What are the factors that lead to the loss of biodiversity in the forests?

Important factors leading to loss of biodiversity are: habitat loss and fragmentation; introduction of non-native species; over exploitation; soil, water and atmospheric pollution; and intensive agriculture and forestry.
(a) Habitat loss and fragmentation: Foremost among the direct threats to biodiversity is the destruction or deterioration of habitats. This occurs due to cutting down of trees, filling a wetland, ploughing grassland or burning a forest. These changes can kill or force out many plants, animals and microorganisms, as well as disrupt complex interactions among the species. The most important example of habitat loss comes from tropical rain forests. Once covering more than 14 % of the earth’s land surface, these rain forests now cover no more than 6 %. Pollution may reduce and eliminate populations of sensitive species. For example, pesticide linked decline of fish-eating birds and falcons. Lead poisoning is another major cause of destruction of many species, such as ducks, swans and cranes, as they take in the spent shotgun pellets that fall into lakes and marshes. Eutrophication (nutrient enrichment) of water bodies drastically reduces species diversity. When large habitats are broken up into small fragments due to various human activities, mammals and birds requiring large territories and certain animals with migratory habits are seriously affected, leading to population decline.
(b) Over-exploitation: Humans have always depended on nature for food and shelter, but when ‘need’ turns to ‘greed’, it leads to over-exploitation of natural resources. Over-hunting has been a significant cause of the extinction of hundreds of species and the endangerment of many more, such as whales, Steller’s sea cow and passenger pigeon. Most extinction in the last 500 years is mainly due to over-harvesting for food, fashion and profit.
(c) Alien species invasions: Another devastating problem is the introduction of non-native or foreign species. New species entering a geographical region are called exotic or alien species. Introduction of such invasive species may cause disappearance of native species through changed biotic interactions. Invasive species are considered second only to habitat destruction as a major cause of extinction of species. A few examples are:
(i) The Nile perch, a voracious predatory fish introduced into South Africa’s Lake Victoria is largely responsible for the extinction of several native species of the small Cichlid fish species that were endemic to this freshwater aquatic system.
(ii) Water hyacinth (Eichhornia crassipes) clogs rivers and lakes and threatens the survival of many aquatic species in lakes and river flood plains in several tropical countries, including India.
(iii) Lantana camara has invaded many forest lands in different parts of India, and strongly competes with the native species.
(d) Co-extinctions: When a species becomes extinct, the plant and animal species associated with it also become extinct. When a host fish species becomes extinct, its distinctive assemblage of parasites also meets the same destiny. Another example is of a coevolved plant-pollinator mutualism where extinction of one always leads to the extinction of the other.

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