Ramesh Kumar Tripathi3,
of Fisheries, Mangalore, 2 College
of Fisheries, Veraval, 3Central
Institute of Fisheries Education, Mumbai, 4College
of Fisheries, Ludhiana,
In its simplest
form, biological diversity is the variety of different types of
organisms present and interacting in an ecosystem. One could say
that more species equals more diversity, although a closer look will
soon require us to qualify that statement. There are, in fact many
more factors beyond a simple count of species that determine whether
biodiversity is higher or lower in any given ecosystem.
indices have been developed that mathematically combine the effects
of richness and eveness. Each has its merits, and may put more or
less emphasis upon richness or eveness. The most widely used is the
Shannon - Weaver Index. This index is explained in the handout titled
"Biodiversity Index." Read this and be familiar with the
concepts behind biodiversity and the Shannon/Weaver index. We will
be using this to calculate indices for several fish habitats.
is the total number of species found in an environment/sample.
is the probability that two randomly selected individuals belong to
two different species/categories.
is measuring the order/disorder in a particular system. This order is
characterized by the number of individuals found for each
species/category in the sample. A high species diversity may indicate
a healthy environment.
a measure of how similar the abundances of different
species/categories are in a community. Evenness is ranged from zero
to one. When evenness is close to zero, it indicates that most of the
individuals belongs to one or a few species/categories. When the
evenness is close to one, it indicates that each species/categories
consists of the same number of individuals.
is a statistic
that increases when the number of types into which a set of entities
has been classified increases, and obtains its maximum value for a
given number of types when all types are represented by the same
number of entities. When diversity indices are used in ecology,
the entities of interest are usually individual plants or animals,
and the types of interest are species
or other taxa.
the entities of interest can be people, and the types of interest
various demographic groups, and in information
the entities can be characters and the types the different letters of
the alphabet. The most commonly used diversity indices are simple
transformations of the effective number of types (also known as 'true
diversity'), but each diversity index can also be interpreted in its
own right as a measure corresponding to some real phenomenon (but a
different one for each diversity index.
Richness is a simple
numerical count of the number of different types of organisms
present. Species richness is a count of the number of species (named
or otherwise) that are present. Taxonomic richness is a count of the
number of different taxons
present. Recall that a taxon is any of our levels of classification.
One would presume that more species equals more diversity. However,
comparing two areas of equal species richness may show that they are
not equally diverse. For example, lets consider a list of tree
species in two forest ecosystems:
Water Oak Water
Post Oak Post Oak
Live Oak Pine
Bur Oak Cedar Elm
Pin Oak Pecan
community has seven different species, in A
all are within the same genus (and thus the same family, order,
class, and division), whereas in B
we have representatives of six genera, four families, two orders, two
classes, and two divisions. Clearly B
is more diverse.
Richness tends to
increase over area. In other words, a larger area will harbor more
different species, probably because of a larger variety of
microhabitats and resources. Additionally, sampling over a larger
area increases the chance of finding rare species. So, how large an
area (or how many samples) is necessary in order to have collected
all the species present? Several mathematical methods are used to
determine this. All are based upon the collector's
Simply put, the number of different species found are graphed
against the number of samples taken. For example, if you collected
100 samples, and each time you added another species to our list, you
should continue to sample, since you are still finding more species.
If, however, you do not discover anything in samples 51 through 100
that you hadn't already found in the first 50 samples, then 50
samples apparently was sufficient to find all the species present.
The term 'Simpson's
Diversity Index' can actually refer to any one of 3 closely related
measures the probability that two individuals randomly selected from
a sample will belong to the same species (or some category other than
species). There are two versions of the formula for calculating
Either is acceptable, but be consistent.
= the total number of organisms of a particular species
the total number of organisms of all species
The value of D
ranges between 0 and 1
With this index, 0
represents infinite diversity and 1, no diversity. That is, the
bigger the value of D, the lower the diversity. This is neither
intuitive nor logical, so to get over this problem, D is often
subtracted from 1 to give:
Index of Diversity
1 - D
The value of this
index also ranges between 0 and 1, but now, the greater the value,
the greater the sample diversity. This makes more sense. In this
case, the index represents the probability that two individuals
randomly selected from a sample will belong to different species.
Another way of
overcoming the problem of the counter-intuitive nature of Simpson's
Index is to take the reciprocal of the Index:
1 / D
The value of this
index starts with 1 as the lowest possible figure. This figure would
represent a community containing only one species. The higher the
value, the greater the diversity. The maximum value is the number of
species (or other category being used) in the sample. For example if
there are five species in the sample, then the maximum value is 5.
The Shannon index
has been a popular diversity index in the ecological literature,
where it is also known as Shannon's diversity index, the
Shannon-Wiener index, the Shannon-Weaver index, the Shannon-Weiner
index and the Shannon entropy. The measure was originally proposed by
to quantify the entropy
(uncertainty or information content) in strings of text The idea is
that the more different letters there are, and the more equal their
proportional abundances in the string of interest, the more difficult
it is to correctly predict which letter will be the next one in the
string. The Shannon entropy quantifies the uncertainty (entropy or
degree of surprise) associated with this prediction. It is calculated
is the proportion of characters belonging to the ith
type of letter in the string of interest. In ecology, pi
is often the proportion of individuals belonging to the ith
species in the dataset of interest. Then the Shannon entropy
quantifies the uncertainty in predicting the species identity of an
individual that is taken at random from the dataset.
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.
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. Simpson's Diversity
Index is a measure of diversity which takes into accounts both
richness and evenness.
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