Habitat destruction resulting from expansion of human population and activities is the primary cause of loss of biodiversity.
Factors causing habitat destruction are large industrial and commercial activities, commercial fishing, cattle ranching, etc.
Habitat fragmentation is the process where large continuous area of habitat is both, reduced in area and divided into two or more fragments. It mits the potential of species for dispersal and colonisation.
Alien Species Invasion
Introduction of exotic species in a new area for their economic and other uses may kill or eat the native species, or alter their habitat in such a way that many natives are no longer able to persist. E.g., Water hyacinth (Eichhornia crassipes) was introduced in Indian waters to reduce pollution but it had clogged water bodies resulting in death of several aquatic plants and animals.
Island ecosystems are most vulnerable due to their small size and less number of species.
Increasing human population has escalated the use of natural resources. Overexploitation of resources also occurs when a commercial market develops for previously unexploited species.
Many species are likely to become endangered or vulnerable to extinction due to overexploitation.
Coextinctions occur when certain obligatory mutualistic relationships exist in nature, e.g., Pronuba yuccasella and Yucca. Extinction of one will automatically cause extinction of the other.
Excessive use of pesticides has polluted both ground water and surface water bodies. Many sensitive species have disappeared.
Pesticide biomagnification in higher concentration with the rise in trophic level has resulted in drastic decline in the population of fish eating birds and falcons.
Run off from fertiliser rich fields causes nutrient enrichment of water bodies, called eutrophication. Sewage and other organic remains also result in eutrophication. There is an additional dense growth of plants and animals followed by depletion of oxygen, death of animals and fouling of water.
Lead and other types of heavy metals poured into water bodies lead to mortality of many animals. Ducks, swans and cranes die of lead poisoning when they take in spent shot gun pellets falling into lakes and marshes. Lead poisoning from industries has killed many cattle drinking that water.
Spill-over of oil in sea causes death of several marine algae, fish and sea birds.
Disturbance and Degradation
They are of 2 types : Natural and man-made.
Natural disturbances include forest fire, tree fall, pest infestation, locust attack, etc.
Man-made disturbances and degradation are more severe as they include felling of trees, use of fire for clearing, collection of litter and overexploitation of other economically important products.
Spread of agriculture is at the cost of wetlands, grasslands and forests. Destruction of habitats results in exinction of species. Intensive agriculture is also based on a few high yielding varieties. As a result there is reduction in the genetic diversity.
It increases vulnerability of the crop plants to sudden attack by pathogens and pests.