Species Richness & Diversity (SL IB Environmental Systems & Societies (ESS))

What is species richness?

• Species richness is the number of species in a community or defined area and can be a useful comparative measure in some cases
• However, in other cases, species richness can be a misleading indicator of diversity as it does not take into account the number of individuals of each species
• Once the abundance of each species in an area has been recorded, the results can be used to calculate the species diversity for that area
  
  • Species diversity looks at the number of different species in an area but also the evenness of abundance across the different species (i.e. their relative abundances)

Species Richness vs Diversity

• An index of diversity is a measurement that describes the relationship between the number of species present and how each species contributes to the total number of organisms that are present in that community
  • It is a much more informative measurement than species richness and conservationists often favour the use of an index of diversity as it takes into account both species richness and evenness
  • A commonly used index of diversity is known as Simpson’s Diversity Index

Example

• 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 evenness (and therefore a higher overall species diversity) than Area 2
• This example illustrates the limitations of using just species richness on its own

Area 1 and Area 2 have the same species richness but different species evenness. As it has a higher species evenness, the overall species diversity of Area 1 is higher than that of Area 2, as species diversity takes both richness and evenness into account

Calculating Simpson’s Diversity Index

• A group of students used the kick sampling technique to collect, identify and count the invertebrates inhabiting a river
• Samples were obtained from different sites along the course of the river
• The data was used to calculate the Simpson's Diversity Index at two different river sites
• This index of diversity is useful when comparing two similar habitats, or the same habitat over time
• The formula for calculating Simpson's Diversity Index, D, is:

Site A

Site B

• Site A was located just 5 km downstream from the river’s source
• Site B was located 50 m downstream from a sewage inlet pipe
• The lower diversity index for site B reflects the stress placed upon the river as a consequence of the pollution load from the sewage inlet point; although the river is recovering from the pollution, sensitive species are unable to tolerate the unfavourable abiotic conditions and species diversity is reduced