Biodiversity

Unit: Ecology

Chapter: Biodiversity

Reference: Biodiversity, patterns of biodiversity, loss of biodiversity, cause of loss of biodiversity

Learning objectives

• To understand pattern of biodiversity ,loss of biodiversity
• To identify the cause of biodiversity loss

Biodiversity                                                

• Biodiversity means diversity or heterogeneity at all levels of biological organization, i.e., from macromolecules of the cells to the Biomass. The term Biodiversity was given by Edward Wilson.
• Diversity at all the levels of biological organisation-

1. Genetic diversity– diversity at genetic level, or at subspecies level, i.e., below species level, in a single species.
2. Species diversity– measurement of species diversity is its richness, i.e., the number of species per unit area. The greater is the species richness the more will be the species diversity.
3. Ecological diversity– diversity at community level. It can be of 3-types a.  Alpha (α) diversity – It is the diversity of organisms within the same community or habitat.

b. Beta (β) diversity – It is the diversity between communities or different habitats. Higher the heterogenicity in the altitude, Humidity and Temperature of a region, the greater will be the dissimilarity between communities, and higher will be the β diversity.

c.  Gamma (γ) diversity – It is the diversity of organisms over the entire geographical area, covering several ecosystems or habitats and various trophic levels and food webs. Such diversity is most stable and productive.

How Many Species are there on Earth and How Many in India?

• According to IUCN (International Union for Conservation of Nature and Natural resources) estimates, the total number of animal and plant species, described so far, is more than 1.5 million. Due to project ‘Species 2000’ and ‘Global Biodiversity Information’, the new species are being discovered faster than ever before.
• According to estimates of Robert May. The global species diversity is about 7 million (1.5 million, i.e., 22% reported till now and 78% are yet to be discovered). More than 70% of all the species recorded are animals. Plants are not more than 22% of the total. Among animals also, about 70% are insects. The Fungi have more species than all the vertebrate’s species combined. In case of vertebrates, the species of fishes are more than that of birds, and of latter, more than reptiles. In case of plant species, the species of fungi > species of angiosperms > species of algae. The all above estimates do not give any idea for the number of species of prokaryotes, whose species diversity may run in millions.
• Number of species in India-India is one of the 12 mega diversity country of the world. It has 2.4%, i.e., 1/40 of world land area, but global species diversity is 8.1 %, i.e., 1/12. In India the number of animal and plant species recorded so far is 90,000 and 45,000 respectively. According to May’s global estimates, about 3,00,000 animal species and 1,00,000 plant species are yet to be discovered from India.

Pattern of biodiversity

Biodiversity varies with the change of altitude and latitude. The species diversity is maximum in plains (low altitude) and equator (low latitude).

1. Latitudinal pattern– species diversity is the maximum at equator and decreases as one moves towards poles. There are 3 explanations or hypothesis for the greater biodiversity in tropics

1. The tropics have remained relatively undisturbed for millions of years. There have been no frequent glaciations as in temperate and hence, long evolution time for species diversification.
2. The tropic environments are more constant or less seasonal.
3. Tropics have more solar energy, contributing to higher productivity, hence greater diversity.

1. Species – Area relationship – richness increased with increasing explored area, but only up to a limit. This relationship between species richness and explored area is a rectangular hyperbola, described by the equation —

S = C Az

Where S is species richness; C is Y-intercept; A is area and z are the slope to the line (regression coefficient). On a logarithmic scale, the relationship is a straight line, described by the equation –

                                       Log S = Log C + Z log A

Most of the ecologists believe that

1. The communities with more species are more stable than those with lesser species.
2.  The stable community means lesser variations in productivity from year to year.
3. The community with more species, is more resistant to occasional (natural or man-made) disturbances.

Such communities are resistant to invasions by alien or exotic species. David Tilman, in his long-term ecosystem experiments, found that plots with more species showed less – year to year variation in total biomass and the increased diversity contributed to higher productivity.

The importance of Species Diversity to the Ecosystem

1. Stability- communities with more species diversity tend to be more stable than those with less species and able to resist occasional disturbance.
2. Productivity- ecosystem with higher biodiversity are more productive than those with lower biodiversity.
3. Ecosystem health- biodiversity is essential for maintenance and health of ecosystem through the occurrence of various checks, controls, negative and positive feedback, critical links, and keystone species.

Note: Paul Ehrlich has proposed a rivet popper hypothesis for effect of decrease in biodiversity on ecosystem. An aeroplane is like an ecosystem, has thousands of rivets (species). Removal of rivets (species) by passengers may not affect flight safety in beginning but the plane will become dangerously weak over a period. Removal of rivets of critical part like wing (species performing major ecosystem functions) will pose a very serious immediate threat to safety.

Loss of biodiversity

• There is continuous loss of the earth’ treasure of species.
• The Red list of IUCN documented the extinction of 784 species in last 500 years. The last 20 years witnessed the disappearance of 27 species. Some important examples of recent extinctions are Dodo (Mauritius), Quagga (Africa), Thylacine (Australia), Steller Sea-cow (Russia), and subspecies of Tiger, like bali, javan and caspian.
• Presently, more than 15,500 species world–wide are facing the threat of extinction. This includes 32% of amphibian species, 23% of mammalian species and 12% of birds’ species.
• About 31% of the gymnosperm species are also facing the extinction. The amphibians are however, more vulnerable in such cases. From origin to evolution of life on earth, i.e., duration about 3 billion yrs., there have been 5-episodes of mass extinction, but the present, the 6th, mass extinction is 100 to 1000 times faster than the pre-human extinctions.
• The ecologists now warn that in next 100 years about 50% of all the species on earth will be wiped out
• The loss in biodiversity of a region leads to-

1. Lowered resistance to environmental changes

2. Decline in the plant production

3. Increased variability in certain ecosystem, pest disease cycles and water use etc.

Cause of biodiversity loss-

‘The Evil Quartet’, for the loss of biodiversity-

• Habitat fragmentation- it is the primary and major reason for the loss of biodiversity. The tropical rain forest is the example of the habitat loss where forest covering has been reduced from 14% of land surface to 6%.

The cutting trees and burning of forest destroy the natural habitat of a species. The construction of mines, dams, harbours, industries and buildings for human settlement has also affected the biodiversity.

• Overexploitation- Many species – extinction, like that of Stellar sea-cow and Passenger pigeon, in last 500 years, are due to overexploitation by humans. Many marine fishes are also being over harvested. Over fi shing from a water body, or over harvesting a product is just like ‘killing a goose laying golden eggs.
• Invasion of Alien or exotic species- When alien species are introduced into an explored area, some of the species turn invasive and cause decline or extinction of indigenous species. Ex.

a. Introduction of Nile perch into Lake Victoria (E. Africa) led to the extinction of more than 200 species of Cichlid fish in the lake

b. Introduction of weed species, like Carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia) has posed threat to the native species and damage to environment.

c. The illegal introduction of African cat fish (Clarias gariepinus) for aquaculture purposes into the river has threatened indigenous cat fishes.

• Co-extinctions – Whenever a plant or animal species becomes extinct, its obligatory-associated species also becomes extinct.

Processes of extinction of biodiversity

1. Natural extinction – With the change of environmental conditions some species have disappeared and the more adaptive species have appeared. This extinction is slow and is called ‘Background extinction’.

2. Mass extinction – It is extinction of large number of species due to natural calamities/catastrophies. The extinction of Dinosaurs is one such example.

3. Anthropogenic extinction – It is the disappearance of species due to human activities. This man-made extinction represents a severe depletion of biodiversity in terms of time. The current rate of extinction is thousand times higher than the background extinction. If the current rate of losses continues the earth may lose up to 50% of the species by the end of 21st Century.

Susceptibility to extinction

The species with the following features are more susceptible (vulnerable) to extinction than the other species-

1. Larger body size (Ex. Elephant, Bengal Tiger and Lion etc.)

2. Smaller population with low reproductive rate (Ex.  Blue whale and Giant panda)

3. Fixed habitat or migratory routes (Ex. Whooping cranes and blue whales).

4. Feeding at higher trophic level in the food chain. (Ex. Bengal tiger and Bald Eagle).

5. Narrow range of distribution (Ex. Island species and woodland caribou).

Solved examples

Example 1. Kudzu has overgrown many parts of southern Missouri and native ground plants have disappeared from the area. This is an example of:

a) Parasitism between kudzu and the native plants

b) A predator prey relationship between ground plants and kudzu

c) Competition occurring between the native and invasive species

d) Commensalism occurring between the kudzu and native plants

Solution 1: c. Kudzu has overgrown many parts of southern Missouri and native ground plants have disappeared from the area. This is an example of competition occurring between the native and invasive species.

Example 2. The main cause of biodiversity loss worldwide is—

a) extinctions b) invasive species c) pollution d) habitat destruction

Solution 2: d. The main cause of biodiversity loss worldwide is habitat destruction

 Summary  

• Life originated on earth nearly 3.8 billion years ago, there had been enormous diversification of life forms on earth
• Biodiversity refers to sum of diversity that exists at all levels of biological organisation.
• More than 1.5 million species have been recorded in world, but there might still nearly 6 million species on earth waiting to be discovered and named.
• Species diversity on earth is not uniformly disturbed but shows interesting patterns. It is generally highest in tropics and decreases towards poles.
• Earth’s rich biodiversity is vital for very survival of mankind. The reasons for conserving biodiversity are narrowly, utilitarian, broadly utilitarian and ethical.