Interactions Between Populations (College Board AP® Biology)

Simpson's Index of Diversity

• The Simpson's Index of Diversity is used to study the composition of communities

• Once the abundance of different species in an area has been recorded the results can be used to calculate the species diversity or biodiversity for that area

• Simpson's diversity takes into account the community effects of species cohabiting an area and interacting with each other

• Species diversity looks at the number of different species in an area (species richness) but also the evenness of abundance across the different species in that area (species evenness)

• Simpson’s index of diversity (D) can be used to quantify the biodiversity of an area

Simpson's index

• The formula is:

 where:

• n = total number of organisms for a single species

• N = total number of organisms for all species

• To calculate Simpson’s Index:

  • Step 1: First calculate n / N for each species

  • Step 2: Square each of these values

  • Step 3: Add them together and subtract the total from 1

• To understand what the value of D means you need to know the following:

  • The value of D can fall between 0 and 1

  • Values near 1 indicate high levels of biodiversity

  • Values near 0 indicate low levels of biodiversity

Using the formula: 

The results and working out are seen in the table below. The figures have been rounded to three decimal places for columns 3 and 4

D = 1 - 0.172 = 0.828

because the value of D is much closer to 1 than 0, it can be said that this is a relatively high value for biodiversity

The Biotic Factors

• Communities change over time as the interactions between different members of the community change

• Relationships among interacting populations can be both positive and negative and can be a major driving force in population dynamics

• These are examples of biotic factors in an ecosystem

• Examples include:

  • Predator/prey interactions; eg. as predators thrive, this puts pressure on prey populations

  • Trophic cascades; the effects of adding or removing a top predator from a food web

  • Niche partitioning; how natural selection drives species to occupy slightly different niches or use resources slightly differently to each other, to reduce competition

  • Competition; competition for resources drives natural selection where those best adapted will survive

  • Symbiosis including:

    • Parasitism; An example of a symbiotic relationship which results in harm to one organism whilst the other derives benefit e.g. ticks that live on rhinos in Africa

    • Commensalism; In this symbiotic relationship, one organism benefits whilst the other neither benefits or is harmed e.g. cattle egrets which benefit from the disturbance of insects from the grass being eaten by cattle - the cattle don't benefit from this relationship, but the egrets do

    • Mutualism; Both organisms benefit from the relationship with one another e.g. oxpeckers feasting on ticks from the back of rhinos in Africa

• The structure of a community is dependent on the energy availability

• Survival of organisms and species relies on there being sufficient energy available in the previous trophic level

• Access to that energy then determines the availability of matter and energy to the next trophic level

• Changes to the structure of the community will have a knock-on effect to all species that are interdependent

Biotic Factors Table