Download Tools and Techniques to study Biodiversity

Measuring Biodiversity
- Foram Mehta
Counting animals and plants, mapping genes, and systematically comparing
ecosystems may seem like a lot of trouble but ultimately can be estimate.
However, the numbers matter. In the field of conservation, biodiversity is
often a consideration within an area; being able to quantify what is being
conserved is essential for good planning and management.
Labeling a species or ecosystem "diverse" becomes relative; an estimate of
biodiversity will have recognizable limitations, like those of imperfect
sampling, but will give a comparison or point of reference.
The creation of indices gives scientists a standardized tool with which to
compare both ecosystem and species health. Therefore, although exact
diversity numbers are difficult to yield, knowing how biological resources are
distributed within a community can be extremely beneficial in determining
both short- and long-term trends.
Measuring biodiversity on an ecosystem level is thought to be a better way of
looking at the shape of the entire system, rather than the particular species.
However, it faces many of the same challenges.
Just as there are many different ways to define biodiversity, there are many
different measures of biodiversity. Most measures quantify the number of
traits, individuals, or species in a given area while taking into account their
degree of dissimilarity. Some measure biodiversity on a genetic level
while others measure within a single habitat or between ecosystems.
Oftentimes, information is not compiled in one specific place, a problem that
can lead to an overlap in the naming of species. Another limitation is an
inconsistency in treating the definition of species: what one scientist may
classify as a new species another may not.
Traditionally there are three levels at which biodiversity has been described.
In effect it uses genetic diversity as a basis for valuing both species diversity
(for their relative richness in different genes) and ecosystem diversity (for the
relative richness in the different processes to which the genes ultimately
contribute). Biodiversity or biological diversity is the variety of life in all its
forms, levels and combinations, including ecosystem diversity, species
diversity and genetic diversity.
Measuring Genetic diversity
It is a genetic diversity which causes tulips to be different colors and
different heights. Typically, researchers measure genetic diversity by
counting how often certain genetic patterns occur.
Measuring biodiversity on the genetic level requires gene map and then
compare them to the genetic make-up of the larger population.
1
Another method of measuring genetic diversity works in the reverse by
evaluating the differences in physical appearance between individuals then
attributes these traits to the most likely genetic roots. Mapping diversity at
the genetic level is currently the most accurate measure of biodiversity,
although it can be costly and time consuming and, thus, impractical for
evaluating large ecosystems. It is most often used to examine managed
populations or agricultural crops which can allow for selective breeding of
the most desirable traits.
Whittaker (1972) described three terms for measuring biodiversity over
spatial scales: alpha, beta, and gamma diversity. Alpha diversity refers to
the diversity within a particular area or ecosystem, and is usually expressed
by the number of species (i.e., species richness) in that ecosystem.
A. Species richness (Alpha diversity):
Biological diversity can be measured 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.
There are also many challenges when measuring species diversity. The
greatest of which is a lack of available data. Conducting a full count of the
number of species in an ecosystem is nearly impossible, so researchers
must use sample plots at a variety of sites but must avoid repetitive
counting.
Species richness is a common way of measuring biodiversity and involves
counting the number of individuals - or even families – within a given area.
This is also expressed as Alpha diversity (α-diversity). This can be
measured by counting the number of taxa (distinct groups of organisms)
within the ecosystem. However, such estimates of species richness are
strongly influenced by sample size, so a number of statistical techniques can
be used to correct for sample size to get comparable values.
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.
There are several keys created to measure species biodiversity; the most
popular are the Simpson Index and the Shannon Index. These indices
focus on the relative species richness and abundance and/or the pattern of
species distribution. The more species present in a sample, the 'richer' the
sample.
Species richness is the number of different species in a given area. It is
represented in equation form as S. It is the fundamental unit in which to
assess the homogeneity of an environment. Typically, species richness is
used in conservation studies to determine the sensitivity of ecosystems and
their resident species. The actual number of species calculated alone is
2
largely a random number. These studies, therefore, often develop a rubric or
measure for valuing the species richness number(s) or adopt one from
previous studies on similar ecosystems.
There is a strong inverse correlation in many groups between species
richness and latitude: the farther from the equator, the fewer species can be
found, even when compensating for the reduced surface area in higher
latitudes due to the spherical geometry of the earth. Equally, as altitude
increases, species richness decreases, indicating an effect of area, available
energy, isolation and/or zonation (intermediate elevations can receive
species from higher and lower).
Evenness:
Evenness is a measure of the relative abundance of the different species
making up the richness of an area.
For Example: If we have sampled two different fields for wildflowers. The
sample from the first field consists of 200 flowers A, 225 flowers B and 265
flowers C. The sample from the second field comprises 20 flowers A, 49
flowers B and 641 flowers C. Now plot the data in a table 1 as shown here.
Numbers of Individuals
Flower
Flower
Flower
Flower
Species
A
B
C
Total
Field 1
200
225
265
690
Field 2
20
29
641
690
Both samples have the same richness of 3 species and the same total
number of individuals (690). 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 of flower. In the
second sample, most of the individuals are flower C, with only a few samples
of flowers A and B present. Sample 2 is therefore considered to be less
diverse than sample 1.
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. Simpson's
Diversity Index is a measure of diversity which takes into accounts both
richness and evenness.
Simpson's Diversity Index: In ecology, it is often used to quantify the
biodiversity of a habitat. It takes into account the number of species
present, as well as the large quantity of each species. It measures the
probability that two individuals randomly selected from a sample will belong
to the same species. It can be measure with the following formula.
3
n = the total number of organisms of
N = the total number of organisms of all species
a
particular
species
Let select table 1 and choose any of the sample species from any of the field,
put the numbers in the index and calculate the diversity of that species in
respective field. 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.
Simpson's 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.
Simpson's Reciprocal 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 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 name 'Simpson's Diversity Index' is often very loosely applied therefore
it is important to find out which index has actually been used in any
comparative studies of diversity.
The Shannon Index, originally developed for use in information science,
accounts for the order or abundance of a species within a sample plot. This
is often used for identifying areas of high natural or human disturbance.
B. Ecosystem diversity (Beta Diversity):
At the ecosystem-level, measures of biodiversity are often used to
compare two ecosystems or to determine changes over time in a given
region. Beta diversity measures the present and changes of species diversity
between ecosystems; this involves comparing the number of taxa that are
unique to each of the ecosystems. In simpler terms, it calculates the number
of species that are not the same in two different environments. The resulting
4
number indicates to researchers whether there is any overlap in the species
found in each group.
There are also indices which measure beta diversity on a normalized scale,
usually from 0 to 1. A high beta diversity index indicates a low level of
similarity, while a low beta diversity index shows a high level of similarity.
At its simplest, beta diversity is the total number of species that are unique
between communities. This can be represented by the following equation:
β = (S1 − c) + (S2 − c)
Where,
S1= the total number of species recorded in the first community/ environment/
ecosystem
S2= the total number of species recorded in the second community/environment/
ecosystem
c= the number of species common to both communities/environment/ecosystem
β = beta diversity
For an example:
Two environments have a total of 12 species: a, b, c, d, e, f, g, h, I, j, k, l
In ecosystem1 there are 10 species: a- j
In ecosystem 2 there are 7 species: f-l
Both environments have 5 species in common i.e. f- j
So β = (10-5) + (7-5) = 7
The beta diversity of the two environments is 7. That is, there are seven
species which are either only in environment one or only in environment
two.
Now to calculate Basic Beta Diversity Index = 2c/ (S1+S2)
Same variables as before: S1, S2, c, and β
Multiply c by two; Divide that number by the sum of S1 and S2
For an example, let take the same situation as before
- C is equal to 5, so twice that is 10
- S1+S2 is 17
- 10 divided by 17 is 0.59, so 0.59 is the Basic beta diversity index
5
Sorensen’s similarity index
The Sorensen index is a very simple measure of beta diversity, ranging from
a value of 0 where there is no species overlap between the communities, to a
value of 1 when exactly the same species are found in both communities.
β = 2C/ (2C + S1 + S2)
Where, S1= the total number of species recorded in the first community
S2= the total number of species recorded in the second community
c= the number of species common to both communities
C. Taxonomic diversity of a region with several ecosystems - (Gamma
diversity):
Gamma diversity (γ-diversity) is a measure of total biodiversity of several
ecosystems within an entire region. It refers to the total species richness over
a large area or region. It is the product of α diversity of component ecosystems
and the β diversity between component ecosystems. It is also define as a
gamma diversity as "geographic-scale species diversity".
According to Whittaker (1972), gamma diversity is the richness in species of a
range of habitats in a geographic area (eg. a landscape, an island) and it is
resulting among them. Like alpha diversity, it is a quality which simply has
magnitude, not direction and can be represented by a single number (a
scalar).
Gamma diversity can be expressed in terms of the species richness of
communities as follows:
(γ = S1 + S2 − c)
Where, S1= the total number of species recorded in the first community
S2= the total number of species recorded in the second community
c= the number of species common to both communities
The internal relationship between alpha, beta and gamma diversity can be
represented as,
(β = γ / α)
With the use of following table one can understand the measuring tools used
to measure biodiversity of an entire region.
6
Species
Field A
A
B
C
D
E
F
G
H
I
J
K
L
M
N
Alpha diversity
(Total no. of individual
species)
Beta diversity
(S1 – c) + (S2 – c)/
(S2 – c) + (S3 – c)/
(S3 – c) + (S1-c)
Gamma diversity
γ = (S1 + S2) − (c12 +c23)
X
X
X
X
X
X
X
X
X
X
Field B
X
X
X
X
X
X
X
Field C
X
X
X
10
7
3
7
8
13
14
7