Edexcel A (SNAB) A Level Biology:复习笔记4.1.2 Measuring Biodiversity Within a Habitat

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:

 

    • 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

 

 

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