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BIODIVERSITY INDICES


Jitendra Kumar1*, Neeraj Pathak2, Ramesh Kumar Tripathi3, Archit Shukla4, Saurabh Dubey1

1 College of Fisheries, Mangalore, 2 College of Fisheries, Veraval, 3Central Institute of Fisheries Education, Mumbai, 4College of Fisheries, Ludhiana, Punjab



What is Biodiversity?

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.


Biodiversity indices

Several biodiversity 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.

Species richness (S) is the total number of species found in an environment/sample.

Simpson's index (D) is the probability that two randomly selected individuals belong to two different species/categories.

Shannon-Wiener index (H) 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.

Evenness (E) is 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.

A diversity index 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. In demography, the entities of interest can be people, and the types of interest various demographic groups, and in information science, 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

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:


Community A Community B

Water Oak Water Oak

Post Oak Post Oak

Blackjack Oak Hickory

Live Oak Pine

Bur Oak Cedar Elm

Pin Oak Pecan

Hickory Black Walnut


 Although each 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 curve. 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.


Simpson's Diversity Indices

The term 'Simpson's Diversity Index' can actually refer to any one of 3 closely related indices.

Simpson's Index (D) 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 D. Either is acceptable, but be consistent.

D =(n / N)2

n = the total number of organisms of a particular species
N = 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:

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 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.


Shannon index

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 Claude Shannon 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 as follows:

wherepi 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

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

Flower Species

Sample 1

Sample 2

Daisy

300

20

Dandelion

335

49

Buttercup

365

931

Total

1000

1000


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.



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