Quadrats: Square frames used to define a specific area for counting sessile or slow-moving organisms; data can be recorded as percentage cover, frequency, or individual counts.
Transects: Lines laid across a habitat to study distribution; a line transect records organisms touching the line, while a belt transect uses quadrats at set intervals along the line.
Simpson's Index of Diversity (): A mathematical tool used to measure diversity by accounting for both richness and evenness.
Formula:
= total number of organisms of a particular species
= total number of organisms of all species
Interpretation: The value of ranges from 0 to 1. A value closer to 1 indicates high diversity and a stable ecosystem, while a value closer to 0 indicates low diversity and potential vulnerability.
Check the Formula: Always ensure you are using the correct version of Simpson's Index; some variations use and for small populations.
Units and Scale: Remember that Simpson's Index is a ratio and has no units; however, the final value must always fall between 0 and 1.
Justify Sampling: In exam questions, always explain why a specific sampling method was chosen (e.g., 'Random sampling was used to avoid bias in a uniform meadow').
Identify the Gradient: If a question mentions a change in light, moisture, or altitude, the correct answer for sampling methodology is almost always a transect.
Confusing and : Students often swap the number of individuals in one species () with the total population () when calculating the index.
Misinterpreting 'High Diversity': A high index value does not just mean 'many species'; it means the ecosystem is robust and less likely to be devastated by a single environmental change.
Ignoring Small Organisms: Sampling often suffers from 'charismatic megafauna' bias, where larger or more colorful species are counted while smaller, cryptic species are overlooked.
