Assessing Biodiversity (HL IB Environmental Systems & Societies (ESS))

Species Diversity

• Species richness is the number of species in a community or defined area

  • In some cases, it can be a useful measure to compare the biodiversity of different areas

• However, in other cases, species richness can be a misleading indicator of diversity

  • This is because 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 species evenness

    • Species evenness is the evenness of abundance across the different species (i.e. their relative abundances)

Species richness vs species diversity

• Species diversity is a much more informative measurement than species richness and conservationists often favour the use of species diversity as it takes into account both species richness and evenness

• For example:

  • Area 1 and Area 2 both contain four 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

Simpson's Diversity Index

• Biological communities can be described and compared through the use of diversity indices

  • These are mathematical tools used to quantify the diversity of species within a community

• These indices provide a measure of the variety of species present, as well as their relative abundances

  • They can be used to compare different communities or to track changes in diversity over time

• A commonly used diversity index is Simpson's index

Calculating Simpson’s diversity index