(c) how to measure species richness and species evenness in a habitat

The idea

Species richness is the number of different species present in a habitat; species evenness describes how similar the abundances of those species are. To measure richness, sample the habitat representatively, for example with random quadrats (or a transect where a gradient exists), identify every species present and count the distinct species. To measure evenness, record the number of individuals of each species, then compare the relative abundances: a community where species have similar counts is even, while one dominated by a single species is uneven. Random sampling avoids bias, and a large enough sample or several quadrats improves reliability. Richness ignores abundance, so two habitats with equal richness can differ in evenness.

What earns the marks

• Species richness is the number of different species in a habitat, counted without regard to how many individuals each species has.
• Species evenness describes the relative abundance of the different species; a community is even when species have similar numbers of individuals.
• Richness is measured by representative sampling (for example random quadrats, or a transect along a gradient), identifying and counting the distinct species present.
• Evenness is measured by recording the number of individuals of each species and comparing their relative abundances.
• Random sampling reduces bias and using more quadrats or a larger sample improves reliability of the estimates.
• Two habitats can have the same richness but different evenness if one is dominated by a single species.

Common mistakes and how to put them right

• Confusing richness with evenness.

  Put it right: Richness counts species, not individuals; evenness compares how many individuals each species has.

• Assuming a habitat with more individuals is always richer.

  Put it right: Abundance does not change the number of species present.

• Ignoring sampling method: non-random or too few quadrats give unreliable, biased estimates of both richness and evenness.

Exam tips

• State clearly that richness counts species and evenness compares relative abundances, using those exact distinctions.
• Link measurement to a method: random quadrats for richness and counts per species for evenness.
• Mention that random sampling and adequate sample size improve reliability when justifying a method.