What does Simpson's Index measure in an ecological context?

Simpson's Index is a measure of diversity used in ecology that quantifies the likelihood of randomly selecting two individuals from a community, where both individuals belong to the same species. This index takes into account both the richness (the number of different species) and evenness (the relative abundances of each species) of species within a community. A higher value indicates lower diversity, meaning a greater probability of selecting individuals from the same species, while a lower value indicates higher diversity, suggesting a more equitable distribution of species.

Understanding this index is crucial for ecologists because it helps evaluate the health and stability of ecosystems. Ecosystems with higher diversity tend to be more resilient to environmental changes and disturbances. In contrast, focusing solely on aspects like total species count, biomass, or energy flow does not provide insight into how individual species contribute to the community's composition and dynamics. Hence, the Simpson's Index specifically measures the probability of finding organisms from the same species, which is fundamental for assessing biodiversity.