Biodiversity and the Index of Diversity

Species richness is the number of species present in a community or sample. Two communities can have the same richness but very different abundances: one may be dominated by a single species while another has a more even distribution.

An index combining species and abundance gives information beyond richness alone. The result still depends on sampling area, effort, identification and the groups counted. A higher value from a longer search is not automatically evidence that the underlying habitat is more diverse; methods must be comparable.

AQA uses d=N(N−1)/Σn(n−1), where N is the total number of organisms and n is the number belonging to each species. Calculate n(n−1) for each species, sum these terms, then divide N(N−1) by that sum.

For this reciprocal form, a higher value generally indicates greater diversity in the sampled community. Other courses use differently arranged Simpson-type indices with different ranges and interpretations. Do not replace the specified formula with 1−D from memory and then interpret the direction as though nothing changed.

Agricultural practices can reduce habitat variety, remove non-crop plants or alter food webs, sometimes reducing biodiversity. Hedgerows, field margins and less disruptive management can provide habitats, but their effects depend on local ecology and practice.

Evaluate conservation alongside food production, economic costs and other land-use needs. Evidence should compare suitably replicated sites and consider confounding, rather than treating all farms or all conservation interventions as identical. An index is a useful summary, not a full account of ecosystem function, genetic variation or the value of a rare species.

Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.