AP Syllabus focus:
‘Simpson’s Diversity Index quantifies community diversity using the relative abundances of different species.’
Species diversity is more informative than a simple species list because it incorporates how individuals are distributed among species. Simpson’s Diversity Index is a standard quantitative tool for comparing communities using relative abundance data.
What Simpson’s Diversity Index measures
Simpson’s Diversity Index combines two key aspects of diversity:
Species richness: how many different species are present
Species evenness: how evenly individuals are distributed among those species
Practice Questions
FAQ
Different conventions report either “dominance” or “diversity”.
$\sum (n_i/N)^2$ increases as dominance increases.
$1-\sum (n_i/N)^2$ increases as diversity increases.
Both are derived from the same components; always check which direction indicates greater diversity.
It stabilises as sampling effort increases because relative abundances become more reliable.
Small samples can over- or under-represent species, especially rare ones, shifting $\frac{n_i}{N}$ values and therefore $D$. Replicated sampling and consistent effort improve comparability.
Yes, if categories are applied consistently (e.g., morphospecies or genus-level groups).
However, lumping taxa can inflate apparent evenness and reduce apparent richness. Comparisons are most defensible when the same identification resolution is used across all communities.
Direct comparison is risky because sampling method affects detectability and relative abundance estimates.
A cautious approach is to compare only within method-matched datasets, or to standardise effort (same area, time, or number of traps) before computing $D$.
It can be, but interpretation depends on design.
Averaging quadrat-level $D$ describes typical local diversity, whereas pooling counts across quadrats and computing one $D$ describes diversity at the combined scale. Choose based on the ecological question and keep the approach consistent.
