The following points highlight the nine main causes for the loss of biodiversity. The causes are: 1. Destruction of Habitats 2. Habitat Fragmentation 3. Hunting 4. Overexploitation 5. Collection for Zoos and Research 6. Alien Species 7. Control of Pest and Predators 8. Co-Extinctions 9. Extinction of Species.
Cause # 1. Destruction of Habitats:
The natural habitat of many species is being destroyed by man for various purposes. Therefore, species must either adapt to change or move out to another location or they may succumb to predation, starvation or disease and eventually die.
Habitat can be destroyed by the following activities:
a. Developmental Activities:
Constructions of human settlements, dams, reservoirs, industries, mines have upset the natural habitats. The laying of roads and railway track in the forests have scared wild animals and limited their movements.
Deforestation results due to population settlement, shifting agriculture, fuel wood demand, demand of wood for industry and other commercial purposes. The Amazon rain forest, popularly known as the ‘lungs of the planet’ which harbours more than millions of species is cut for cultivating soya beans and growing feed for beef cattle.
Pollution affects and alters the habitat. For example, water pollution affects the aquatic ecosystem by upsetting the food chain. Presence of pesticides, insecticides, sulphur and nitrogen compounds, acid rain, ozone depletion and global warming can adversely affect plant and animal species. Coral reefs have been threatened by pollution from offshore mining along coastal areas. Noise pollution can also lead to wildlife extinction.
Cause # 2. Habitat Fragmentation:
It is the fragmentation of a large habitat into smaller habitats. Many species of mammals such as bears and large cats and species of birds which need large areas are unable to cope with this change. Population is divided into smaller groups that are vulnerable to disease and succumb to inter-specific and intra-specific competitions.
Cause # 3. Hunting:
Wild animals are hunted for products such as hides and skin, tusk, antlers, fur, meat, medicines, perfumes, cosmetics and ornamental purposes. The loss in biodiversity due to excessive hunting is severe.
A few examples are listed below:
a. Harpooning of whales has reduced the whale population considerably.
b. Extensive poaching of large mammals such as chimpanzee, orangutan and gorilla in central Africa and South East Asia has reduced the population in drastic numbers.
c. The Steller’s sea cow and the American passenger pigeon has become extinct due to hunting and habitat destruction.
A few examples of animals hunted for various products are as follows:
a. The black rhino for its horn
b. The elephant for ivory
c. Spotted cats for skin
d. Musk deer for musk
e. Gharials for skin
f. The spiny tailed lizard for extracting oil for its aphrodisiac properties
g. Many animals are hunted to trade as pet animals and museum specimens.
Cause # 4. Overexploitation:
Overexploitation is the main cause of loss of species. Many economically important species and biologically interesting species such as the insectivorous plants are overexploited. Many plants of medicinal value were also overexploited leading to drastic decrease in numbers.
Cause # 5. Collection for Zoos and Research:
Animals and plants are collected for zoos and laboratories. Monkeys and chimpanzees are sacrificed for research as they have anatomical, genetic and physiological similarity to man.
Cause # 6. Exotic or Alien Species:
Exotic species are organisms introduced from another place to a local area. Exotic species competes for food and space with the native species and eliminate them.
a. Introduction of goats and rabbits in the Pacific and Indian regions has resulted in the destruction of habitat of several plants, birds and reptiles.
b. Introduction of the exotic predatory fish, Nile perch into Lake Victoria in East South Africa, led to the elimination of about 200 several native species of the small Cichlid fish species endemic to the lake.
c. In the Waterberg region of South Africa, cattle grazing over the past six centuries has allowed invasive scrub and small trees to displace much of the original grassland, resulting in a massive reduction in forage for native bovids and other grazers.
d. The weed species such as the carrot grass (Parthenium), Lantana and the water hyacinth (Eicchornia) has posed a threat for many native species of plants.
e. The illegal introduction of the African catfish (Clarias gariepinus) for aquaculture is a threat for the indigenous cat fishes in the Indian rivers.
Cause # 7. Control of Pest and Predators:
The various pest and predator control measures cause severe imbalance in the ecosystem, which affects other species present in the area.
Cause # 8. Co-Extinctions:
When a species becomes extinct, other associated species of plants and animals also become extinct. This is known as co- extinction.
Some examples are as follows:
a. When a host fish species becomes extinct, the parasites that depend on it also become extinct.
b. When an insect which pollinates a plant becomes extinct, it invariably leads to the extinction of the plant species.
Cause # 9. Extinction of Species:
Extinction is the disappearance of a species from earth. A species may become extinct naturally because of sudden environmental changes and population characteristics.
The population characteristics include the following traits:
a. Large size, e.g. lion, elephant and rhinoceros
b. Small population size and low reproductive potential, e.g. blue whale and the giant panda
c. High levels in the food chain, e.g. tiger, lion, bear
d. Narrow range of distribution, e.g. island species
e. Poor adaptations and lack of genetic variability
Extinction may be of the following types:
a. Natural Extinction:
This is also called background extinction. It is the slow process of replacement of a species with better adapted species due to change in environmental conditions.
b. Mass Extinction:
The elimination of a large number of species because of a catastrophe is known as mass extinction. This has occurred several times in history. For example, in the Permian period of the geologic time scale, 90% of the marine invertebrates became extinct. During the Pleistocene period, many mammals such as the mammoth, mastodon and the giant sloth became extinct.
The extinction of the dinosaur is also an example of mass extinction. It is believed that mass extinction occurs because of climatic changes. There is serious concern about the increasing greenhouse gases and depletion of ozone layer because these have the potential to cause catastrophic effects in the future.
c. Anthropogenic Extinction:
The extinction of species due to human activities is called anthropogenic extinction. The extinction of the Dodo and the Indian cheetah are the examples of extinction due to hunting by man. About 533 animal species and 384 plant species have become extinct due to human activities. The disappearance of many plants and animals at an alarming rate from earth since the last 300 years is known as biodiversity crisis.