Ch 2 · 18.2 Biodiversity · AS & A Level Biology 9700
Topic 2 of 3 in Classification, biodiversity and conservation — AS & A Level Biology 9700 — 87 lessons.
Introduction
Biodiversity describes the variety of life at several levels, from genes to whole habitats, and is central to A2 conservation topics. Cambridge examiners expect a precise definition covering all three levels of biodiversity, fluent use of Simpson's index of diversity with the correct formula and units, and a clear account of sampling methods, and the use of correlation to link species to their environment. (Syllabus section 18.2.)
Key ideas — Ecosystems and niches
An ecosystem is a relatively self-contained, interacting community of organisms (all the populations of all the species in an area) together with the environment in which they live and with which they interact, e.g. a pond or a forest. A niche is the role of a species in its ecosystem: how it gets its energy and nutrients, where it lives, when it is active, and how it interacts with other species and with the physical environment. Two species cannot share exactly the same niche for long; one will out-compete the other.
🧑🏫 Teacher's explanation
Imagine a busy city. The city itself, including the buildings, the weather, and all the people living there, is like an ecosystem. Every person has a specific 'job' or 'role'—a baker, a bus driver, a student. This specific role, including where they work and how they interact with others, is their niche. If two people tried to do the exact same job in the exact same way in the exact same shop, one would eventually replace the other.
📖 Definitions
🔍 Worked example — Easy
A student uses the Lincoln index to estimate a population of snails. 15 snails were caught, marked, and released. A week later, 25 snails were caught, and 5 of these were marked. Calculate the estimated population size (N).
🔍 Worked example — Standard
In a sample of a pond, there are 4 species of aquatic invertebrates. The number of individuals for each species is: Species A = 10, Species B = 20, Species C = 5, Species D = 5. Calculate Simpson's index of diversity (D) using the formula D = 1/Σ x(x-1), where x is the number of individuals of each species.
🔍 Worked example — Exam-style
A researcher wants to determine if there is a relationship between soil pH and the abundance of a specific fern. They collect data for 10 sites. The Spearman's rank correlation coefficient (r_s) is calculated to be 0.85. Interpret this result.
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🎯 Exam question
Key ideas — Levels of biodiversity
Biodiversity is the variety of life and is assessed at three levels. Ecosystem and habitat diversity is the number and range of different ecosystems and habitats in an area (for example a coastline may include rock pools, sand dunes and salt marsh). Species diversity is the number of different species, and their relative abundance, in an area. Genetic diversity is the variety of alleles within a species, or within a population of that species, which determines its capacity to adapt to environmental change. All three levels must be considered together, since an area with many species but low genetic diversity within each is still vulnerable to environmental change.
🧑🏫 Teacher's explanation
Biodiversity is not just a count of how many different animals you see in a park; it is a multi-layered measurement of life's complexity. Think of it like a library: you can measure it by how many different buildings (ecosystems) the library system has, how many different genres of books (species) are on the shelves, or even how many different versions/editions (genetic variation) of a specific book exist.
📖 Definitions
🔍 Worked example — Standard
In a field, a researcher uses the Lincoln Index to estimate a population of snails. 50 snails were marked and released. Later, 100 snails were captured, and 10 of them were marked. Calculate the estimated population size (N) using the formula N = M × C/R, where M is the number marked initially, C is the total caught in the second sample, and R is the number of marked individuals recaptured.
🔍 Worked example — Exam-style
An ecologist wants to determine if there is a relationship between soil pH (abiotic factor) and the abundance of a specific lichen species. They collect data for 10 sites. The Spearman's rank correlation coefficient (r_s) is calculated to be 0.85. Interpret this value in the context of biodiversity and correlation.
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🎯 Exam question
(a) Calculate the Simpson's Index of diversity (D) for the following data: Species A: 40 individuals Species B: 40 individuals Species C: 10 individuals Species D: 10 individuals Total (N) = 100
Key ideas — Species richness and index of diversity
Species richness is simply the number of different species present in a habitat; it ignores how many individuals of each species there are. Two habitats can have identical species richness but very different evenness (how equally individuals are distributed among species): a habitat dominated by one species is less diverse than one where abundance is spread evenly. An index of diversity combines richness and evenness into a single number, giving a more complete measure of biodiversity than richness alone, and allows valid comparison between habitats or between the same habitat over time.
🧑🏫 Teacher's explanation
Imagine you are looking at two different classrooms. In Classroom A, there are 10 students, but all 10 are wearing blue shirts. In Classroom B, there are 10 students, but 2 are wearing red, 2 are wearing blue, 2 are wearing green, 2 are wearing yellow, and 2 are wearing white. If you only count 'how many different colors are present' (species richness), both rooms have a score of 1 and 5 respectively. But if you want to know how 'diverse' the group is, Classroom B is clearly more varied because the individuals are spread out across many groups rather than being concentrated in one.
📖 Definitions
🔍 Worked example — Easy
In a small pond, you find 3 species: Algae (15 individuals), Water snails (5 individuals), and Dragonfly larvae (5 individuals). Calculate the Simpson's index of diversity (D) using the formula D = N^2/Σ n^2, where N is the total number of individuals and n is the number of individuals of each species.
🔍 Worked example — Standard
Compare two habitats. Habitat X has 2 species with 10 individuals each. Habitat Y has 2 species, one with 18 individuals and one with 2 individuals. Calculate D for both and explain which is more diverse.
🔍 Worked example — Exam-style
An ecologist finds 4 species in a forest plot: Species A (50), Species B (30), Species C (15), and Species D (5). Calculate D and interpret the result in terms of biodiversity.
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🎯 Exam question
Key ideas — Simpson's index of diversity
Cambridge uses Simpson's index of diversity:
🧑🏫 Teacher's explanation
Imagine you are looking at two different fruit bowls. In Bowl A, you have 10 apples and 1 orange. In Bowl B, you have 5 apples, 3 oranges, and 2 bananas. Even though both bowls have 11 pieces of fruit, Bowl B is much more 'diverse' because the fruit is spread out more evenly. If you only have one type of fruit, your diet is very vulnerable to a single disease; if you have many, you are safer.
📖 Definitions
🔍 Worked example — Easy
Calculate Simpson's index of diversity (D) for a pond containing 15 tadpoles, 5 water beetles, and 5 dragonfly nymphs. Use the formula D = 1 - Σ ( n/N )^2.
🔍 Worked example — Standard
A field contains 40 grasshoppers, 10 butterflies, and 10 ants. Calculate the Simpson's index of diversity (D).
🔍 Worked example — Exam-style
In Habitat X, there are 50 individuals of Species A and 50 individuals of Species B. In Habitat Y, there are 99 individuals of Species A and 1 individual of Species B. Compare the Simpson's index of diversity (D) for both habitats and explain the biological significance.
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Species A: 20 individuals Species B: 15 individuals Species C: 5 individuals
Key ideas — Sampling with quadrats and transects
Frame quadrats are square frames of known area placed on the ground to record the species present, and their percentage cover or number of individuals, within that area; results from many quadrats are used to estimate species richness and abundance across a whole habitat. Transects are lines laid across a habitat, often where an environmental gradient exists (for example from a shore into a woodland); a line transect records species touching the line, while a belt transect places quadrats at set intervals along the line, giving quantitative data on how species composition and abundance change along the gradient.
🧑🏫 Teacher's explanation
Imagine you want to know how many different types of candy are in a giant, messy ball pit. You can't pick up every single piece, or you'll spend all day! Instead, you take a small scoop (a quadrat) from several random spots. If you only take scoops from the top, you might only find the big candies, which wouldn't represent the whole pit. This is why we use 'random sampling'—to avoid bias and get a fair picture of the whole area.
📖 Definitions
🔍 Worked example — Easy
In a field, a student uses a quadrat to count individuals of three species: Grass (n_1=50), Clover (n_2=30), and Daisies (n_3=20). Calculate Simpson's Index of Diversity (D) using the formula D = 1/Σ n(n-1).
🔍 Worked example — Standard
A biologist captures 40 snails in a garden (first sample). After marking them and releasing them, a second sample of 50 snails is caught. Calculate the estimated total population size (N) using the Lincoln Index: N = M × n/m, where M is the number marked, n is the second sample size, and m is the number of marked individuals in the second sample. Assume m=5.
🔍 Worked example — Exam-style
A student uses a belt transect to study plant distribution along a salinity gradient. They record the percentage cover of Species A at 5 locations: 10\%, 20\%, 30\%, 40\%, 50\%. They want to see if there is a linear relationship between distance and cover. If the Spearman's rank correlation coefficient (r_s) is calculated to be 0.99, interpret this value.
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Key ideas — Random sampling
Sampling positions must be chosen using random sampling, for example by laying a grid over a map of the habitat and generating coordinates with random numbers, to decide where each quadrat is placed. This avoids sampling bias, such as a person unconsciously placing quadrats where a particular species looks abundant, and ensures that every part of the habitat has an equal chance of being sampled. Random sampling of a sufficiently large number of quadrats allows the resulting data to be treated statistically and compared validly between habitats or between sampling dates.
🧑🏫 Teacher's explanation
Imagine you want to know if a large bag of mixed sweets is mostly chocolate or mostly fruit chews. If you only reach into the top layer where you see lots of chocolate, your conclusion will be biased. To get a true picture, you need to reach into different parts of the bag without looking. In biology, we can't count every single organism in a forest or an ocean—it's physically impossible. Instead, we use small 'samples' to represent the whole.
📖 Definitions
🔍 Worked example — Easy
A student wants to estimate the abundance of clover in a 100 m^2 field. They use a 1 m^2 quadrat. If they find an average of 12 clover plants per quadrat, calculate the estimated total number of clover plants in the field.
🔍 Worked example — Standard
A researcher uses a grid system to select 20 random coordinates in a 50 m × 50 m woodland to place quadrats. Why is this method preferred over placing quadrats where the researcher sees the highest density of ferns?
🔍 Worked example — Exam-style
A scientist is studying biodiversity. They use 50 quadrats of 0.5 m^2 each. If the total area being studied is 2500 m^2, calculate the percentage of the total area that has been sampled.
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Key ideas — Mark-release-recapture
Quadrats work for plants and slow animals. For mobile animals (beetles, woodlice, fish, small mammals) use mark-release-recapture:
🧑🏫 Teacher's explanation
Imagine you want to know how many jellybeans are in a giant jar, but you can't count them all. Instead, you take out a handful, mark them with a tiny dot of ink, and put them back. You shake the jar so they mix well. Then, you take a second handful. If a huge proportion of your second handful has ink on them, you know there were a lot of jellybeans to begin with. If only one has ink, the jar must be massive.
📖 Definitions
🔍 Worked example — Easy
A biologist captures 60 snails in a garden. Each snail is marked with a tiny dot of non-toxic paint and released. A week later, a second sample of 40 snails is caught, and 12 of these are marked. Calculate the estimated population size (N).
🔍 Worked example — Standard
In a pond, 150 fish are caught, marked, and released. In a subsequent sampling, 200 fish are caught, of which 30 are marked. Estimate the population size.
🔍 Worked example — Exam-style
A researcher estimates a population of woodlice using mark-release-recapture. The first sample contains 80 individuals. The second sample contains 100 individuals, but only 5 are marked. Calculate the estimated population size and explain why the estimate might be an overestimate if the mark makes the woodlice more visible to predators.
⚠️ Common errors
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🎯 Exam question
(a) Calculate the estimated population size of the beetles. [2] (b) State one reason why the actual population might be smaller than the estimate calculated in (a). [1] [3 marks]
Key ideas — Correlation: Spearman's rank and Pearson's
To test whether the distribution or abundance of a species is linked to a biotic factor (e.g. number of grazing animals) or an abiotic factor (e.g. light intensity, soil pH, distance from the sea), collect paired data (often along a transect) and calculate a correlation coefficient. It ranges from −1 (perfect negative correlation) through 0 (none) to +1 (perfect positive correlation). Correlation does not prove that one variable causes the other.
🧑🏫 Teacher's explanation
Imagine you are looking at two things that seem to change together, like the height of plants and the amount of sunlight they get. If they both increase together, they are 'correlated'. Correlation is like noticing that every time it rains, people carry umbrellas; it shows a relationship, but it doesn't prove that the umbrellas *cause* the rain.
📖 Definitions
🔍 Worked example — Easy
A student measures light intensity (lux) and the number of daisies in a field. The data for n=3 pairs is: (10, 5), (50, 15), (100, 25). Calculate Spearman's rank correlation coefficient (r_s).
🔍 Worked example — Standard
A biologist wants to know if there is a significant relationship between soil pH and the abundance of a specific fungus. They calculate r_s = 0.85 for n=10 pairs. The critical value for n=10 at p=0.05 is 0.71. State whether the correlation is significant.
🔍 Worked example — Exam-style
A researcher finds a correlation between temperature and species richness. The data shows a non-linear, curved relationship (as temperature increases, richness increases, then decreases). Which correlation coefficient should be used and why?
⚠️ Common errors
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🎯 Exam question
(a) State whether the correlation between light intensity and algae abundance is significant. [1] (b) Explain why the student used Spearman's rank correlation instead of Pearson's linear correlation. [2] [3 marks]
Worked example
Question [6]: A student sampled a grassland using quadrats and recorded the following numbers of individuals of five plant species: A, 25; B, 12; C, 8; D, 3; E, 2.
🔍 Worked example — Easy
A student uses a quadrat to sample a pond. They find 10 algae, 5 snails, and 5 water lilies. Calculate the species richness.
🔍 Worked example — Standard
In a forest patch, the following counts were recorded: Species X: 40, Species Y: 10, Species Z: 5. Calculate Simpson's Index of Diversity (D) using the formula D = 1 - Σ(n/N)^2, where N is the total number of individuals.
🔍 Worked example — Exam-style
A student sampled a grassland using quadrats and recorded the following numbers of individuals of five plant species: A, 25; B, 12; C, 8; D, 3; E, 2. (a) Calculate the species richness. (b) Calculate Simpson's index of diversity, D, for this sample. (c) State what this value of D suggests about the diversity of this grassland.
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