Species Richness & Heterozygosity Index (Edexcel International A Level Biology): Revision Note

Species Richness & Heterozygosity Index

• The place where an organism lives is called its habitat

• Habitats can vary in size, with some being very large (e.g. grasslands) while others are very small (e.g. a puddle of water that formed after heavy rain)

• Measuring the species diversity within different habitats can be useful in making comparisons between them or when studying how habitats change over time

• One way to determine species diversity is to measure species richness in a habitat

• Species richness is the number of species within a community

  • A community is a group of populations of different species living in the same place at the same time that interact with each other

• Species richness is the simplest way to measure species diversity

• A community with a greater number of species will have a greater species richness score

  • For example, a tropical rain forest has a very high number of different species so it would be described as species-rich

• Species richness can be a misleading indicator of diversity as it does not take into account the number of individuals of each species

• Species abundance is a measure of the relative number of individuals in the different species within a given area

  • In the example below, Area 1 and Area 2 both contain 4 tree species

  • However, Area 2 is actually dominated by one species and in fact, one of the species is very rare (only one individual)

  • Although the two areas have exactly the same species richness, Area 1 has a higher species abundance (and therefore a higher overall species diversity) than Area 2

  • This example illustrates the limitations of using just species richness on its own

• Conservationists often favour the use of an index of diversity as it takes into account species number and evenness

Area 1 and 2 have the same species richness but different species abundance. Area 1 will have a higher overall species diversity as it has a higher species abundance. Species diversity takes both richness and abundance into account.

• Measuring the different levels of biodiversity within an ecosystem can be challenging

• Finding out which species live in an ecosystem and the size of the populations requires the identification and cataloguing of all organisms present to build a species list

• This is possible for areas that are very small or where the species are very large like trees

• However, for larger and more complex ecosystems like rainforests, it is simply impossible to find, identify and count every organism that exists there

• When this is the case, different samples of the area can be taken and used to make an estimate for the total species numbers in the area

Random sampling

• Some ecosystems are very complex with large numbers of different species of different sizes

• For the sake of logistics, random sampling is often used to estimate the distribution and abundance of species

  • The distribution of a species describes how it is spread throughout the ecosystem

  • The abundance of a species is the number of individuals of that species

• When carrying out sampling, square frames called quadrats can be used to mark off the area being sampled

  • Quadrats are square frames made of wood or wire

  • They can be a variety of sizes eg. 0.25m² or 1m²

  • They are placed on the ground and the organisms within them are recorded

  • They can be used to measure the distribution and abundance of plants 

Using a quadrat to investigate population size or distribution

• Quadrats of different sizes can be used depending on what is being measured and what is most suitable in the space the samples are being made in

• Quadrats must be laid randomly in the area to avoid sampling bias

  • This random sampling can be done by converting the sampling area into a grid format and labelling each square on the grid with a number

  • Then a random number generator is used to pick the sample points

• Once the quadrat has been laid on the chosen sample point the abundance or percentage cover of all the different species present can be recorded

Using a quadrat to investigate the percentage cover of two species of grass. Some squares may be lacking any species while other squares may have multiple species in them - this means that the total percentage cover may sometimes be over or under 100%

• Quadrats are suitable for sampling plants 

• For many animal species, however, it is not possible to use quadrats to measure their distribution and abundance

• In these cases, other techniques involving other items of equipment are necessary, including:

  • Sweeping nets: these are large, strong nets with a fine material (very small holes) that are used to catch flying insects and insects that live in long grass by sweeping the net back and forth through the grass

  • Pitfall traps: these are cans or jars that are buried in the ground that are used to catch ground-dwelling (often nocturnal) insects and other invertebrates as they fall into the trap

  • Kick-sampling: this technique is used to catch freshwater invertebrates living in streams or rivers. A net in placed on the stream-bed so that the water is flowing into it and the stream-bed just above the net is churned up by the scientist (using their foot) for a set period of time. The invertebrates are carried by the stream into the net

• Take as many samples as possible to get a more accurate indication of the entire habitat

• The results can be used to estimate the total number of individuals or species richness in the habitat

• It is important to use the same sampling method when gathering data to compare different habitats with one another

Example of how a pitfall trap can be used

Example of how kick-sampling is done

Measuring genetic diversity within a species

• Genetic diversity refers to the different alleles that occur within the gene pool of a species or a population

• The greater the allele variety within a species, the higher the genetic diversity will be

• Measurements of genetic diversity is useful to investigate changes occurring in a population over time or when comparing two species with one another

• To measure genetic diversity two factors can be considered:

  • Phenotype

  • Genotype

• The phenotype of an organism refers to its observable features

• Different alleles are responsible for the variety of phenotypes that can be observed within a species

• Therefore, the greater the variety of phenotypes within a species, the higher the genetic diversity

• An organism's genotype is determined by the different alleles that are found within the cells

• Different alleles will have a different order of bases in the DNA molecules

• These base orders can be determined by sequencing the DNA of individuals in a species

• The higher the number of different alleles for a characteristic, the greater the genetic diversity within that species

• Another way to determine the genetic diversity within a species is to use the heterozygosity index

Heterozygosity index

• Organisms that have two different alleles at a particular gene locus are known as heterozygotes

• The higher the proportion of heterozygotes in a population, the greater the genetic diversity of that population will be

• We can use the following formula to calculate the heterozygosity index (H):