Community Ecology

Simpson’s Index of Diversity ($D$) is a mathematical tool used to quantify the biodiversity of a habitat by accounting for both richness and evenness.

The formula is expressed as: $D = 1 - \sum \left(\frac{n}{N}\right)^2$ where $n$ is the total number of organisms of a particular species and $N$ is the total number of organisms of all species found in the community.

The resulting value of $D$ ranges from $0$ to $1$. A value closer to $1$ indicates high biodiversity and a more stable, complex community, while a value closer to $0$ indicates low biodiversity and potential vulnerability to environmental changes.

Community structure is heavily influenced by energy availability, as the survival of any trophic level depends on the energy captured and passed on by the level below it.

A trophic cascade is a powerful indirect interaction that can control entire ecosystems; for example, removing a top predator can lead to an explosion of herbivore populations, which then overconsume primary producers.

Because all species in a community are interdependent, a significant change in the population of one species often triggers a ripple effect across the entire food web.

Interpreting Simpson's Index: Always check if the question asks for the index of diversity ($D$) or the dominance index. In the $1 - \sum(n/N)^2$ version, a higher number always means higher diversity.

Identifying Symbiosis: When analyzing scenarios, look for the net effect on each participant ($+/+$, $+/0$, or $+/-$). If one organism is harmed, it is likely parasitism or predation, not commensalism.

Trophic Cascade Logic: When a predator is removed, follow the logic step-by-step: Predator $\downarrow \rightarrow$ Prey $\uparrow \rightarrow$ Producers $\downarrow$. Students often skip the middle step and get the final result wrong.

Sanity Check: If a community has 10 species but 99% of the individuals belong to just one species, the Simpson's Index should be very low, despite the high species richness.