is the variation of life forms within a given ecosystem, biome, or
for the entire Earth. Biodiversity is often used as a measure of the
health of biological systems.
Once we have
produced the range maps for the all the species found at a particular
site (see our page
on species distribution maps), we can use the range map polygons
to calculate the biodiversity of the site.
Some of the biodiversity
indices which we can calculate are:
diversity index - the Shannon's diversity index
is an “information index” which measures biodiversity
based on the principle that diversity can be measured much
like the information contained in a code or message (e.g., the longer and
more complex the code, the greater the diversity). The Shannon's diversity
index is affected by both the number of species and their equitability, or
evenness. A greater number of species and a more even distribution of species
both result in an increase in Shannon's diversity. The maximum Shannon's
diversity for a sample is found when all species are equally abundant. Values
of the Shannon's diversity index for real communities typically fall between
1.5 and 3.5.
evenness - Shannon's evenness is derived from Shannon's
diversity index. Evennes is a measure of how similar the abundances
of different species are. When there are similar proportions
of all species, evenness approaches a value of 1.0. When the
abundances are very dissimilar (some rare and some common species),
then the value for evenness decreases.
dominance index - the Simpson's dominance index
measures biodiversity based on the probability that two individuals
randomly selected from a sample will belong to the same species
(or some category other than species). Simpson's dominance index
ranges from 0 (all taxa are equally present) to 1.0 (one taxon
dominates the community completely).
richness - species richness is the simplest measure
of biodiversity, and is simply a count of the number of different
species in a given area. Species richness is also referred to
as alpha-diversity. Species richness is commonly used, along
with other factors, as a measure for determining the overall
health of different biological ecosystems. High species richness
for a given area indicates a high level of ecosystem stability,
thus allowing the ecosystem to better withstand natural or anthropogenic
disturbanced (i.e. fires, floods, disease, deforestation, etc.).
Therefore, high levels of species richness in ecosystems typically
characterize these ecosystems as healthy and robust.
Biodiversity Analysis Example
Click image to enlarge.
is an example of the variation of species richness throughout a site.
The bottom map shows all the species range polygons
overlaid on one another. The top map shows the species richness for
the site based on the range polygons. Red indicates areas of high species
richness, whereas green indicates areas of low species richness.
values were calculated for this site:
richness = 16
diversity index = 2.44
evenness = 0.88
dominance index = 0.30
indicate that this site has a relatively even distribution of species
and a moderate level of biodiversity.