Biodiversity: (a) how biodiversity may be considered at different levels

The idea

Biodiversity is considered at several levels. At the habitat level, the range of habitats in an area is assessed. At the species level, species richness (the number of different species) and species evenness (how similar their abundances are) are considered. At the genetic level, variation within a species is assessed, which underpins adaptation and resilience. These levels are linked: a habitat with many niches can support more species, and genetically diverse populations are better able to survive environmental change. A single measure such as species richness alone does not capture genetic or habitat diversity, so biodiversity is described by combining levels.

What earns the marks

• Habitat diversity: the range of different habitats present in an area.
• Species diversity: combines species richness (number of different species) and species evenness (relative abundance of each species).
• Genetic diversity: variation in alleles or genes within a species or population.
• The levels are linked: varied habitats provide niches for more species, and genetic variation supports adaptation and resilience.
• Species richness alone is an incomplete measure because it ignores evenness, genetic variation and habitat variety.

Common mistakes and how to put them right

• Equating biodiversity only with the number of species present.

  Put it right: Evenness, genetic variation and habitat variety are also levels to consider.

• Confusing species richness with species evenness.

  Put it right: Richness counts species, evenness describes how similar their abundances are.

• Treating genetic diversity as a separate unrelated idea.

  Put it right: It is one of the levels of biodiversity and links to population resilience.

Exam tips

• Name each level explicitly (habitat, species, genetic) and give a one-line definition for each.
• When comparing areas, mention both richness and evenness rather than richness alone.
• Link genetic diversity to the ability of a population to adapt to change to show why the level matters.