Biodiversity is an abbreviation for biological diversity and is the variety of living organisms in an area. It can be measured at different levels.
Methods of determining the diversity of species
Species richness. This is the number of different species in an ecosystem. It takes no notice of abundance and gives as much weight to those species which have very few individuals as to those who have thousands.
Species evenness. This is a measure of the relative abundance of the different species. A community dominated by one or two species is considered to be less diverse than one in which several different species have a similar abundance.
Percentage cover. If we want to know what kind of plants are in a particular habitat, and how many there are of each species, it is usually impossible to go and count each and every one present. Therefore, we use a quadrat and estimate percentage cover.
Percentage frequency. This is the probability that a species will be found within a single quadrat.
Percentage frequency = (number of quadrats in which the species is found/total number of quadrats) x 100
Simpsons Diversity Index (D). This measures the probability that two individuals randomly selected from a sample will belong to the same species. If you need extra help on calculating this, then head to the Simpsons Diversity page.
∑ = sum of
n = number of individuals
N = total number of individuals
The value of Simpsons Diversity Index ranges between 0 and 1. What does an index closer to 1 represent?
What are some problems associated with the percentage cover method of determining species diversity?
Comparing ecosystems across spatial and temporal scales
Ecosystems can be compared across spatial and temporal scales.
To understand how an ecosystem will respond requires accounting for the myriad interactions between biological and physical processes across spatial scales ranging from very local to global, and temporal scales ranging from within a day to decades.
Wikle, 2017
A spatial scale refers to the size of the ecosystem being considered. Ecosystems can be studied at a number of different sizes. Broadly:
Large scale: for example, an entire continent or ocean.
Medium scale: for example, a mountain range or desert.
Small scale: for example, a single local area such as a specific forest, paddock or mountain.
A temporal scale considers an ecosystem over a particular timeframe.
This can range from very small scale (i.e. hours to days) to medium scale (seasonal) to very large scale (years). A good example of this is succession, where pioneer species are the first to populate an area, but over time, a climax community will develop.
We can use a number of different factors to compare ecosystems.
Species diversity indices
This includes Simpsons Diversity Index (D), but there are a number of different indices that can be used. For example, the Menhinick Index:
D = species diversity
s = number of different species (i.e. species richness)
N = total number of individuals
On a walk in the Daintree rainforest, we saw 1 cassowary, 2 tree frogs, 2 golden orb spiders, 1 feral pig and 3 rufous owls. a) What is the species richness? b) Calculate D using the Menhinick Index.
Ecologists studied the process of succession in an area of wasteland over a period of fifteen years. They calculated the index of diversity of the area every year. After three years, the index of diversity was 1.7. After ten years, it had risen to 4.6. What information concerning the organisms present in the area is suggested by the increase in the index of diversity?.
Species interactions (biotic factors)
Abiotic factors
Remember that all of these abiotic factors can be considered both spatially (i.e. considering the substrate of a forest near Brisbane vs a forest near Mount Isa) or temporally (i.e. considering the climate of a forest near Brisbane in June; or 2009 vs 2020).
Tolerance Limits
Environmental factors can limit the distribution and abundance of species in an ecosystem. This is because each organism has their own specific tolerance limit or range that restricts where that organism could potentially live.
- Available area/size of habitat
- Climate
- Precipitation
- Temperature
- Wind direction and/or speed
- Cloud cover
- Sunshine
- Soil structure
- Thickness
- Structure and porosity
- pH
- Nutrient content
- Salinity
- Aquatic factors
- Light level
- Depth
- Turbidity (i.e. ‘cloudiness’ of the water)
- Salinity
- Oxygenation
- Temperature
- Water flow
- pH