Diversity is defined as the no of species present in a community termed as "species richness". We can represent species biodiversity at different geographical scales as alpha, beta and gamma diversity.
(a) Alpha diversity: it indicates diversity within a particular area or ecosystem, and is usually expressed by no of the species. Thus alpha diversity is measure of "species richness".
(b) Beta diversity: it refers to the changes in species richness between ecosystem. The higher heterogeneity in the habitats in a region or greater dissimilarity between communities exhibit higher beta diversity.
Gamma diversity: it refers, to the diversity of the habitats over the total land scope or geographical area. The higher diversity at community levels provides stability and higher productivity. In temperate, grasslands, it has been observed that diverse communities are functionally more productive and stable, even under environment stresses such as prolonged dry conditions.
Extinction of species: A species is said to be extinct, when there is no reasonable doubt that its last individual has died.
The characteristic which make a species susceptible to extinction are listed below:
(i) Large body size eg. Elephant, lion and bengal.
(ii) Small population size and low reproductive rate eg. Blue whale, giant panda.
(iii) Feeding at high trophic levels in food chain eg. Bald eagle and bengal tiger.
(iv) Fixed migratory routes and habitat eg. Blue whale and whooping crane.
(v) Localized and narrow range distribution e.g. woodland caribou and island species.
Species become extinct by these three processes:
(i) Natural extinction
(ii) Mass extinction
(iii) Anthropogenic extinction
According to the world conservation monitoring centre (WCMC),533 animal (mostly vertebrates) and 384 plant species (mostly flowering plant) have become extinct since the year 1600.