Species richness is a measure of the number of different species in a…

The idea

Species richness is a count: the number of different species present in a community. It is quick to obtain and easy to compare between sites, but it treats every species as equal, so a community of a thousand individuals spread evenly across ten species has the same richness as one in which nine of those species are represented by a single individual. That is the limitation the index of diversity exists to overcome. To measure richness you have to identify every species in a sample, which is why organisms that cannot be told apart, or that are too small or too numerous to count, cause problems. Random sampling matters, because one quadrat or one field of view is not representative of a whole site, and taking repeat samples and calculating a mean reduces that error.

What earns the marks

• Defining species richness as the number of different species in a community, and stating that it takes no account of the number of individuals of each species.
• Comparing the species richness of two sites or two years using values read from a figure, and stating that richness alone does not show how abundance is distributed.
• Identifying random sampling, repeat samples and calculation of a mean as the way to make a richness estimate representative of a habitat.
• Explaining why identification is difficult for organisms that are too small, too numerous or cannot be told apart, and how this limits a richness estimate.
• Explaining that two communities with the same species richness can differ in diversity because relative abundance differs, which is why an index of diversity is also used.

Common mistakes and how to put them right

• Defining species richness as the number of organisms rather than the number of species.

  Put it right by counting different species, not individuals.

• Using the word biodiversity where the question asks specifically about species richness.

  Put it right by keeping richness as the species count and reserving biodiversity for the broader concept, which in AQA also covers genetic diversity and ecosystem diversity, not just species richness plus abundance.

• Assuming a higher species richness must mean a higher index of diversity.

  Put it right by checking relative abundance, since evenness can change the index independently.

• Sampling in one place, or examining one field of view, and treating the result as representative of the whole habitat.

  Put it right by using random sampling, repeat samples and a mean.

• Claiming a difference between two richness values is significant with no statistical support.

  Put it right by describing the difference as apparent from the data unless a statistical test has been applied.

Exam tips

• Count species, not organisms, and say so in your definition; the word 'different' carries the mark.
• When species richness and an index of diversity are plotted together, compare both before concluding that diversity has changed.
• If asked how to improve a richness estimate, give three separate points: random sampling, more samples, and calculate a mean.