Diversity - the great variety of life
diversity can be quantified in many different ways. The two main factors taken into
account when measuring diversity are richness and evenness. Richness is a measure of the
number of different kinds of organisms present in a particular area. For example, species
richness is the number of different species present. However, diversity depends not only
on richness, but also on evenness. Evenness compares the similarity of the population size
of each of the species present.
The number of species per sample is a measure of richness. The more
species present in a sample, the 'richer' the sample.
Species richness as a measure on its own takes no account of the
number of individuals of each species present. It gives as much weight to those species
which have very few individuals as to those which have many individuals. Thus, one daisy
has as much influence on the richness of an area as 1000 buttercups.
Evenness is a measure of the relative abundance of the different
species making up the richness of an area.
To give an example, we might have sampled two different fields for
wildflowers. The sample from the first field consists of 300 daisies, 335 dandelions and
365 buttercups. The sample from the second field comprises 20 daisies, 49 dandelions
and 931 buttercups (see the table below). Both samples have the same richness (3 species)
and the same total number of individuals (1000). However, the first sample has more
evenness than the second. This is because the total number of individuals in the sample is
quite evenly distributed between the three species. In the second sample, most of the
individuals are buttercups, with only a few daisies and dandelions present. Sample 2 is
therefore considered to be less diverse than sample 1.
||Numbers of individuals
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.
As species richness and evenness increase, so diversity increases.
||To calculate a diversity index for a particular area, the area must
first be sampled. The number of individuals of each species present in the samples must be
noted. (This can be easier said than done! more here)
For example, the diversity of the ground flora in a woodland, might be tested
by sampling random quadrats. The number of plant species within each quadrat, as well as
the number of individuals of each species is noted. There is no necessity to be able to
identify all the species, provided they can be distinguished from each other.
The data can then be put into the formula for the Diversity Index
As an example, let us work out the value of D
for a single quadrat sample of ground vegetation in a woodland.
The higher the value of D, the
greater the diversity.
Of course, sampling only one quadrat would not give you
a reliable estimate of the diversity of the ground flora in the wood. Several samples
would have to be taken and the data pooled to give a better estimate of overall
diversity. How many samples?
There are many different diversity
indices, of which Simpson's Diversity Index is one of the best
(The index of diversity used on this page
is Simpson's Reciprocal Index)