Measuring Biodiversity Within a Habitat
- 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 organisms genotype is determined by the different alleles that is 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):
Worked example
In pea plants the gene controlling seed shape exist in two forms: Allele R codes for round seeds and is dominant over allele r which codes for wrinkled seeds. It is found that within a population of 620 pea plants, there are 350 heterozygous (Rr) individuals.
Calculate the heterozygosity index for the pea plants at the locus for seed shape.
The formula is:
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- It is possible to determine the value for H at many loci, then calculate an average value
- This value can then be used as an estimation of the genetic diversity of the entire genome of the population