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4.4.4Unit 4: Energy, Environment, Microbiology and Immunity

Species Richness and Heterozygosity Index — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 5 parts Lesson 24 of 28 Unit quiz

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Part 1 of 5Richness and Abundance

4.4.4 Species Richness and Heterozygosity Index

A community is a group of populations of different species living together, and its diversity can be described in several ways.

Richness and Abundance
Species richness
  • The simplest measurethe number of species present in the community.
Abundance
  • The relative number of individuals of each species in an area.
Limitation
  • Two areas can have equal richness but different diversity if one is dominated by a single species and has very rare others.
Index of diversity
  • Conservationists prefer an index that combines the number of species with their evenness.

Part 2 of 5Sampling Methods

It is impossible to count every organism in a large ecosystem, so random samples are used to estimate numbers.

Random quadrats
  • Square frames are placed at points chosen by a random number generator on a grid to avoid bias,
    • and abundance or percentage cover is recorded.
Sweep nets
  • Strong fine-mesh nets swept through long grass catch flying insects and those living in vegetation.
Pitfall traps
  • Cans or jars buried in the ground catch ground-dwelling invertebrates, often nocturnal ones, which fall in.
Kick sampling
  • A net is held on a stream bed facing the flow while the bed is churned for a set time so invertebrates wash in.
Good practice
  • Take as many samples as possible and use the same method when comparing habitats.

Part 3 of 5Heterozygosity Index

Genetic diversity can be judged from phenotypes, DNA sequences or the heterozygosity index.

1
Organisms with two different alleles at a gene locus are heterozygotes, and a population with more of them has greater genetic diversity.
2
The heterozygosity index H is the number of heterozygotes divided by the number of individuals in the population.
3
H can be calculated at many loci and the values averaged to estimate diversity across the genome.
4
A greater variety of phenotypes, or of base sequences found by DNA sequencing, also indicates greater diversity.

Part 4 of 5Worked Example

Question

  • A population of 240 plants is studied at one gene locus.
  • Of these plants, 90 are heterozygous.
  • Calculate the heterozygosity index.

  1. H = number of heterozygotes ÷ number of individuals in the population.
  2. H = 90 ÷ 240 = 0.375.
  3. Rounded to 2 decimal places, H = 0.38.

Answer H = 0.38, meaning 38 % of the plants are heterozygous at this locus.

Part 5 of 5Exam Checklist & Tips

4.4.4 – Key Points for the Exam

  • Community Populations of different species living and interacting in the same place.
  • Quadrat A square frame used to sample plants or slow-moving organisms.
  • Heterozygote An individual with two different alleles of a gene.
  • Heterozygosity index The proportion of individuals in a population that are heterozygous at a locus.

Common Mistakes

  • Using non-random placement of quadrats, which introduces sampling bias.
  • Dividing heterozygotes by the number of homozygotes rather than the total number of individuals.
  • Using a quadrat to sample fast-moving animals, when nets or traps are more suitable.
  • Saying high species richness always means high diversity, ignoring abundance.

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