indicate?
Worked example
A survey records 6 species in 10 quadrats. Explain why systematic transects may be more suitable than randomly placed quadrats if the question is how abundance changes with shore height.
Show the worked solution
- Identify shore height as a spatial gradient.
- Use a transect to sample at defined positions along that gradient.
- Repeat transects and quadrat counts to assess variation.
Answer Systematic transects preserve the position along the shore gradient, so abundance can be compared with height. Repeated transects and quadrats improve reliability; random quadrats alone may not sample the gradient evenly.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A study records organism counts at 0, 2, 4, 6 and 8 m up a shore. Propose a sampling plan to test whether abundance changes with height.
Give me a hint
Keep positions, area and effort comparable, and repeat the transect.
Compare my reasoning
- Run several transects perpendicular to the shoreline.
- Place equal-sized quadrats at the same measured heights on each transect.
- Identify organisms consistently and compare counts or distributions by height.
Use repeated transects from low to high shore and place equal-sized quadrats at fixed, recorded heights on each. Keep identification and counting effort consistent, record tidal timing, and report variation among transects. Follow local safety and organism-protection rules.
Look for these in your work
- Uses systematic positions along the gradient
- Controls quadrat size and effort
- Includes repeats and safe field practice
Two sites have the same number of species. Site A has similar numbers of each species; Site B is dominated by one species. Explain why their diversity may differ.
Give me a hint
Separate richness from the relative abundance of species.
Compare my reasoning
- State that species richness is equal if both sites contain the same number of species.
- Compare evenness or relative abundance.
- Explain that an index incorporating abundance can distinguish the sites.
The sites have equal richness, but Site A is more even. A diversity index that incorporates relative abundance may therefore give Site A greater diversity; calculate using the formula and convention provided in the question.
Look for these in your work
- Distinguishes richness from evenness
- Uses relative abundance
- Checks the index definition
Spearman’s rs between wave exposure and barnacle abundance is −0.78. State the pattern and explain one reason it does not prove that waves caused the abundance pattern.
Give me a hint
Interpret strength and direction, then identify confounding or alternative variables.
Compare my reasoning
- Read the negative sign as an inverse rank association.
- Describe the magnitude as a strong association, with caution about context.
- Name a possible correlated factor such as desiccation, substrate or predation.
The data show a strong negative rank association: sites ranked higher for exposure tend to rank lower for barnacle abundance. This does not prove causation; substrate, desiccation, predators or sampling conditions may contribute.
Look for these in your work
- States direction correctly
- Interprets association without overclaiming
- Provides a plausible alternative factor
Common mistakes
- Confusing species richness with abundance
- Using random sampling to answer a directional-gradient question
- Interpreting a Simpson value without checking its stated convention
- Treating correlation as a cause-and-effect result
What can you explain now?
What does a Spearman rank coefficient near −1 indicate?
Compare with the explanation
A strong negative association between the ranked variables
State the mechanism, use relevant evidence and include suitable units. Explain what the evidence cannot establish.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
When is a transect more informative than a set of randomly placed quadrats?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.