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The major causes of biodiversity loss (accelerated rates of species extinctions) is largely due to 'The Evil Quartet' which includes over-exploitation, co-extinction, alien species invasion and habitat loss and fragmentation.

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Tropical regions show greatest level of species richness because of the following reasons:
(I) Tropical latitudes have remained relatively undisturbed for millions of years and thus, had a long evolutionary time for species diversification.
(II) Tropical environments are less seasonal, relatively more constant and predictable. Such constant environments promote niche specialisation and lead to a greater species diversity.
(III) There is more solar energy available in the tropics, which contributes to higher productivity; this in turn might contribute indirectly to greater diversity.

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The Rivet popper hypothesis was proposed by Stanford ecologist Paul Ehrlich as an analogy. He compared the rivets in an airplane to species within a genus. According to him, in an airplane (ecosystem), all parts are joined together using thousands of rivets (species). If every passenger travelling in it starts popping a rivet (causing a species to become extinct), it may not affect flight safety (proper functioning of the ecosystem) initially, but as more and more rivets are removed, the plane becomes dangerously weak over a period of time. Loss of rivets on the wings (species that drive major ecosystem functions) is obviously a more serious threat to flight safety than loss of a few rivets on the seats or windows inside the plane.

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Keystone species are those species which has significant and disproportionately large influence on the community structure and characteristics. It has often considerably low abundance and biomass as compared to dominant species. Removal of such species causes serious disruption in structure and function of community.

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There are four major causes of biodiversity losses which are collectively called as the evil quartet. These are habitat loss and fragmentation, over-exploitation, alien species invasion and co-extinctions. However, other factors are also intensifying extinctions like disturbance and degradation, pollution, intensive agriculture and forestry.

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Habitat loss and fragmentation is the most important cause driving animals and plants to extinction. The most dramatic examples of habitat loss come from Amazon tropical rain forest.

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There are four major causes (The Evil Quartet) of biodiversity losses: habitat loss and fragmentation, over exploitation, alien species invasion and co-extinctions.

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Habitat loss and fragmentation is the most important cause of driving the animals and plants to extinction. When large habitats are broken into small fragments due to various human activities, mammals and birds requiring large territories and certain animals with migratory habitats are badly affected, leading to population declines. The same can be applicable to the plant (forest) loss and degradation as millions of species are being cut and cleared for the expansion of agricultural land, harvesting timber, forest fire, as well as overgrazing.

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Alexander von Humboldt described species area relationship for the first time. He observed that within a region, species richness increases with increasing explored area, but only upto a limit.

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Parthenium hysterophorus is commonly known as congress grass or carrot weed. It is herbaceous annual plant of Family Asteraceae. It is a deadly invasive, noxious weed infesting cropped and non-cropped areas. It rapidly colonises area replacing the native vegetation and causes a number of human health related problems such as skin allergy, rhinitis and eye irritations. Also, being toxic and unpalatable it causes fodder scarcity. Hence, it is considered a threat to the biodiversity.

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A red data book or red list is a catalogue of taxa facing risk of extinction. Red data book or red list was initiated in 1963.

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River dolphin found in holy river Ganga, Brahmaputra, Indus and its tributaries is the National aquatic animal of India. Presence of river dolphin in Ganga indicates pure and freshwater.

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Destruction of natural habitat causes the most serious threat to the biodiversity. Over-population, urbanisation and industrialisation lead to the destruction or fragmentation of natural habitats to fulfill the requirement of additional land. Loss of habitat results in annihilation of plants, microorganisms and forcing out of animals which in alien lands die out after some time. Fragmentation of habitats results in disruption of complex interactions amongst species, destruction of species in the cleared regions, annihilation of species restricted to deeper undisturbed parts of forests and decreased biodiversity in the habitat fragments.

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The taxon under critically endangered category are facing very high risk of extinction in the wild and can become extinct at any moment in the immediate future.

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IUCN is International Union of Conservation of Nature and Natural Resources which is now called World Conservation Union (WCU). It has its headquarters at Morges, Switzerland. It maintains a red data book or red list which is a catalogue of taxa facing risk of extinction. Red data book or red list was initiated in 1963. The Red list of year 2000 has made assessment of 18,000 species.

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