Diversity Indices (SL IB Environmental Systems & Societies (ESS))

• Communities can be described and compared through the use of diversity indices, which 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, and can be used to compare different communities or to track changes in diversity over time

• When comparing communities that are similar, low diversity could be indicative of several factors, such as pollution, eutrophication and recent colonisation of a site
  • For example, pollution can have a negative impact on the diversity of a community by reducing the number of species that can survive in the area
  • Similarly, eutrophication, which is the process of nutrient enrichment in aquatic ecosystems, can lead to a decrease in diversity as a few dominant species outcompete others
  • Finally, when a site is recently colonised, the diversity of the community is naturally low as it takes time for a diverse range of species to establish themselves

• Some commonly used diversity indices include the Shannon-Wiener index, Simpson's index, and the evenness index
  • The Shannon-Wiener index is a measure of the diversity of a community that takes into account both the number of species present and their relative abundances
  • Simpson's index, on the other hand, is a measure of the dominance of a few species within a community, and is calculated based on the probability that two individuals selected at random from the community belong to the same species

• Overall, diversity indices provide a powerful tool for describing and comparing communities
• By quantifying the diversity of species within a community, researchers can gain insight into the ecological processes that drive community structure, as well as track changes in diversity over time
• Additionally, when comparing similar communities, a low diversity index may indicate that one of the communities has undergone some kind of disturbance (usually caused by some kind of damaging and detrimental human activity)

Photo by Dustan Woodhouse on Unsplash

 A low biodiversity index may indicate detrimental human activity in a community

Case Study Example

• A study conducted by researchers in India compared the diversity of phytoplankton in a lake impacted by industrial pollution and a control lake (a similar lake that was not impacted by pollution
• The control lake, L1, was located in a rural area and had no known sources of pollution, while the impacted lake, L2, received effluent discharges from an industrial area

• The study was conducted over a period of six months, during which time the researchers collected samples of phytoplankton from each lake and calculated the Simpson Index values for each site
• The table below shows the Simpson Index values for each lake:

Simpson’s Diversity Index Values

Lake	Month 1	Month 2	Month 3	Month 4	Month 5	Month 6
L1	0.84	0.82	0.83	0.81	0.84	0.85
L2	0.62	0.63	0.61	0.61	0.62	0.64

• The results of the study showed that the reference lake, L1, had a higher diversity of phytoplankton compared to the impacted lake, L2
• The Simpson’s Index value for L1 ranged from 0.81 to 0.85, indicating a high level of diversity, while the Simpson Index value for L2 ranged from 0.61 to 0.64, indicating a low level of diversity

• The data also showed that certain taxa of phytoplankton were more abundant in the reference lake, while others were more abundant in the impacted lake
• For example, the reference lake had a higher abundance of diatoms and green algae, which are considered to be indicators of good water quality
• In contrast, the impacted lake had a higher abundance of blue-green algae, which are considered to be indicators of poor water quality

• This study provides a clear example of how the Simpson’s Index can be used to compare the diversity of the communities in two similar habitats
• The low Simpson’s Index value for the impacted lake indicates a reduced diversity of phytoplankton, which is likely due to the negative impact of pollution on the community