Simpson’s reciprocal diversity index
Diversity is not just about how many species are present. Simpson’s reciprocal diversity index combines species richness and species evenness into one value.
Comparing the diversity of ecosystems
You may be asked to compare the diversity of two ecosystems and then infer what that could mean about their stability or resilience.
In general, greater biodiversity can provide more possible pathways and functional overlap within a system, which may help it absorb disturbance. But diversity alone does not prove that one ecosystem is definitely more resilient.
Which ecosystem is more diverse?
Both ecosystems contain exactly five species and 100 organisms. The scene is illustrative; use the counts below each ecosystem for the calculation.
Ecosystem A
5 species · 100 organisms
Ecosystem B
5 species · 100 organisms
Both ecosystems have the same richness: 5 species.
Ecosystem B is much more even. The 100 organisms are distributed equally across the five species.
Richness + evenness in one number
How many different species are present?
How evenly are the individuals spread between those species?
Capital N = the big total. Add every organism in the whole sample.
Lowercase n = one species. Do n(n−1) for every species, then add all of those answers together. Σ means “add them all”.
Ecosystem A, step by step
| Species | Eco A | Eco B |
|---|---|---|
| Penguin | 70 | 20 |
| Seal | 10 | 20 |
| Fish | 8 | 20 |
| Krill | 7 | 20 |
| Seabird | 5 | 20 |
Try the top before revealing the answer
Ecosystem B also contains 100 organisms.
Which ecosystem is more diverse?
D = 1.97
D = 5.21
Ecosystem B is more diverse. Both ecosystems contain five species, but Ecosystem B has much greater evenness.
A higher diversity value may suggest greater resilience because a more diverse system can have more species and potentially more functional pathways available if one population declines. But the index alone does not prove resilience.
“Ecosystem B has a higher Simpson’s reciprocal diversity index than Ecosystem A, indicating greater species diversity. Both communities contain five species, but the individuals in Ecosystem B are distributed more evenly between the species.”
A four-step routine
Add every organism in the sample.
N(N−1)
For every species: n(n−1), then add them all.
Top ÷ bottom.
N(N−1)
Σn(n−1)
This is worth putting on a flashcard. In the exam, it is often the order of the steps that trips people up, not the multiplication itself.
Calculate D for Ecosystem C
| Species | Individuals |
|---|---|
| Species 1 | 18 |
| Species 2 | 15 |
| Species 3 | 12 |
| Species 4 | 9 |
| Species 5 | 6 |
D ≈ 4.72. This indicates relatively high diversity for a community containing five species because the individuals are distributed fairly evenly between the species.