Biodiversity, endemism and measuring diversity

Biodiversity and threats

Biodiversity is the variety of living organisms in an area. It includes the number of species and the genetic diversity within each species.

An endemic species is found naturally in only one geographical area: for example kiwis are endemic to New Zealand. Endemic species are at particular risk because they have nowhere else to live.

Over millions of years evolution has produced enormous diversity, but human activity now threatens it through habitat destruction, pollution, overhunting, invasive species and climate change.

Within a habitat and within a species

• Species richness: the number of different species in a habitat. Simple, but it ignores how many of each there are.
• Genetic diversity within a species can be measured with the heterozygosity index:

H = number of heterozygotes / number of individuals in the population.

A higher H means more genetic diversity.

Comparing habitats: index of diversity

The index of diversity takes account of both the number of species and how many individuals of each:

D = N(N - 1) / Σn(n - 1)

where N is the total number of organisms of all species and n is the number of organisms of each species. A higher D means a more diverse habitat.

Worked example

A habitat has 10 of species A, 6 of B and 4 of C. Calculate D.

1. N = 20, so N(N - 1) = 20 x 19 = 380.
2. Σn(n - 1) = 10 x 9 + 6 x 5 + 4 x 3 = 90 + 30 + 12 = 132.
3. D = 380 / 132 = 2.88

Answer: D = 2.88

Check yourself

1. A pond has 8 water snails, 4 water boatmen and 3 mayfly nymphs. Calculate the index of diversity D to 2 decimal places.
   Show answer

   2.84. N = 15, N(N - 1) = 210; Σn(n - 1) = 56 + 12 + 6 = 74; D = 210 / 74 = 2.84.

2. A habitat has 5 individuals of each of 4 species. Calculate D to 2 decimal places.
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   4.75. N = 20, 380; Σn(n - 1) = 4 x 20 = 80; D = 4.75.

3. In a population of 60 animals, 18 are heterozygous at a particular gene locus. Calculate the heterozygosity index.
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   0.30. 18 / 60 = 0.3.

4. A survey finds 12 woodlice, 5 beetles, 5 spiders and 2 centipedes. What is the species richness?
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   4. Four different species.

5. What does endemic mean?
   • Found naturally in only one geographical area
   • Found everywhere on Earth
   • Introduced by humans
   • Extinct in the wild
   Show answer

   Found naturally in only one geographical area. Endemic species are restricted to one area.

6. Why is the index of diversity more useful than species richness?
   • It only counts the most common species
   • It is easier to calculate
   • It takes account of the number of individuals of each species as well as the number of species
   • It ignores rare species
   Show answer

   It takes account of the number of individuals of each species as well as the number of species. D includes evenness (abundance) as well as richness.

7. Which is a major threat to biodiversity caused by humans?
   • Habitat destruction
   • Natural selection
   • Photosynthesis
   • Seed dispersal
   Show answer

   Habitat destruction. Clearing habitats removes the places organisms live.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• The Science Hive: Biodiversity (Edexcel A)

Last updated 7 November 2026. Spotted a mistake? Tell us.