Community Ecology (College Board AP® Biology) : Study Guide

Simpson's index of diversity

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

  • Once the abundance of different 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; species richness

  • the evenness of abundance across the different species in that area; species evenness

• Simpson's index of diversity is used to study the structure of communities, including:

  • species composition, i.e. which species are present

  • species diversity

Calculating 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: calculate n / N for each species

  • Step 2: square each of these values

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

• 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

Interactions between populations

• Communities change over time due to changing interactions between different members of the community

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

• Examples of biotic factors in an ecosystem that can alter population dynamics include:

  • predator/prey interactions, e.g. as predators thrive, this puts pressure on prey populations

  • trophic cascades; the effects of adding or removing a top predator from a food web which impacts all species and energy distribution

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

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

  • symbiosis:

    • Parasitism; 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; 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 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 a community will have a knock-on effect on all species that are interdependent