Syllabus Edition
First teaching 2015
Last exams 2025
Species Richness & Diversity
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
Species Richness & Diversity
What is species richness?
Species richness is the number of species in a community or defined area and can be a useful comparative measure in some cases
However, in other cases, species richness can be a misleading indicator of diversity as 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 evenness of abundance across the different species (i.e. their relative abundances)
Species Richness vs Diversity
An index of diversity is a measurement that describes the relationship between the number of species present and how each species contributes to the total number of organisms that are present in that community
It is a much more informative measurement than species richness and conservationists often favour the use of an index of diversity as it takes into account both species richness and evenness
A commonly used index of diversity is known as Simpson’s Diversity Index
Example
Area 1 and Area 2 both contain 4 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
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
Calculating Simpson’s Diversity Index
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:
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 |
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 |
Site A was located just 5 km downstream from the river’s source
Site B was located 50 m downstream from a sewage inlet pipe
The lower diversity index for site B reflects the stress placed upon the river as a consequence of the pollution load from the sewage inlet point; although the river is recovering from the pollution, sensitive species are unable to tolerate the unfavourable abiotic conditions and species diversity is reduced
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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