LOSS OF BIODIVERSITY
Biological wealth of our planet is declining rapidly. This is due to human activities as they derive benefits from the living world either directly or indirectly. This is due to the diversity of organisms. E.g., Colonisation of Tropical Pacific Island by humans led to the extinction of more than 2000 species of native birds.
IUCN Red List (2004): Extinction of 784 species i.e., 338 vertebrates, 359 Invertebrates, 87 Plants, have occurred in the last 500 years.
Recent extinctions are:
- Dodo in Mauritius.
- Quagga in Africa.
- Thylacine in Australia.
- Steller sea cow in Russia.
- 3 species of Tiger: Bali, Java & Caspian.
In the last 20 years, 27 species have disappeared. Extinction across taxa is not random, some groups like Amphibians are more vulnerable. More than 15,500 species worldwide are facing the threat of extinction.
Presently 12% of all bird species, 23% of all animal species, 32% of all amphibian species & 31% of all gymnosperm species are under threat of extinction.
History of life on earth: Fossil records show that large-scale loss of species has happened earlier. More than 3 billion years since origin & diversification of life on earth, five episodes of mass extinction of species have occurred & sixth is in progress.The current rate of species extinction is 100 -1000 times faster than pre-human times. This can wipe out nearly half of all species within next 100 years.
Loss of Biodiversity may lead to:
- Decline in plant production
- Resistance to environmental perturbances decreases
- Increased variability in certain ecosystem processes like plant productivity, water use, pest & disease cycles.
CAUSES OF BIODIVERSITY LOSS: The Evil Quartet
- HABITAT LOSS & FRAGMENTATION: Most important reason for the extinction of plants & animals.
- Natural habitats of a species are either changed or destroyed due to the cutting of trees, filling a wetland, plowing a grassland or burning a forest. It kills or forces out many plants, animals & micro-organisms. This disrupts the complex interactions between the species.
- Fragmented habitats are forest patches surrounded by cropland, orchards, plantations or urban areas.
- When large forests are fragmented, the species found in deeper parts of the forest disappears first. Mammals & birds requiring large territories and certain animals with migratory habits are affected, causing a decline in their population.
- Tropical rainforest that occupied 14% of earth’s land surface has been reduced to less than 6%. Amazon rainforest is being cleared for soyabean cultivation.
- Man made disturbances differ from natural disturbances in intensity, rate & spatial extent.
- Degradation of habitat occurs due to pollution. This can reduce or even eliminate populations of sensitive species. Use of chemicals, the release of radiations, oil spill over sea can change the quality of the habitat. E.g., pesticides caused the decline of fish-eating birds & Falcons.
- Humans depend on nature for food & shelter, due to which it is overexploiting its natural resources. Overexploitation of a particular species reduces the size of its population to an extent that it becomes vulnerable to extinction.
- In last 500 years, many species extinctions have occurred due to overexploitation by humans. E.g., Steller’s sea cow, passenger pigeon. Overharvesting of marine fishes has endangered the existence of some commercially important species.
3. ALIEN SPECIES INVASION: The second major reason for the extinction of species.
- Due to some reasons when an alien species is introduced, some of them turn invasive & cause decline or extinction of indigenous species.
- E.g., Introduction of Nile perch into Lake Victoria in East Africa caused the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish.
- Eicchornia, Parthenium, Lantana camara are examples of invasive weed species.
- African catfish Clarias gariepinus introduced for aquaculture purposes is posing a threat to the indigenous catfishes.
- When one species becomes extinct it leads to the extinction of plant and animal species that are obligatorily associated with it.
- E.g., When a host fish species becomes extinct, its parasites also become extinct.
- In case of co-evolved plant-pollinator mutualism, extinction of one leads to the extinction of the other.