Syllabus Edition

First teaching 2024

First exams 2026

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Assessing Biodiversity (DP IB Environmental Systems & Societies (ESS))

Revision Note

Species Diversity

  • Species richness is the number of species in a community or defined area

    • In some cases, it can be a useful measure to compare the biodiversity of different areas

  • However, in other cases, species richness can be a misleading indicator of diversity

    • This is because it does not take into account the number of individuals of each species

  • Once the abundance of each species in an area has been recorded, the results can be used to calculate the species diversity for that area

    • Species diversity looks at the number of different species in an area but also the species evenness

      • Species evenness is the evenness of abundance across the different species (i.e. their relative abundances)

Species richness vs species diversity

  • Species diversity is a much more informative measurement than species richness and conservationists often favour the use of species diversity as it takes into account both species richness and evenness

  • For example:

    • Area 1 and Area 2 both contain four tree species

    • However, Area 2 is actually dominated by one species and in fact, one of the species is very rare (only one individual)

    • Although the two areas have exactly the same species richness, Area 1 has a higher species evenness (and therefore a higher overall species diversity) than Area 2

    • This example illustrates the limitations of using just species richness on its own

Diagram showing the difference between species richness and evenness
Area 1 and Area 2 have the same species richness but different species evenness. As it has a higher species evenness, the overall species diversity of Area 1 is higher than that of Area 2, as species diversity takes both richness and evenness into account

Simpson's Diversity Index

  • Biological communities can be described and compared through the use of diversity indices

    • These are mathematical tools used to quantify the diversity of species within a community

  • These indices provide a measure of the variety of species present, as well as their relative abundances

    • They can be used to compare different communities or to track changes in diversity over time

  • A commonly used diversity index is Simpson's index

Calculating Simpson’s diversity index

Worked Example

  • A group of students used the kick sampling technique to collect, identify and count the invertebrates inhabiting a river

  • Samples were obtained from different sites along the course of the river

  • The data was used to calculate the Simpson's diversity index at two different river sites

    • This index of diversity is useful when comparing two similar habitats, or the same habitat over time

  • The formula for calculating Simpson's Diversity Index, D, is:

  • Where:

    • D = Simpson's diversity index

    • N = total number of individuals sampled

    • n = number of individuals of each species

Data Collection Table

Species

Mean number of organisms per m2 of river bed

Site A

Site B

Mite

14

0

Snail

9

0

Leech

3

26

Worm

0

6

Flat worm

132

9

Mayfly nymph

43

0

Olive mayfly nymph

154

0

Midge Larva

0

10

Blackfly larva

77

0

Caddis larva

15

1

Fish

1

0

Freshwater shrimp

211

6

Water hog louse

0

40

Site A

Species

Number (n) 

n (n-1)

Mite

14

182

Snail

9

72

Leech

3

6

Worm

0

0

Flat worm

132

17 292

Mayfly nymph

43

1 806

Olive mayfly nymph

154

23 562

Midge Larva

0

0

Blackfly larva

77

5 852

Caddis larva

15

210

Fish

1

0

Freshwater shrimp

211

44 310

Water hog louse

0

0

Total

N= ∑n= 659

∑n(n-1)= 93 292

bold italic D equals bold italic space fraction numerator N stretchy left parenthesis N minus 1 stretchy right parenthesis over denominator capital sigma n stretchy left parenthesis n minus 1 stretchy right parenthesis end fraction bold italic equals fraction numerator space 659 stretchy left parenthesis 658 stretchy right parenthesis over denominator 93292 end fraction bold italic space equals space 4.65

Site B

Species

Number (n) 

n (n-1)

Mite

0

0

Snail

0

0

Leech

6

30

Worm

26

650

Flat worm

9

72

Mayfly nymph

0

0

Olive mayfly nymph

0

0

Midge Larva

10

90

Blackfly larva

0

0

Caddis larva

1

0

Fish

0

0

Freshwater shrimp

6

30

Water hog louse

40

1 560

Total

N= ∑n= 98

∑n(n-1)= 2 432


bold italic D equals bold italic space fraction numerator N stretchy left parenthesis N minus 1 stretchy right parenthesis over denominator capital sigma n stretchy left parenthesis n minus 1 stretchy right parenthesis end fraction bold italic equals fraction numerator space 98 stretchy left parenthesis 97 stretchy right parenthesis over denominator 2432 end fraction bold italic space equals space 3.91

  • By comparing the diversity indices for Site A and Site B, we can see that Site B has a lower species diversity

    • The value of D will be higher where there is greater richness (number of species) and evenness (similar abundance)

    • The lowest possible value for D is 1

Examiner Tips and Tricks

Remember, this index of diversity is only useful when comparing two similar habitats, or the same habitat over time.

You will be provided with the formula for Simpson’s Index in the exam but you need to know how to use it to calculate Simpson’s Diversity Index for example sets of data.

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Alistair Marjot

Author: Alistair Marjot

Expertise: Biology & Environmental Systems and Societies

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

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