Biodiversity
Cambridge A-Level Biology (9700) · Unit 18: Classification, biodiversity and conservation · 18 flashcards
Biodiversity is topic 18.2 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 18 — Classification, biodiversity and conservation , alongside Classification and Conservation. In one line: An ecosystem is a dynamic complex of plant, animal, and microorganism communities and their non-living environment interacting as a functional unit.
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 18 flashcards — 5 definitions, 11 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 5 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The term 'ecosystem'
An ecosystem is a dynamic complex of plant, animal, and microorganism communities and their non-living environment interacting as a functional unit.
What the Cambridge 9700 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Define the terms niche and ecosystem.
- Explain that biodiversity can be measured at several levels, including: how many different ecosystems and habitats there are and how wide a range they cover; how many species there are and how abundant each is relative to the others; and the genetic variation found inside each species.
- Explain why random sampling matters when working out how biodiverse an area is.
- Describe and use appropriate methods for assessing how organisms are distributed in an area and how abundant they are, limited to: frame quadrats; line and belt transects; mark-release-recapture with the Lincoln index (you will be given the Lincoln index formula, as set out in the Mathematical requirements).
- Use Pearson's linear correlation and Spearman's rank correlation to analyse how two variables are related, including how biotic and abiotic factors influence the abundance and distribution of species (formulae will be provided, as set out in the Mathematical requirements).
- Calculate an area's biodiversity using Simpson's index of diversity (D), and state what different values of D indicate (the formula will be provided, as set out in the Mathematical requirements).
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Biodiversity
- › Use a belt transect with quadrats at regular intervals when investigating how species distribution changes across a clear environmental gradient.
Define the term 'ecosystem'.
An ecosystem is a dynamic complex of plant, animal, and microorganism communities and their non-living environment interacting as a functional unit.
Define the term 'niche'.
A niche describes the functional role of an organism within its ecosystem, including its habitat, resource use, and interactions with other species.
Describe three levels at which biodiversity can be assessed.
Biodiversity can be assessed by examining the number and range of ecosystems and habitats, the number of species and their relative abundance, and the genetic variation within each species. A high biodiversity area demonstrates considerable variability at all three levels.
Explain the importance of random sampling in determining biodiversity.
Random sampling eliminates bias, ensuring that the sample is representative of the whole area. This increases the accuracy and reliability of biodiversity estimates, enabling more valid comparisons between different areas or over time.
Describe how a frame quadrat is used to assess the abundance of organisms.
A frame quadrat is a square frame of known area used to sample organisms in a habitat. The quadrat is placed randomly, and the number or percentage cover of each species within the frame is recorded. This is repeated multiple times to obtain an average density or frequency.
Distinguish between a line transect and a belt transect.
A line transect involves recording all the species that touch a line laid across a habitat, providing a quick but limited survey. A belt transect involves sampling organisms within a defined width (e.g. 1 meter) along the transect line, providing a more detailed survey of abundance.
Describe the mark-release-recapture method for estimating population size.
A sample of organisms is captured, marked, and released back into the population. After a period, a second sample is captured. The proportion of marked individuals in the second sample is used to estimate the total population size.
State the Lincoln index formula and define each term.
The Lincoln index is: N = (n₁ × n₂) / m₂, where N = estimated total population size, n₁ = number of individuals captured and marked in the first sample, n₂ = number of individuals captured in the second sample, and m₂ = number of marked individuals recaptured in the second sample.
What are the assumptions made when using the Lincoln index?
The Lincoln Index assumes: marked individuals mix randomly within the population; there is no immigration or emigration; there are no births or deaths; marking does not affect survival or behaviour; and marks are not lost or overlooked.
What does Spearman's rank correlation measure?
Spearman's rank correlation measures the strength and direction of the monotonic relationship between two ranked variables. It assesses how well the relationship between two variables can be described using a monotonic function.
Describe how Spearman's rank correlation can be used to analyze the impact of an abiotic factor on species distribution.
Spearman's rank correlation can assess if there is a relationship between the rank of an abiotic factor (
What does Pearson's linear correlation measure?
Pearson's linear correlation measures the strength and direction of a linear relationship between two continuous variables. It indicates how well the data points fit a straight line.
Explain how Pearson's linear correlation can be used to analyse the impact of a biotic factor on another species' distribution.
Pearson's correlation tests whether there is a linear relationship between the abundances of two species measured at the same sampling points, both normally distributed. A significant positive correlation may suggest the species depend on similar conditions or on each other (e.g. mutualism). A negative one may suggest competition, but correlation alone does not prove cause.
What is Simpson's Index of Diversity (D)?
Simpson's Index of Diversity (D) is a measure of biodiversity that takes into account both the number of species present and their relative abundance. A higher D value indicates greater diversity.
Write the formula for Simpson's Index of Diversity (D).
D = 1 - Σ(n/N)², where n is the number of individuals of a particular species and N is the total number of all individuals of all species.
What does a Simpson's Index of Diversity (D) value close to 1 indicate?
A D value close to 1 indicates high biodiversity. It suggests that the habitat has a relatively even distribution of species and is likely stable and resilient to environmental changes.
What does a Simpson's Index of Diversity (D) value close to 0 indicate?
A D value close to 0 indicates low biodiversity. This suggests that the habitat is dominated by a few species and may be vulnerable to disturbances or disease.
Explain how abiotic factors can affect species abundance, and how this could be analyzed.
Abiotic factors such as temperature, pH, and light intensity can directly affect an organism's survival and reproduction, thus impacting abundance. Data can be collected and analyzed using Spearman's rank to test if there is a correlation between factor values and abundance.
Review the material
Read full revision notes on Biodiversity — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 18 — Classification, biodiversity and conservation
Biodiversity sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Biodiversity deck
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