An index of diversity describes the relationship between the number of…

The idea

An index of diversity describes the relationship between the number of species and the number of individuals in each species. Unlike species richness, which only counts the number of species present, the index accounts for relative abundance. AQA uses the formula d = N(N - 1) / Σn(n - 1), where N is the total number of organisms of all species and n is the total number of organisms of each species. The formula is provided in exams. A community with individuals spread evenly across many species yields a high index, suggesting a stable ecosystem with many interdependent species. Conversely, a community dominated by a single abundant species gives a low index, often indicating a harsh or disturbed environment.

What earns the marks

• Stating that the index accounts for both the number of species and the number of individuals in each species, unlike species richness.
• Correct substitution into d = N(N - 1) / Σn(n - 1), with N as the total of all species and each n as the count for one species.
• Calculating the value of d, given to an appropriate number of significant figures, with no unit because the index is dimensionless.
• Explaining why two communities with the same species richness can have different indices of diversity, because relative abundance differs.
• Interpreting a higher index as indicating greater diversity, and noting that a change in relative abundance alone can alter the index without any species being lost.

Common mistakes and how to put them right

• Using N as the number of species rather than the total number of individual organisms.

  Put it right by summing all individuals across all species.

• Summing the n values before multiplying, instead of working out n(n - 1) for each species and then adding.

  Put it right by tabulating n and n(n - 1) per species.

• Giving the index a unit, when the index of diversity has none.

  Put it right by leaving the value as a dimensionless number.

• Assuming a fall in the index must mean species have been lost, when a change in relative abundance alone can cause it.

  Put it right by checking the species counts as well as the index.

• Treating a high index as proof of stability rather than an interpretation.

  Put it right by hedging, for example 'usually indicates' rather than 'proves'.

Exam tips

• Lay the calculation out as a table of n and n(n - 1) per species; marks are usually lost to arithmetic errors, not to method.
• Quote N as the grand total and check that it equals the sum of your n values before you divide.