SPEC 3.4.6 • GENETIC INFORMATION — BIODIVERSITY
Biodiversity, Species Richness & Index of Diversity
What is biodiversity? Why species richness alone isn't enough. Calculating the index of diversity. How farming practices reduce biodiversity — and the balance with conservation.
What is Biodiversity?
Biodiversity
The variety of living organisms in an area. AQA spec notes that biodiversity can relate to habitats at any scale — from a small local habitat to the whole Earth.
Three Scales of Biodiversity
Local (habitat): a single pond, woodland or meadow
Regional: a biome, country, or ecosystem type
Global: the entire Earth
Why It Matters
High biodiversity makes ecosystems more stable, more resilient to change, and more productive. A loss of biodiversity reduces an ecosystem's ability to recover from disturbance (e.g. disease outbreak, drought, pollution).
Measure 1 — Species Richness
Species Richness (AQA)
A measure of the number of different species in a community.
Simple Example
Community A contains 3 different species.
Community B contains 12 different species.
Community B has higher species richness.
Examiner Alert
⚠️ Species richness only counts the number of species — it ignores how many individuals of each species are present. This is why it is often an incomplete measure of biodiversity.
Why Species Richness Isn't Enough
Imagine two communities, both with 3 species and 100 individuals:
The Problem
Both communities have the same species richness (3) and the same number of individuals (100). But Community A is clearly more diverse — Community B is dominated by one species with the other two hanging on by a thread.
Species richness alone cannot distinguish these two communities. We need a measure that accounts for evenness as well as species count.
Measure 2 — Index of Diversity
Index of Diversity (AQA)
A measure that describes the relationship between the number of species in a community AND the number of individuals in each species.
AQA SPEC FORMULAd = N(N − 1) ÷ Σn(n − 1)where:N = total number of organisms of all species
n = total number of organisms of each species
Σ = "sum of"
How Σ Works
The Σ (sigma) means you calculate n(n−1) for each species separately, then add those values together. So for 3 species you get three terms in the denominator.
Worked Example 1
Scenario
A woodland contains: 10 sparrows, 6 robins, 4 blackbirds. Calculate the index of diversity.
1Find N (total of all species): N = 10 + 6 + 4 = 20
2Calculate numerator: N(N − 1) = 20 × 19 = 380
3Calculate n(n − 1) for each species:
Sparrows: 10 × 9 = 90
Robins: 6 × 5 = 30
Blackbirds: 4 × 3 = 12
4Sum for denominator: Σn(n − 1) = 90 + 30 + 12 = 132
5Divide: d = 380 ÷ 132 = 2.88 (to 2 dp)
Exam Tip
⚠️ Always show all working. Examiners award marks for the method even if the final answer is wrong. Write out each step, keep units where relevant, and round only at the end.
Worked Example 2 — Two Species
Scenario
A pond contains 60 water fleas and 40 mayfly nymphs. Calculate d.
N = 60 + 40 = 100N(N − 1) = 100 × 99 = 9,900Σn(n − 1) = (60 × 59) + (40 × 39)
= 3,540 + 1,560 = 5,100d = 9,900 ÷ 5,100 = 1.94 (to 2 dp)
Compare with an Extreme Case
If instead the pond had 99 water fleas and just 1 mayfly nymph (very uneven):Σn(n − 1) = (99 × 98) + (1 × 0) = 9,702 + 0 = 9,702
d = 9,900 ÷ 9,702 ≈ 1.02d ≈ 1 — showing that a community dominated by one species has very low diversity. As d approaches 1, diversity approaches its minimum.
Interpreting the Index of Diversity
What d Values Mean
Value of dInterpretationd = 1 (minimum)Only one species present — d only reaches exactly 1 in a true monocultured just above 1One species heavily dominates (e.g. 99 + 1) — very low biodiversityd much greater than 1Many species AND individuals distributed evenly — high biodiversity
Key Insight
A higher d value means greater biodiversity. This is why d increases when:
The number of species in the community increases, OR
The distribution of individuals across species becomes more even
Examiner Alert
⚠️ d is dimensionless — it has no units. Don't add units to your final answer. But do state "d = X" clearly.
What Affects d?
Two independent factors drive d upwards:
1. Number of Species
More species → more non-zero n terms in the denominator, each contributing less than one dominant term would. Denominator gets smaller relative to N(N−1) → d rises.
2. Evenness
For a fixed number of species, spreading individuals evenly minimises each n(n−1) term (since n(n−1) grows quadratically with n). More even → denominator smaller → d rises.
Worked Illustration — Same Species Richness (N = 100, 2 species)
DistributionDenominatord50 + 50 (even)50×49 + 50×49 = 4,9002.0260 + 4060×59 + 40×39 = 5,1001.9480 + 2080×79 + 20×19 = 6,7001.4899 + 1 (very uneven)99×98 + 1×0 = 9,7021.02Same number of species (2), same N (100), but d changes dramatically with evenness.
Farming Reduces Biodiversity — Monocultures
Monoculture
A farming practice in which a single crop species is grown over a large area.
Impact on Biodiversity
Only one plant species on the field — species richness of plants drops to 1
Few niches available → reduced diversity of insects, birds, and small mammals
Uniform habitat → few specialist species can thrive
Pests spread easily because all plants are genetically similar → more pesticides needed → further biodiversity loss
In the index of diversity: d approaches 1 (the minimum) because plant species richness is reduced to one crop
Examples
Vast fields of wheat, maize, soybean, oil palm. Also applies to forestry (e.g. conifer plantations) where a single tree species replaces natural mixed woodland.
Farming — Hedgerow Removal
Why Hedgerows Matter
A hedgerow is a narrow linear woodland habitat between fields. Hedgerows contain many plant species (hawthorn, blackthorn, bramble) and act as:
Habitat for small mammals, insects, and nesting birds
Wildlife corridors connecting isolated patches of woodland
Food source — berries, nectar, pollen for pollinators
Shelter from wind and predators
Impact of Removal
When farmers remove hedgerows to enlarge fields and use bigger machinery more efficiently:
Many species lose their habitat and food source → population declines
Populations become isolated → reduced gene flow → lower genetic diversity
Pollinator numbers fall → reduced crop pollination
Pest-control species (e.g. ladybirds, hoverflies) are lost → more pesticide needed
Pesticides, Fertilisers & Drainage
Pesticides
Chemicals that kill pests. Problem: most pesticides are not fully selective and kill non-target species — beneficial insects (bees, ladybirds), soil invertebrates, and predators higher up the food chain via bioaccumulation.
Inorganic Fertilisers
Add nitrate and phosphate to boost crop yield. Problem: runoff into waterways causes eutrophication — algal bloom → oxygen depletion when algae die and decompose → fish and aquatic invertebrates die → sharp loss of aquatic biodiversity.
Drainage of Wetlands
Wetlands and marshes are among the most biodiverse habitats. Draining them to create arable farmland eliminates entire specialised communities of wading birds, amphibians, and wetland plants.
Synoptic Link
Eutrophication links to Topic 5.6 (Nutrient Cycles) — excess nitrate/phosphate disrupts the nitrogen and phosphorus cycles, with knock-on effects across the ecosystem.
Summary — Farming Impacts on Biodiversity
Farming PracticeMechanismEffect on d
MonocultureOne crop species dominates; few niches; genetically uniformd falls sharply (approaches 1)
Hedgerow removalHabitat loss; isolation of populations; loss of corridorsd falls
PesticidesKill non-target species; bioaccumulation up food chaind falls
Inorganic fertilisersRunoff → eutrophication → hypoxic waterwaysd falls in nearby waterways
Wetland drainageDestruction of specialised habitatd falls dramatically
Conservation vs Farming — The Balance
Farming is essential for feeding a growing population — but unchecked it destroys biodiversity. AQA requires you to appreciate the balance between conservation and farming.
Conservation Measures Compatible with Farming
Maintain or replant hedgerows between fields
Set aside uncultivated field margins as wildlife strips
Use crop rotation to break pest cycles and reduce pesticide needs
Plant intercrops or cover crops to increase species diversity within fields
Reduce pesticide use (integrated pest management, biological control)
Protect wildlife corridors linking habitat fragments
Organic farming: avoids synthetic pesticides/fertilisers entirely
The Trade-off
Many conservation measures reduce short-term yields — farmers may need government subsidies or environmental payment schemes to adopt them. The question is whether the long-term benefits (pollinators, soil health, ecosystem stability) outweigh the short-term yield loss.
DEEP Answer — Biodiversity & Farming
Typical 6-mark question: "Describe and explain how intensive farming reduces biodiversity. Use the index of diversity in your answer."
DDefine: Biodiversity is the variety of living organisms in an area. It is measured using species richness (number of different species) and the index of diversity d = N(N−1)/Σn(n−1), which also accounts for the number of individuals of each species.
EExplain mechanisms: Monoculture reduces plant species to one, eliminating niches for insects, birds and mammals. Hedgerow removal destroys habitat and wildlife corridors. Pesticides kill non-target species. Fertiliser runoff causes eutrophication of waterways, killing aquatic organisms.
EEvidence: In a monoculture field with 100 wheat plants and 2 individuals of one weed species (total N=102, 2 species), d = 102×101 / (100×99 + 2×1) = 10,302/9,902 ≈ 1.04 — close to the minimum value of 1, indicating severely reduced diversity.
PPractical/Quantitative: Students could compare d between a farmed field and an adjacent hedgerow using quadrats and random sampling. Typically d is 2-4× higher in hedgerows than in adjacent monoculture fields — a directly measurable effect of intensive farming.
Practice Question 1
Q1. A student sampled two fields using quadrats and recorded the following:
Field A (conventional): 35 wheat plants, 3 poppies, 2 thistles
Field B (set-aside margin): 15 grass plants, 13 clover, 12 dandelions
(a) Calculate the index of diversity (d) for Field A. Show your working. [2 marks]
(b) Calculate the index of diversity (d) for Field B. [2 marks]
(c) Using your values, explain what these results suggest about the effect of set-aside farming on biodiversity. [2 marks]
(a)
N = 35 + 3 + 2 = 40. N(N−1) = 40 × 39 = 1,560. Σn(n−1) = (35×34) + (3×2) + (2×1) = 1,190 + 6 + 2 = 1,198. d = 1,560 ÷ 1,198 = 1.30.
(b)
N = 15 + 13 + 12 = 40. N(N−1) = 1,560. Σn(n−1) = (15×14) + (13×12) + (12×11) = 210 + 156 + 132 = 498. d = 1,560 ÷ 498 = 3.13.
(c)
(1) Field B has a much higher d (3.13 vs 1.30) even though both have 3 species and 40 individuals — because individuals are distributed much more evenly. (2) Set-aside margins support higher biodiversity than conventional monoculture-dominated fields because no single species dominates.
Practice Question 2
Q2. A farmer is considering two changes to increase biodiversity on their land: (i) planting hedgerows along field boundaries, and (ii) reducing pesticide use.
(a) Explain how each measure would be expected to increase biodiversity. [4 marks]
(b) Suggest one reason why a farmer might be reluctant to adopt these measures. [1 mark]
(c) State what species richness and the index of diversity each measure, and explain why both are used. [3 marks]
(a)
Hedgerows: (1) Provide habitat, food and shelter for many species of insects, birds and small mammals. (2) Act as wildlife corridors connecting isolated habitats → gene flow maintained. Reduced pesticides: (3) Fewer non-target species killed, so populations of beneficial insects and predators recover. (4) Less bioaccumulation of pesticide through the food chain, protecting higher trophic levels.
(b)
Reduced pesticide use may decrease short-term crop yield / increase pest damage, reducing farmer's income. Hedgerows take land out of production. (Either point = 1 mark.)
(c)
(1) Species richness measures only the number of different species in a community. (2) The index of diversity (d) accounts for both the number of species AND the number of individuals of each species (evenness). (3) Both are used because species richness alone cannot distinguish between a community dominated by one species and a community with an even distribution.
Practice Question 3 — Synoptic
Q3. A monoculture wheat field is sampled and contains 200 wheat plants, 1 poppy, and 1 dandelion. A neighbouring meadow is sampled with a single quadrat and contains 8 grass plants, 7 buttercups, 6 daisies, 5 plantains, and 4 clovers.
(a) Calculate d for both the wheat field and the meadow. Give answers to 2 decimal places. [4 marks]
(b) Compare the two d values and explain the biological significance of the difference. [3 marks]
(c) Suggest, using your understanding of natural selection (Topic 4.5), why monocultures are more vulnerable to pest outbreaks than mixed communities. [3 marks]
(a) Wheat field:
N = 202. N(N−1) = 202 × 201 = 40,602. Σn(n−1) = (200×199) + (1×0) + (1×0) = 39,800. d = 40,602 ÷ 39,800 = 1.02.
Meadow: N = 30. N(N−1) = 870. Σn(n−1) = (8×7) + (7×6) + (6×5) + (5×4) + (4×3) = 56 + 42 + 30 + 20 + 12 = 160. d = 870 ÷ 160 = 5.44.
(b)
(1) The meadow has a much higher d (5.44 vs 1.02) — far higher biodiversity. (2) The wheat field is close to the minimum of d=1 because one species (wheat) dominates almost completely. (3) Higher biodiversity makes the meadow ecosystem more stable, more resilient to disturbance, and supports more ecological niches.
(c)
(1) All wheat plants in a monoculture are genetically similar (same cultivar, same alleles), so if a pest or pathogen adapts to overcome their defences, all plants are equally susceptible. (2) Once a pest population arises that can exploit this host, it faces no directional selection towards different host-plant traits — the whole crop becomes food. (3) In a mixed community, different species have different defences; a pest specialised on one species finds only a small proportion of plants are suitable, limiting its spread.
Summary
Must-Know Definitions
Biodiversity — the variety of living organisms in an area (local → Earth).
Species richness — the number of different species in a community.
Index of diversity — accounts for number of species AND number of individuals in each species.
d = N(N − 1) ÷ Σn(n − 1)N = total of all organisms n = total per species
Higher d Means
More species AND/OR more even distribution → higher biodiversity.
Farming Reduces d By
Monoculture, hedgerow removal, pesticides, fertiliser runoff, wetland drainage.
Top Exam Tip
When calculating d: always show every step (N, N(N−1), each n(n−1), sum, division). When interpreting d: state that d = 1 is the minimum, and higher d values indicate greater biodiversity through more species AND/OR greater evenness.
Next: What is Biodiversity?