Measuring biodiversityAQA A-Level Biology: Revision notes

Section 1

Biodiversity and scale

Biodiversity is the variety of living organisms in an area. It can be considered at a range of scales, from a small local habitat such as a pond, hedgerow or meadow, up to a whole country or the Earth as a whole.

The number of different species in a community is its species richness. A species richness count is simple to make, but it only tells you how many species are present.

Key termsbiodiversityhabitatspecies richness

Section 2

Why species richness is not enough

Imagine two ponds, each with four species and 100 individuals. In pond A the individuals are shared equally, with 25 of each. In pond B one species has 97 individuals and each of the others has one. Both have a species richness of 4, but pond A is clearly more diverse.

An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species. It therefore takes account of how evenly the individuals are distributed.

Key termsindex of diversityevenness

Exam tip

Always say that species richness ignores the number of individuals in each species. This is the point most mark schemes look for.

Section 3

Calculating the index of diversity (d)

$d = \frac{N(N-1)}{\sum n(n-1)}$

$N$ = total number of organisms of all species
$n$ = total number of organisms of each species
$\sum$ means add together the values for every species.

Worked example. A sample has three species with 10, 6 and 4 individuals.

$N = 20$ , so $N(N-1) = 20 \times 19 = 380$
$\sum n(n-1) = (10 \times 9) + (6 \times 5) + (4 \times 3) = 90 + 30 + 12 = 132$
$d = 380 \div 132 = 2.9$

Key termsNnsum of n(n-1)

Common mistake

Do not forget to work out n(n-1) for every species before adding. A species with one individual contributes 0, not 1.

Section 4

Interpreting the value of d

The higher the index of diversity, the more diverse the community: it has more species and/or individuals more evenly spread between them. A community containing only one species has the lowest possible value, d = 1.

A high value of d suggests a stable community with many food sources and niches. If one species declines, others can fill its role. A low value suggests a community dominated by one or a few species, which is more vulnerable to changes such as disease or pollution.

A value of d is most useful when it is compared: with another habitat, or the same habitat at different times.

Key termsstable communitydominance

Section 5

Reliable measurements

A calculated index depends on the sample. Small samples, a single visit, or samples taken at one time of year may not represent the whole habitat. Random sampling, large samples and repeated surveys improve reliability.

Key termsrandom sampling

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Measuring biodiversity

Two ponds were each surveyed with the same sampling effort. Each pond contained four species and 100 individuals in total. In pond A the individuals were shared equally between the four species. In pond B, one species made up 97 of the individuals and each of the other three species was represented by a single individual.
Explain why species richness alone is not a sufficient measure of the biodiversity of the two ponds.2 marks
An ecologist calculated the index of diversity (d) for ground beetles in two habitats. The value for a hedgerow was 1.8 and the value for an area of ancient woodland was 14.5.
Suggest why the community in the woodland is likely to be more stable than the community in the hedgerow when the environment changes.2 marks
A survey of ground beetles in a woodland used randomly placed pitfall traps and recorded five species, with 12, 8, 5, 3 and 2 individuals of each. A sample collected in the same way in a neighbouring conifer plantation contained three species, with 25, 4 and 1 individuals of each.
Calculate the index of diversity (d) for the woodland sample, using $d = \frac{N(N-1)}{\sum n(n-1)}$ . Show your working.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).