Levels of biodiversity (Plant Biodiversity)

Introduction, types, Explanation of each type of biodiversity levels.
For M.Sc, B.Sc students.

1. Levels of Biodiversity

2. Biodiversity literally means diversity of life that exists on earth. The term biodiversity was given by Walter G. Rosen (1985) as a compound word for the term ‘Biological Diversity’ originally proposed by Lovejoy (1980) as the number of species in a community. Biodiversity has been defined variously depending upon the objectives of the study. Its definitions vary in their scope from simply referring it to the number of species present in an area, to the variety and variability of life and its processes that exist on earth. It occurs at all the levels of biological organization beginning from the level of population to the level of biosphere. Therefore, it can be defined as the variation and the variability presenting the living world (the biosphere) within and among the populations, species, or the ecosystems.

3. Genetic Diversity It is basically the variety of species expressed at the  genetic  level by each individual in a species. No two individuals belonging to the same species are exactly similar. For example, in the species of  human beings , each human shows a lot of  diversity  in comparison to another  human . People living in different regions show a great level of variation. This variation forms the basis for natural selection and adaptations to changing environmental conditions and therefore, is highly important for survival of a species. Such that, if in future, there is an outbreak of any disease in a crop, all plants will be affected because all are more or less genetically similar. Genetic diversity is also crucial for evolution of new species.

4. It is the variation in the genetic material among organisms of a species. It is found both within as well as between populations of a species. For example, variation within and between populations of humans ( Homo sapiens ) found in different continents like Asia, and Africa, or various varieties of rice ( Oryza sativa ), or mango ( Mangifera indica ). This variation is attributed to the differences in the genetic make-up (nucleotide sequence of DNA)of organisms of species. This involves occurrence of variety of versions of a gene (alleles), and also numerous combinations of different genes among individuals of a species. Sources of this variation include phenomenon like genetic recombination (meiosis, crossing over, and sexual reproduction) and spontaneous mutations in the DNA. Therefore, no two individuals of a population are ever identical (unless they are identical twins); there is always slight difference in their genetic make-up.

5. Species Diversity It refers to the variety of species present in a community. However, not all the species present in a community are found in equal proportions; usually a few are relatively more abundant and most are less abundant. Therefore, the species diversity of an area or a community has two components, a)  species richness  or  variety  component, that represents the total number of species per unit area (species density), and b) the  relative abundance  or  apportionment  component that represents the relative proportion of each species in an area (i.e. number of individuals of each species are also considered to estimate relative abundance of each species over others in an area). The second component is very important to be included while comparing two communities for species diversity. Imagine two communities having equal number of species per unit area for example 5, however community 1 has 10 individuals of each species (total 50 individuals), while community 2 has 30 individuals of one species and 5 individuals of each of the rest four species.

6. So, though the two communities have same species richness but they differ in relative proportion of each species or in other words, community 2 is less even in terms of distribution of species over number of individuals. This property of a community is also called as  species evenness . Species diversity is of three types depending upon the spatial scale : i ) Alpha-(α)-diversity is the species diversity within a community, ii) Beta-(β)-diversity is the measure of variation in species diversity between two or more adjacent communities, and iii) Gamma-(γ)-diversity is represented as total number of species in a landscape or geographic region.

7. Ecosystem Diversity It is the measure of ecological variation. It includes variation within an ecosystem in terms of variety of niches, trophic levels, interactions among organisms viz. parasitism, predation, mutualism, etc. and ecological processes that are vital for sustaining ecosystems. For example , a forest community is more ecologically diverse than a wheat field because it has variety of interactions among different species, different species are having different trophic positions and functions. It also includes variation in terms of variety of ecosystems found within larger geographical region like forests, prairies, deserts, freshwater wetlands, lakes, rivers, coral reefs, estuaries etc. in a landscape, country, sub-continent, or a continent. For example, India has rich ecological diversity.