Loss of Biodiversity: Causes

Alongside the immense value biodiversity provides, it is also being lost at a rate that conservation biologists consider alarming, and, unlike the five great mass extinction events recorded in Earth's deep geological past, the current episode of biodiversity loss is understood to be driven overwhelmingly by human activity rather than by natural planetary processes such as asteroid impacts or volcanic upheaval. Four principal, interacting human causes are together responsible for the great majority of recorded and projected species losses, and are collectively referred to, somewhat memorably, as the 'Evil Quartet'.

Habitat loss and fragmentation is generally considered the single most significant cause of biodiversity loss worldwide. Habitat loss occurs when a natural ecosystem -- a tract of tropical rainforest, a wetland, a native grassland -- is converted to another land use entirely, most often agriculture, urban development or infrastructure, leaving the species that depended on it with no suitable place to live at all. Habitat fragmentation is a related but distinct process, in which a large, continuous natural area is broken up by roads, farmland or settlements into a number of smaller, disconnected patches, even where none of the original habitat area is entirely destroyed. Fragmentation is especially damaging to species that need a large, continuous territory to find sufficient food, to maintain a genetically viable breeding population, or to migrate seasonally, since a population confined to one small isolated fragment can become too small to survive over the long term, and is effectively cut off from other, similarly isolated populations of the same species in nearby fragments, with whom it can no longer freely interbreed.

Over-exploitation refers to harvesting a species -- whether by hunting, fishing, logging or collection -- at a rate faster than the species' population can naturally replace itself through reproduction, so that its total numbers steadily decline toward extinction. History offers stark examples of over-exploitation driving a once-abundant species to complete extinction: Steller's sea cow, a large marine mammal, was hunted to extinction within around 27 years of its scientific discovery, and the passenger pigeon, once so numerous in North America that migrating flocks were described as darkening the sky for days, was hunted to complete extinction by the early twentieth century.

Alien species invasion occurs when a species is introduced, whether deliberately or accidentally, into a region outside its natural native range, and goes on to out-compete, prey upon, or otherwise harm the native species already living there, which have no evolutionary history of coping with this new competitor or predator. The introduction of the Nile perch, a large predatory fish, into Lake Victoria in East Africa is one of the most frequently cited examples: the Nile perch preyed so heavily on the lake's extraordinarily diverse native cichlid fish community that it is credited with driving several hundred native cichlid species to extinction, one of the largest documented losses of vertebrate biodiversity caused by a single introduced predator. Invasive alien plant species are an equally serious problem within India itself, where species such as Parthenium (carrot grass), Lantana camara and water hyacinth (Eichhornia crassipes) have spread aggressively across large areas, out-competing and displacing native vegetation. …