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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