Syllabus Edition
First teaching 2024
First exams 2026
Assessing Biodiversity (DP IB Environmental Systems & Societies (ESS))
Revision Note
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
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
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 |
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 |
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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