Systematic work on identifying and naming species has been in progress for the last 250 years. But still, we have collected, described and named far less number of species that the actual number present.
The known and described number of species of all organisms on the earth is between 1.7 and 1.8 million, which is fewer than 15percent of the actual number. The predicted number of total species varies from 5 to 50 million and averages at 14 million. About 61 percent of the known species are insects. Only 4650 species of mammals are known to science.
A large number of plant species and vertebrates are known. There are many more species that have not yet been described, especially in the tropics.
Information about bacteria, viruses, protists and Archaea is just fragmentary. However, new species are being discovered faster than ever before due to the efforts of projects like Global Biodiversity Information Facility and the Species 2000.
The number of species of different taxonomic groups, described from India.
There is a fascinating variety of organisms, complex ecological relationship among organisms, genetic diversity within species and a great variety of ecological systems. To develop conservation plans for biodiversity, we must be clear about the concept of biodiversity.
Community and ecosystem diversity
These levels of biodiverstity are interrelated, yet distinct enough to be studied separately to understand the interconnections that support life of earth.
The diversity of basic units of hereditary information (genes) which are passed down the generations, found with a species.
The genetic diversity within a species is expressed by many terms, subspecies, breeds, races, varieties and forms,. This diversity arises from ‘‘variations in the sequence of four base pairs which as components of nucleic acids which constitute the genetic code.’’
Genetic information is stored in genes. In Mycoplasma, there are about 450-700 genes; in Escherichia coli 4000, in Dropsophila melanogaster 13000, in Oryza sativa 32000-50000 and in Homo sapiens. The number of genes ranges in between 35000 and 45000.
Due to genetic diversity, population adjusts in its environment and respond to natural selection. More the genetic diversity in an organism, better is the adaptation to changed environmental conditions. A sort of uniformity is obtained with lower genetic diversity.
This happens with large monocultures of genetically similar crops. This condition is considered to be better for increased production of crops. However, it may create a problem when crops are attacked by insects or fungal disease. Genetic variation leads to evolution of new species (speciation).
Species diversity refers to variety of species in a region. Number of species per unit area is called species richness.
Eveness or equitability differs due to difference in number of individuals in an area.With increase in area, number of species increase. Usually, species diversity increases, if species richness is higher. Somehow, number of individuals among species may differ. This may lead to differences in evenness or equitability.
This also results to change in diversity.
The diversity of populations of organisms which interbreed or are reproductively isolated from other such populations (WCMC, 1992). Species are the most commonly used unit describing biodiversity. Comparisons of the diversity find in different countries or ecosystems, for instance, are almost always based on species number. According to investigation into Amazonian rainforest canopy (Wilson, 1988), worldwide some 5 to 30 million species are believed to exist. Nearly, 1.7 million have so far been described (Myers, 1993). Species diversity has frequently been used as an indicator of the conservation significance of an area.
This demarcates three levels of components (Cowling et. al. 1989, WCMC 1992).
Fig. : Three perspectives of diversity : Alpha, Beta, Gamma diversity
The diversity of ecological complexes or biotic communities found in a given area (McNeely et.al. 1990). Indian ecosystem diversity is described at each of the three levels (biogeographical region, biotic province and biome). Largest of identified ecosystems has been the biogeographical zone. The vast area covered by biogeographical zone contains a wide diversity of smaller units called biotic provinces. Finally, within each biotic province, various kinds of biomes are distinguished. Biome classification broadly follows commonly used terminology, distinguishing between forests, grasslands, wetlands, deserts, and other such ecosystems on the basis of their physical appearance and dominant biotic or abiotic element.
Fig. : Biogeographical regions of India