The relationships of ecosystems, both within and between, are very complex, sensitive to changes and have a profound influence on the other two levels of biodiversity.

      Species diversity represents the numbers and the degree of variability of families, genera and species. However, only species richness comes to be regarded for purposes of measuring species diversity.

      Species are now generally regarded as populations within which gene flow occurs under natural conditions. By this definition, the members of one species do not interbreed freely and do not produce fertile offspring, with the members of other species. New species originate due to a variety of causes which determine their structure and reproductive behaviour. For this reason, no single definition of a species is universally applicable. The concept of species is one of the much debated and inconclusive issues in biology.

      The exact number of species of all living organisms is not known, even approximately. About 2,75,000 species of flowering plants are scientifically known and some estimates (or guesstimates) indicate that an equal number of species of flowering plants awaits scientific discovery. Wilson (1988) estimated that the number of species of all organisms falls between 5 and 30 million. A recent estimate puts the known species at a little over 1.4 million (Gadgil, 1996). At present, probably about 1.5 million species of all organisms have been scientifically described and named. A far greater lot more is yet to be known to science. Ignorance, is thus the basic cause for the enormity and insolvability of several aspects of the species problem.

       Habitat variability promotes species richness. Wide ranges of distribution result in genetic diversity. In comparison to the temperate parts of the world, the tropical regions harbour a vast range of biodiversity. The global distribution patterns indicate that species richness decreases with latitude. Diversity also decreases with increasing altitude. In the marine habitats, diversity negatively correlates with depth.

       Genetic diversity is the amount of genetic variability occurring within a species, which is the sum total of genetic information contained in the genes of all individual organisms of that species. This is the most important aspect of biodiversity.

       Some amount of genetic diversity is readily discernible in the form of visible differences in the expression of a particular character. The medicinally important species, Catharanthus roseus has usually pink flowers but a white flowered variety also occurs along with the pink flowered variety. In this species, the flower colour is also associated with the distribution of anthocyanin (the pink pigment) on the vegetative parts. Another example is Clitoria ternatea where both blue and white flowered varieties coexist. Such visibly apparent variability may or may not have a genetic basis, as some variability is due to an interaction of the same genotype with different types of the environment (phenotypic plasticity). Live plants of the ‘black’ variety of the sacred basil (Ocimum sanctum, krishna tulasi), collected from the Western Ghats, became green in a couple of months at Bangalore, on profuse watering. Some amount of variability is due to the differences in the genetic constitution of individuals/populations of a species. Such gene based variability is faithfully transmitted to the subsequent generations. Only this gene based variability is important in issues of biodiversity.