International A Level Biology / Biodiversity and conservation
Species richness and the heterozygosity index revision notes
Edexcel International A Level Biology (YBI11), chapter 12: Biodiversity and conservation, specification point 4.17. Free lesson.
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Species richness counts the different species in a habitat, while the heterozygosity index measures genetic diversity within a population. You need to know how to collect the data fairly and calculate or rearrange the index.
What species richness measures
Species richness is the number of different species in a habitat. It is a count of kinds of organism, not a count of how many organisms there are. Use the phrase “number of different species” for the definition. The number of organisms or the number of individuals of one species is not species richness.
How to measure species richness
You usually cannot search every part of a habitat, so you use a sample. Define the habitat first so its boundary and area are clear. Then sample at random, identify every species found and count the different kinds. Use quadrats for plants, and nets or traps for animals. Include plants and animals, rather than counting only one group. To compare two habitats fairly, sample the same area in the same way in each.
How random quadrat sampling works
For plants, two tape measures can mark the edges of the defined habitat. They act as axes, so each point has a pair of coordinates. A random number generator selects coordinates, which prevents you choosing a point because it looks interesting. Place one corner of the quadrat on the selected point, identify every species inside it, then count the kinds. The habitat boundary must be fixed before sampling, so the area being studied is clear. Count different species inside the quadrat, not the number of individual plants.
What the heterozygosity index tells you
Genetic diversity is the variety of alleles within one species. The heterozygosity index, H, measures the proportion of individuals that are heterozygous for the gene being studied. A heterozygote has two different alleles of that gene. H = number of heterozygotes ÷ number of individuals in the population. The denominator is every individual in the population, not the number of species. H is a proportion from 0 to 1, so do not multiply it by 100. A higher H means more genetic diversity. Fewer alleles can mean less chance that some individuals survive a new disease.
Calculate or rearrange H
Put the number of heterozygotes on top and the total number of individuals underneath. To work backwards from H, multiply it by the number of individuals. A heterozygote count should be a whole number.