Measures of Biodiversity

This lesson covers: 

  1. Species richness and species evenness
  2. The Simpson’s diversity index
  3. How to use polymorphism to quantify diversity

Species richness and species evenness

Species richness and species evenness are measures of biodiversity.


Species richness:

  • The total number of different species in a habitat.
  • It is quantified by taking random samples and counting the species present.
  • A higher species richness indicates greater diversity.


Species evenness:

  • A comparison of the numbers of individuals of each species in a community.
  • It is measured by taking samples and counting individuals of each species.
  • More even abundances mean higher species evenness and diversity.

The Simpson’s diversity index

The Simpson's index of diversity calculates biodiversity using the number of species and their relative abundances. That is, it takes both species richness and species evenness into account.


It is calculated as:

D=1−(∑(Nn​)2)


Where:

  • n = number of individuals of each species
  • N = total number of all individuals


D ranges from 0 to 1, with 1 representing maximum diversity.

Worked example - Calculating the Simpson’s diversity index

A biologist is assessing the biodiversity of two fields. field A contains 150 individual plants, consisting of 85 daisies, 35 buttercups, and 30 clovers. field B contains 150 individual plants, consisting of 55 daisies, 45 buttercups, and 50 clovers.


Calculate and compare the diversity indices for each habitat.


Step 1: Equation

D=1−(∑(Nn​)2)


Step 2: Construct a table to calculate ∑(Nn​)2 for field A

SpeciesNumber of individuals (n)(nN)2\left(\frac{n}{N}\right)^2
Daisies85(85150)2=0.3211...\left(\frac{85}{150}\right)^2 = 0.3211\text{...}
Buttercups35(35150)2=0.0544...\left(\frac{35}{150}\right)^2 = 0.0544\text{...}
Clovers30(30150)2=0.04\left(\frac{30}{150}\right)^2 = 0.04
SumN=∑n=150N = \sum{n} = 150∑(nN)2=0.4156...\sum\left(\frac{n}{N}\right)^2 = 0.4156\text{...}


Step 3: Substitution and correct evaluation for field A

diversity of field A =1−(∑(Nn​)2)

diversity of field A =1−0.4156...

diversity of field A =0.584 (to 3 s.f.)  


Step 4: Construct a table to calculate ∑(Nn​)2 for field B

SpeciesNumber of individuals (n)(nN)2\left(\frac{n}{N}\right)^2
Daisies55(55150)2=0.1344...\left(\frac{55}{150}\right)^2 = 0.1344\text{...}
Buttercups45(45150)2=0.09\left(\frac{45}{150}\right)^2 = 0.09
Clovers50(50150)2=0.1111...\left(\frac{50}{150}\right)^2 = 0.1111\text{...}
SumN=∑n=150N = \sum{n} = 150∑(nN)2=0.3356...\sum\left(\frac{n}{N}\right)^2 = 0.3356\text{...}


Step 5: Substitution and correct evaluation for field B

diversity of field B =1−(∑(Nn​)2)

diversity of field B =1−0.3356...

diversity of field B =0.664 (to 3 s.f.)


Step 6: Interpretation of result

the Simpson’s diversity index for field A is 0.584 and for field B it is 0.664, which indicates that field B has a higher species diversity compared to field A

Quantifying genetic biodiversity

Genetic biodiversity can be assessed by calculating the percentage of gene variants (alleles) in a genome within isolated populations.


A high genetic biodiversity means there is a large variety of alleles in the population's gene pool. This increases a population's ability to adapt to environmental changes, and so helps them avoid extinction.

Increasing genetic biodiversity

Genetic biodiversity of a population can be increased by:

  1. Gene flow - This is interbreeding between different populations, introducing new alleles.
  2. Mutations in the DNA of an organism - This creates new alleles.

Decreasing genetic biodiversity

Genetic biodiversity of a population can be decreased by:

  1. Selective breeding - Humans choose organisms with advantageous characteristics to breed (e.g. for rare dog breeds).
  2. Captive breeding programmes - A small group of individuals are bred in captivity.
  3. Artificial cloning - Using asexual reproduction to artificially produce large numbers of a particular (e.g. cloning crop plants).
  4. Natural selection - Species evolve and advantageous characteristics increase in a population.
  5. Genetic bottlenecks - A sudden decrease in population size means only a few individuals remain.
  6. The founder effect - A few individuals form a new population split from the original one.
  7. Genetic drift - Alleles are randomly passed through generations and some may disappear by chance.


Note: You will learn more about these processes in greater detail in later lessons.

Calculating the proportion of polymorphic loci

Polymorphism is when a gene locus has multiple alleles. A monomorphic gene is one where a single allele exists, keeping the basic structure of individuals in a species consistent.


The proportion of polymorphic loci measures diversity, where a higher proportion of polymorphic gene loci means there is a greater genetic diversity.


It is calculated as follows:

proportion of polymorphic gene loci =total number of locinumber of polymorphic loci​

Worked example - Calculating the proportion of polymorphic loci

A geneticist is studying a population of beetles and has identified a total of 50 loci. Out of these, 18 loci are found to be polymorphic.


Calculate the proportion of polymorphic gene loci in the beetle population.


Step 1: Equation

proportion of polymorphic gene loci =total number of locinumber of polymorphic loci​


Step 2: Substitution and correct evaluation

proportion of polymorphic gene loci =5018​

proportion of polymorphic gene loci =0.36

this means that 36% of the loci are polymorphic