BIODIVERSITY
$\displaystyle \small \bullet$ Biodiversity can be defined as ‘the totality of genes, species and ecosystems of a region’.
$\displaystyle \small \bullet$ All the species do not occur at one place. The site of occurrence of a species is determined by the environmental conditions of the site and the range of tolerance of a species.
$\displaystyle \small \bullet$ The term biodiversity was popularized by a sociobiologist Edward Wilson.
LEVELS OF BIODIVERSITY
$\displaystyle \small \bullet$ In our biosphere immense diversity (or heterogeneity) exists not only at the species level but at all levels of biological organisation, ranging from macromolecules within cells to biomes.
1. Genetic diversity: Diversity shown by a single species at genetic level. E.g. Rauwolfia vomitoria (Himalaya) shows genetic variation in the potency & concentration of the chemical reserpine. India has more than 50,000 different strains of rice and 1000 varieties of mango.
2. Species diversity: Diversity at species level. E.g. Western Ghats have greater amphibian species than Eastern Ghats.
3. Ecological diversity: Diversity at ecosystem level. E.g. In India, deserts, rain forests, mangroves, coral reefs, wet lands, estuaries & alpine meadows are seen.
TOTAL NUMBER OF SPECIES ON EARTH
$\displaystyle \small \bullet$ According to IUCN (2004) more than 1.5 million species described so far.
$\displaystyle \small \bullet$ According to Robert May’s Global estimate, about 7 million species would be on earth.
$\displaystyle \small \bullet$ Animals are more diverse (above 70%) than plants including Plantae and Fungi (22%).
$\displaystyle \small \bullet$ Among animals, insects are most species rich group (70%, i.e. out of every 10 animals, 7 are insects).
$\displaystyle \small \bullet$ Number of fungi species is more than the combined total of the species of fishes, amphibians, reptiles & mammals.
$\displaystyle \small \bullet$ India has only 2.4% of world’s land area, but has 8.1% of the species diversity. India is one of the 12 mega diversity countries of the world. Nearly 45,000 plant species and twice as many of animals have been recorded from India.
$\displaystyle \small \bullet$ Applying May’s global estimates, India would have more than 1 lakh plant species and 3 lakh animal species.
$\displaystyle \small \bullet$ Biologists are not sure about total number of prokaryotic species because
• Conventional taxonomic methods are not suitable for identifying microbial species.
• In laboratory, many species cannot be cultured.
PATTERNS OF BIODIVERSITY
(i) Latitudinal gradients
$\displaystyle \small \bullet$ Species diversity decreases as we move away from the equator towards the poles.
$\displaystyle \small \bullet$ With very few exceptions, tropics (latitudinal range of 23.5° N to 23.5° S) harbour more species than temperate or polar areas.
$\displaystyle \small \bullet$ Number of bird species in different latitudes:
$\displaystyle \small \circ$ Colombia (near equator): about 1400 species.
$\displaystyle \small \circ$ India (in tropics): > 1200 species.
$\displaystyle \small \circ$ New York (41° N): 105 species.
$\displaystyle \small \circ$ Greenland (71° N): 56 species.
$\displaystyle \small \bullet$ Tropical forest region like Equador has up to 10 times of vascular plant species as compared to a temperate forest region like the Midwest of USA.
$\displaystyle \small \bullet$ The largely tropical Amazonian rain forest in South America has the greatest biodiversity on earth- it is home to more than 40,000 species of plants, 3,000 of fishes, 1,300 of birds, 427 of mammals, 427 of amphibians, 378 of reptiles and of more than 1,25,000 invertebrates.
WHY TROPICAL REGIONS ARE RICH IN BIODIVERSITY THAN TEMPERATE ONES
(a) Speciation is generally a function of time, unlike temperate regions subjected to frequent glaciations in the past, tropical latitudes have remained relatively undisturbed for millions of years and thus, had a long evolutionary time for species diversification,
(b) Tropical environments, unlike temperate ones, are less seasonal, relatively more constant and predictable. Such constant environments promote niche specialization and lead to a greater species diversity.
(c) There is more solar energy available in the tropics, which contributes to higher productivity.
(ii) Species- Area relationship
$\displaystyle \small \bullet$ According to the study of Alexander von Humboldt in South American jungles, within a region, species richness increases with increasing explored area, but only up to a limit.
$\displaystyle \small \bullet$ Relation between species richness and area gives a rectangular hyperbola. On a logarithmic scale, the relationship is a straight line described by the equation Log S = log C + Z log A
$\displaystyle \small \bullet$ Generally, for small areas, the Z value is 0.1 to 0.2.
$\displaystyle \small \bullet$ But for large areas (e.g. entire continents), slope of the line is steeper (Z value: 0.6 to 1.2).
$\displaystyle \small \bullet$ E.g. for frugivorous birds and mammals in the tropical forests of different continents, the Z value is 1.15.
$\displaystyle \small \bullet$ Communities with more species are more stable than those with less species.
$\displaystyle \small \bullet$ Stable community do not show variation in productivity, resistance to disturbances and invasions by alien species.
$\displaystyle \small \bullet$ Characteristics of stable community:
i) Do not show too much variation in productivity from year to year.
ii) Community must be either resistant or resilient (back into original shape) to occasional disturbances of natural or man-made.
iii) It must also be resistant to invasions by alien species.
$\displaystyle \small \bullet$ According to David Tilman, plots with more species shows less year-to-year variation in total biomass.
$\displaystyle \small \bullet$ Increased diversity contributes to higher productivity. It is essential for ecosystem health and survival of human race.
‘Rivet popper hypothesis’
$\displaystyle \small \bullet$ Proposed by Stanford ecologist Paul Ehrlich.
$\displaystyle \small \bullet$ In an airplane (ecosystem) all parts are joined together using thousands of rivets (species).
$\displaystyle \small \bullet$ If every passenger traveling in it starts popping a rivet to take home (causing a species to become extinct), it may not affect flight safety (proper functioning of the ecosystem) initially.
$\displaystyle \small \bullet$ But the removal of more and more rivets, the plane becomes dangerously weak over a period of time.
$\displaystyle \small \bullet$ Furthermore, which rivet is removed may also be critical. Loss of rivets on the wings (key species that drive major ecosystem functions) is obviously a more serious threat to flight safety than loss of a few rivets on the seats or windows inside the plane.
LOSS OF BIODIVERSITY
$\displaystyle \small \bullet$ The biological wealth of our planet has been declining rapidly and the main reason for species loss is human activities.
$\displaystyle \small \bullet$ Begin Human life in tropical Pacific Island led to the extinction of more than 2000 species of native birds.
$\displaystyle \small \bullet$ The IUCN Red List (2004) documents the extinction of 784 species (including 338 vertebrates, 359 invertebrates and 87 plants) in the last 500 years.
$\displaystyle \small \bullet$ E.g. Dodo (Mauritius), Quagga (Africa), Thylacine (Australia), Stellar’s sea cow (Russia) and subspecies (Bali, Javan, Caspian) of tiger.
$\displaystyle \small \bullet$ 27 species have been disappeared in the last 20 years.
$\displaystyle \small \bullet$ More than 15,500 species are facing threat of extinction.
$\displaystyle \small \bullet$ 12% birds, 23% mammals, 32% amphibians, 31% gymnosperm species face the threat of extinction.
$\displaystyle \small \bullet$ The current extinction rate is 100 - 1000 times faster than in the pre-human times. If this trend continues, nearly 50% species might be extinct within next 100 years.
$\displaystyle \small \bullet$ Problems due to loss of biodiversity
(a) Decline in plant production.
(b) Lowered resistance to environmental perturbations such as drought.
(c) Increased variability in certain ecosystem processes such as plant productivity, water use, pest and disease cycles.
CAUSES OF BIODIVERSITY LOSSES (The 'Evil Quartet')
(i) Habitat loss and fragmentation
$\displaystyle \small \bullet$ This is the most important cause for the extinction of animals and plants.
$\displaystyle \small \bullet$ Once tropical rain forests was covering more than 14% of the earth’s land surface, these rain forests now cover no more than 6%. They are being destroyed fast.
$\displaystyle \small \bullet$ The Amazon rain forest (it is so huge that it is called the ‘lungs of the planet’) was supporting for millions of species, it had been cut and cleared for cultivating soya beans or for conversion to grasslands for raising cattle.
$\displaystyle \small \bullet$ It results in the total loss of many species, the degradation of many habitats, and pollution also threatens the survival of many species.
$\displaystyle \small \bullet$ 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 badly affected, leading to population declines.
(ii) Over-exploitation
$\displaystyle \small \bullet$ Many species extinctions in the last 500 years (Steller’s sea cow, passenger pigeon) were due to over-exploitation by humans.
$\displaystyle \small \bullet$ Presently many marine fish populations around the world are over harvested, endangering the continued existence of some commercially important species.
(iii) Alien species invasions
$\displaystyle \small \bullet$ When alien species are introduced unintentionally some of them turn invasive, and cause decline or extinction of indigenous species.
$\displaystyle \small \bullet$ The Nile perch introduced into Lake Victoria in east Africa led to the extinction of more than 200 species of cichlid fish in the lake.
$\displaystyle \small \bullet$ The environmental damage caused to our native species by invasive weed species like carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia).
$\displaystyle \small \bullet$ The recent illegal introduction of the African catfish Clarias gariepinus for aquaculture purposes is posing a threat to the indigenous catfishes in our rivers.
(iv) Co-extinctions
$\displaystyle \small \bullet$ When a species becomes extinct, the plant and animal species associated with it in an obligatory way also become extinct.
$\displaystyle \small \bullet$ When a host fish species becomes extinct, its dependent parasites are also extinct.
$\displaystyle \small \bullet$ Another example is the case of a coevolved plant-pollinator mutualism, where extinction of one invariably leads to the extinction of the other.
0 Comments