AQA A-level Biology: Biodiversity, Index of Diversity and Farming (7402)

This guide covers AQA A-level Biology (7402) section 3.4.6, Biodiversity within a community, as set out in the AQA AS and A-level Biology specification for first teaching 2015. Section 3.4.6 sits in topic 4, which is also AS content. The assessment overview lists topics 1 to 4 for A-level Paper 1, and topics 1 to 8 for Paper 3. For the other pages in this unit see the revision notes and the practice questions. Course pages: AQA A-level Biology hub and the printable checklist. To find weak spots first, try the free diagnostics. Genetic diversity within a species is covered separately in mutation and meiosis.

What the specification asks for

Statement in 3.4.6	What you must be able to do
Biodiversity across habitats	Explain that biodiversity can be considered from a small local habitat up to the whole Earth
Species richness	Define it as a count of the different species in a community
Index of diversity	State that it relates the number of species to the number of individuals in each species
Calculation (maths skill MS 2.3)	Substitute into d = N(N − 1) ÷ Σn(n − 1) and interpret the value
Farming	Explain how farming techniques reduce biodiversity
Balance	Discuss the balance between conservation and farming

The specification says you could be given data from which to calculate an index of diversity and then interpret its significance. That is the main skill to practise.

Biodiversity across habitat scales

Biodiversity is the variety of living organisms. The specification says it can relate to “a range of habitats, from a small local habitat to the Earth”. A habitat is the place where an organism lives, and a community is made up of the populations of every species that share one habitat.

Scale	Example	What you could measure
Small local habitat	A rotting log, a pond, the base of a hedge	Species caught or counted in one sample
Larger habitat	A woodland, a lake, a field	Several samples combined
Region or country	All the heathland in a county	Species lists built up from many surveys
The Earth	All living species	An estimate only, because not every species is known

Two points follow. First, you can only compare biodiversity fairly at the same scale and with the same sampling effort. A pond sample cannot be compared with a whole-country species list. Second, the index of diversity in this unit is calculated for one community, from a sample.

Species richness

Species richness is a count of how many different species live in a community. It is simple to find: count the species and ignore how many individuals there are of each.

That is also its weakness. Compare two communities, each with 24 individuals.

Community	Counts (individuals per species)	Species richness
P	6, 6, 6, 6	4
Q	21, 1, 1, 1	4

Both have a richness of 4, yet P has an even spread of individuals and Q is dominated by one species. Richness cannot tell them apart, so you need an index that uses the abundance of each species as well.

The index of diversity

An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species. The formula you must be able to use is:

d = N(N − 1) ÷ Σn(n − 1)

where N is the total number of organisms of all species and n is the total number of organisms of each species. Σ means “the sum of”.

One way to see what the formula does: n(n − 1) ÷ N(N − 1) is the chance that two individuals picked at random from the sample, without putting the first back, belong to the same species. The denominator adds this up across all species, so d is large when that chance is small, meaning the community is varied.

Method

1. Add all the individuals to get N, then work out N(N − 1).
2. For each species work out n(n − 1). A species with one individual gives 0.
3. Add the n(n − 1) values to get Σn(n − 1).
4. Divide N(N − 1) by Σn(n − 1). Give a sensible number of significant figures, usually 2 or 3 unless told otherwise.

Worked example 1: communities P and Q

For P: N = 24, so N(N − 1) = 24 × 23 = 552. Each species gives 6 × 5 = 30, so Σn(n − 1) = 4 × 30 = 120. Then d = 552 ÷ 120 = 4.6.

For Q: N(N − 1) is again 552. The terms are 21 × 20 = 420, then 0, 0 and 0, so Σn(n − 1) = 420. Then d = 552 ÷ 420 = 1.3 (2 s.f.).

Same richness, same N, very different d. P is far more diverse in the sense that matters ecologically.

Worked example 2: hedge base and field edge

A student set pitfall traps for the same number of nights in two places and identified the invertebrates caught.

Species group	Hedge base (n)	n(n − 1)	Field edge (n)	n(n − 1)
Ground beetle	15	210	33	1056
Woodlouse	10	90	5	20
Centipede	7	42	3	6
Spider	5	20	1	0
Slug	3	6	0	not counted
Earwig	2	2	0	not counted
Total	N = 42	370	N = 42	1082

Both traps caught 42 individuals, so N(N − 1) = 42 × 41 = 1722 for each.

Hedge base: d = 1722 ÷ 370 = 4.7 (2 s.f.). Field edge: d = 1722 ÷ 1082 = 1.6 (2 s.f.).

The hedge base has 6 species and the field edge 4. Richness is higher at the hedge, and the index shows the field edge is also dominated by ground beetles. The index of diversity is about 66% lower at the field edge: (1.59 − 4.65) ÷ 4.65 × 100 = −65.8%.

Interpreting the value

• Higher d means a more diverse community: more species, spread more evenly. Lower d means fewer species or one species dominating.
• d cannot be below 1. It equals 1 when there is only one species.
• If every species has just one individual, Σn(n − 1) = 0 and d cannot be calculated. The sample is too small to say anything.
• A community with a low d is often described as more vulnerable to change, because if the dominant species is lost, most of the individuals go with it. Treat this as a likely effect, not a certainty.
• A species with n = 0 is not in the sample. Leave it out.

Worked example 3: before and after a change

A field margin was sampled before and after a strip of native flowering plants was sown. The same trapping effort was used each time.

	Counts per species	N	N(N − 1)	Σn(n − 1)	d
Before	18, 4, 2, 1	25	600	306 + 12 + 2 + 0 = 320	1.9
After	12, 9, 8, 6, 4, 3	42	1722	132 + 72 + 56 + 30 + 12 + 6 = 308	5.6

The index has risen from 1.875 to 5.59. That is a rise of (5.59 − 1.875) ÷ 1.875 × 100 = 198%. Richness has gone from 4 to 6 and the individuals are spread more evenly.

Limits when interpreting

• Small samples give unreliable values. A different sample from the same place can give a different d, so use random sampling and a large enough sample.
• One sample is a snapshot. Numbers change with season and time of day.
• The formula needs individuals to be counted. For grasses and other spreading plants, counting individuals is hard, so another measure such as percentage cover may be used instead.
• Species must be identified correctly. Mistaking two species for one lowers richness and changes d.
• A high d does not show that the species present are the ones you want to conserve. Non-native or pest species also count.
• The maths skills list for the A-level gives Simpson’s index of diversity as an example of a given equation: D = 1 − Σ(n ÷ N)². This is a different index. For P it gives 0.75 and for Q 0.23. If an exam question supplies an equation, substitute into the one given.

How farming reduces biodiversity

The specification states that farming techniques reduce biodiversity but does not list the techniques, so explain the mechanism for any example you use.

Technique	How it reduces biodiversity
Monoculture (one crop over a large area)	Fewer plant species means fewer food sources and shelter types, so fewer animal species
Removing hedgerows and enlarging fields	Loss of habitat, nesting sites and shelter, and fewer routes between habitats
Herbicides	Kill wild plants, removing food for insects and the animals that feed on them
Pesticides	Kill non-target insects as well as pests, and reduce food for insect-eating animals
Draining wet areas, ploughing grassland	Habitat destroyed or changed to one few species can live in
Fertilisers	Nutrients washing into water can cause the loss of species there
Overgrazing	Fewer plant species survive, so the community is less varied

In every case the chain is: habitat or food source removed or simplified, then fewer niches, then fewer species and a lower d.

The balance between conservation and farming

Farming provides food for a growing human population and a living for farmers. Conservation protects species and the habitats they depend on. The two compete for the same land, so the specification asks you to consider the balance.

Points for intensive farming: higher yield per hectare, lower food prices, and less land needed to produce the same food.

Points for conservation: species are protected for their own value, ecosystems stay resilient, and some wild species help farming, for example insects that pollinate crops or prey on pests.

Ways to reach a balance

• Keep or replant hedgerows and leave uncultivated field margins and buffer strips.
• Rotate crops and leave some land fallow.
• Use fewer pesticides, or use biological control where it is suitable.
• Reduce stocking density to avoid overgrazing.
• Protect wetlands and woodland on or near farms.

A good evaluation uses evidence. If a farm trial shows that sown margins raise d a lot while lowering yield only slightly, that supports the margins. If yield falls heavily for a small rise in d, the case is weaker. Say what the data show, what they do not show, and who is affected.

Common errors

• Writing N² or n² in the formula instead of N(N − 1) and n(n − 1).
• Dividing before adding up all the n(n − 1) terms, or dividing the wrong way round (Σn(n − 1) ÷ N(N − 1) gives a small fraction, not d).
• Counting a species with n = 0 as a term, or counting it in richness.
• Saying a higher d means “more individuals”. It means a more varied community.
• Defining species richness as the number of organisms.
• Writing that farming “kills wildlife” without a mechanism such as loss of food or habitat.
• Comparing two values of d from samples taken with different effort and drawing a firm conclusion.

Next steps

Use the revision notes for quick recall, then try the practice questions. All the sample data on this page are invented for practice.

Official syllabus

AQA AS and A-level Biology specification (7401, 7402), for first teaching 2015, published by AQA. This page is aligned to section 3.4.6 and maths skill MS 2.3.