Biodiversity is a critical indicator of ecosystem health, and ecologists use various mathematical indices to quantify it. One of the most widely used indices is the Simpson Index, which captures both species richness and evenness. If you’ve encountered the equation 1-∑Pi² in ecological studies, you’re looking at a core formula for measuring diversity—specifically, the Simpson Index of Diversity.

What Does 1-∑Pi² Represent?

The equation 1-∑Pi² is used to calculate the Simpson Index of Diversity. Here’s what each term means:

• Pi is the proportion of individuals belonging to the ith species in the community (i.e., the probability of randomly selecting an individual of that species).

• ∑Pi² is the sum of the squared proportions for all species in the sample.

The formula calculates the probability that two individuals randomly selected from a sample will belong to different species. The value ranges from 0 to 1, where values closer to 1 indicate higher diversity.

Why Is the Simpson Index Important?

• Accounts for Richness and Evenness: Unlike simple species counts (species richness), the Simpson Index incorporates both the number of species and the relative abundance of each species.

• Sensitive to Dominance: It gives more weight to common or dominant species. If a community is dominated by one or a few species, the index value decreases, reflecting lower diversity.

• Intuitive Interpretation: A higher Simpson Index of Diversity means a more diverse community, while a lower value indicates less diversity.

How Is It Used in Ecology?

Ecologists use the Simpson Index to:

• Compare biodiversity between different habitats or over time.

• Assess the impact of environmental changes or disturbances on species diversity.

• Guide conservation efforts by identifying areas of high or low diversity.

Simpson Index vs. Other Indices

• Shannon-Weaver Index: Uses a different formula involving the natural logarithm and focuses more on information content and evenness.

• Brillouin Index: Similar to Shannon but used for completely censused communities.

• Hill’s Equation: Refers to a family of diversity indices, not specifically 1-∑Pi².

Conclusion

The equation 1-∑Pi² is the formula for the Simpson Index of Diversity. It is a robust and widely used measure for quantifying biodiversity, especially when both species richness and evenness are important factors.

Correct answer:
(2) Simpson index