No communities are having all the species in equal abundance. Ecologists have devised numerous indices and ecological models as indicators of diversity, yet diversity remains hard to be defined specifically and constrained within such definitions.
The earlier conservation approaches in biodiversity were based on the principle that if the communities have more diversity, the ecosystem is more stable. But this is not the case always and conservation approaches has now replaced this simple approach with more complex mathematical models integrating and analyzing the species diversity indices. These indices are also used as indicators for changes in the community.
A good conservation approach consists of identifying the different indices which when put together gives the most fitting model that explains the stability of ecosystem. Alternatively a simple comparison of the different diversity indices or application of parametric statistical tests can be done to gain more reliable and significant results regarding the characteristics of ecosystem under observation.
Competitive exclusion reduces the species diversity in uniform and unstructured ecosystems. However, the species diversity would increase if the intra and interspecies interactions are more.
Diversity is measured by counting the number of species, assessing the relative abundance of the species, or by using an index which cumulatively analyzes the species richness and abundance.
The most common indices of species diversity are dominance index, Simpson's index and Shannon's index. Apart from these basic indices, there are more modified approaches and newer indices used in ecological studies. Some of them are listed below.
a. Pielou's evenness index
The index measures equitability and allows comparison of Shannon Weaver index with the distribution of individuals in the observed species that would have the maximum diversity. It is calculated as
J= H'/ log (S)
H- Shannon Weaver index and S- total number of observed species in the community
H can take a maximum value which is equal to log (S). Hmax is the theoretical maximum value for H(s) when all the species in the sample were equally abundant. The index measures how equal a community is numerically. The index can have values ranging from 0 to 1. When there are frequent variations in the community, the index has higher values.
b. Brillouin index
The index is more sensitive to species abundance.
It is calculated as:
HB = ln(N!) - Î£ln(ni!)
Where HB = the Brillouin index,
N is the total number of individuals in the sample,
ni is number of individual of species i,
ln(x) refers to natural logarithm of x
c. Fisher's alpha index
It is a tool to measure the diversity within a population. It is a parametric diversity index which assumes that species abundance follows log distribution. It is a scale independent indicator of diversity, but can be underestimated in communities where clustered distribution of species is found.
It is calculated by the formula S=a*ln(1+n/a)
where S- number of taxa, n- number of individuals and a- the Fisher's alpha.
d. Menhinick's index
It is the ratio of the number of taxa to the square root of sample size.
e. Margalef's index
It is given by (S-1)/ln(n)
Where S is the number of taxa, and n represents the number of individuals.
f. Shannon's Equitability index
It is Shannon diversity divided by the logarithm of number of taxa. This measures the evenness with which individuals of the community are divided among the taxa present. For a given equitability, the Simpson's index increases as the species richness increases.
Similarly for a given species richness, the Simpson's index increases as the species diversity increases.
This can also be calculated by expressing the Simpson's index as a proportion of the maximum value. Equitability value of 0 refers to complete evenness.
g. Berger- Parker Dominance Index: It is a simple mathematical expression relating the species richness and abundance
It takes into account only the commonest species in the sample and is calculated as
d = Nmax/N
where Nmax is the number of individuals in the most abundant species and N is the total number of species.
h. Sorensen's coefficient of community
It is used to measure similarities between communities. It is calculated as
CC = 2c / (s1+s2)
Where c is the number of species common to both communities; s1 number of species of community 1 and s2 is the number of species in community 2.
i. Percent Similarity (PS)
To calculate PS, add the lowest percentage for each species that the communities have in common. Percentage similarity is based on relative abundance of the species.
j. Buzas and Gibson's evenness measure is given by eH/S
where H' is the Shannon Weaver index and S is the species number
These diversity indices are now being used as indicators of pollution, for biomonitoring, for deciphering conservation models and to simply observe the trends of changes in ecosystems.
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