Exercises
Name the three important components of biodiversity.
Answer
(i) Genetic diversity – variation in genes within a species, e.g. different potency of reserpine in Rauwolfia vomitoria from different Himalayan ranges; over 50,000 strains of rice and 1,000 varieties of mango in India.
(ii) Species diversity – variety of species in a region, e.g. the Western Ghats have greater amphibian species diversity than the Eastern Ghats.
(iii) Ecological diversity – variety of ecosystems, e.g. India with deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows has greater ecosystem diversity than Norway.
How do ecologists estimate the total number of species present in the world?
Answer
Species inventories are more complete in temperate countries than in the tropics, where most undiscovered species live. Ecologists therefore make a statistical comparison of the temperate–tropical species richness of an exhaustively studied group (e.g. insects) and extrapolate this ratio to other groups of plants and animals to get a gross estimate of the total. Estimates range from 20 to 50 million in extreme cases, while Robert May's more conservative and scientifically sound estimate puts global species diversity at about 7 million. Only about 1.5 million species have been described so far.
Give three hypotheses for explaining why tropics show greatest levels of species richness.
Answer
(i) Speciation is a function of time: unlike temperate regions, which faced frequent glaciations, the tropics have remained relatively undisturbed for millions of years, giving a long evolutionary time for species diversification.
(ii) Tropical environments are less seasonal and more constant and predictable; such conditions promote niche specialisation, leading to greater species diversity.
(iii) More solar energy is available in the tropics, which gives higher productivity; this indirectly supports greater diversity.
What is the significance of the slope of regression in a species – area relationship?
Answer
On a log scale the species–area relationship is a straight line: log S = log C + Z log A, where S = species richness, A = area, C = Y-intercept and Z = slope (regression coefficient).
Z tells how fast species richness increases with area. For smaller areas within a region, Z lies between 0.1 and 0.2, remarkably similar regardless of taxonomic group or region. For very large areas such as whole continents, the slope is much steeper (Z = 0.6 to 1.2; e.g. 1.15 for frugivorous birds and mammals of tropical forests). A steeper slope means species richness rises sharply with area – so loss of area causes a large loss of species, which is important for conservation planning.
What are the major causes of species losses in a geographical region?
Answer
The four major causes ("The Evil Quartet"):
(i) Habitat loss and fragmentation – the most important cause; e.g. tropical rain forests once covered over 14% of the land surface but now cover no more than 6%; the Amazon is cleared for soya beans and cattle. Fragmentation especially harms animals needing large territories and migratory species. Pollution also degrades habitats.
(ii) Over-exploitation – when "need" turns to "greed", e.g. Steller's sea cow and passenger pigeon became extinct; many marine fish are over-harvested.
(iii) Alien species invasions – introduced species become invasive and eliminate native species, e.g. Nile perch in Lake Victoria caused extinction of more than 200 cichlid fish species; Parthenium, Lantana and water hyacinth; African catfish Clarias gariepinus threatening native catfishes.
(iv) Co-extinctions – when a species becomes extinct, species obligately associated with it also die out, e.g. parasites of an extinct host fish, or a co-evolved plant and its pollinator.
How is biodiversity important for ecosystem functioning?
Answer
Communities with more species are believed to be more stable – showing less year-to-year variation in productivity, being resistant or resilient to disturbances and resistant to invasion by alien species. David Tilman's long-term outdoor plot experiments showed that plots with more species had less year-to-year variation in total biomass and higher productivity. Paul Ehrlich's "rivet popper hypothesis" explains this: species are like rivets holding an airplane (ecosystem) together; removing a few may not matter at first, but losing more weakens it dangerously, and losing key species (rivets on the wings) is especially serious. Loss of biodiversity leads to decline in plant production, lower resistance to perturbations such as drought, and greater variability in processes such as productivity, water use and pest and disease cycles. Biodiversity also supports ecosystem services such as oxygen production, pollination, flood control and nutrient cycling.
What are sacred groves? What is their role in conservation?
Answer
Sacred groves are tracts of forest set aside on religious and cultural grounds, in which all trees and wildlife are venerated and given total protection by the local community. In India they are found in the Khasi and Jaintia Hills (Meghalaya), Aravalli Hills (Rajasthan), Western Ghat regions of Karnataka and Maharashtra, and the Sarguja, Chanda and Bastar areas of Madhya Pradesh.
Role: they are a form of in situ conservation that protects whole ecosystems undisturbed; in Meghalaya they are the last refuges for a large number of rare and threatened plants.
Among the ecosystem services are control of floods and soil erosion. How is this achieved by the biotic components of the ecosystem?
Answer
Plants are the main biotic components providing this service. (i) Roots of trees and grasses bind soil particles and hold the soil firmly, preventing it from being washed or blown away. (ii) The canopy and leaf litter intercept rain and reduce the force with which raindrops strike the soil. (iii) Vegetation, litter and humus (formed by decomposers and detritivores) increase the porosity and water-holding capacity of soil, so more water soaks into the ground and less runs off. (iv) Plants slow down the speed of surface run-off, reducing the volume and speed of water reaching rivers and thus preventing floods; forests also absorb water and release it slowly through transpiration and seepage.
The species diversity of plants (22 per cent) is much less than that of animals (72 per cent). What could be the explanations to how animals achieved greater diversification?
Answer
(i) Animals can move, so they can occupy a wider variety of habitats and niches and escape unfavourable conditions. (ii) Animals have complex nervous and sensory systems and varied behaviour, allowing adaptation to many different modes of life. (iii) Animals have very varied feeding habits (herbivores, carnivores, parasites, etc.), and different body parts and food sources of a single plant can support many animal species. (iv) Insects in particular – more than 70% of all animals – are small, have short life cycles and high reproductive rates, which give rapid speciation and adaptation to minute niches. Plants, being fixed, are limited in the ways they can adapt to their environment.
Can you think of a situation where we deliberately want to make a species extinct? How would you justify it?
Answer
Yes – for disease-causing organisms such as viruses and bacteria (e.g. variola virus causing smallpox, poliovirus, Plasmodium causing malaria) and their vectors in some situations. Justification: these organisms cause severe illness and death in millions of humans and animals; eliminating them protects human health and saves lives and money, as was done when smallpox was eradicated. They play no essential role in ecosystem functioning that cannot be compensated. However, any such step must be carefully weighed against possible ecological effects, and the general aim of conservation remains to protect all other species.
Important Extra Questions
What is a biodiversity hotspot? Name those covering India.
Answer
A region with very high species richness and a high degree of endemism, also facing accelerated habitat loss. Of the 34 hotspots, three cover India: Western Ghats and Sri Lanka, Indo-Burma, and Himalaya.
Why is India called a mega diversity country?
Answer
With only 2.4% of the world's land area, India has 8.1% of global species diversity – about 45,000 plant species and twice as many animal species – making it one of 12 mega diversity countries.
How is the "Sixth Extinction" different from earlier mass extinctions?
Answer
Present extinction rates are 100 to 1,000 times faster than in pre-human times, and human activities are responsible for them.
Differentiate between in situ and ex situ conservation.
Answer
In situ conservation protects species in their natural habitat by protecting the whole ecosystem (biosphere reserves, national parks, sanctuaries, sacred groves). Ex situ conservation removes threatened species from their habitat to special settings such as zoos, botanical gardens, safari parks, seed banks and cryopreservation of gametes.
What are the three categories of arguments for conserving biodiversity?
Answer
Narrowly utilitarian (direct products like food, fibre, drugs), broadly utilitarian (ecosystem services like oxygen production and pollination) and ethical (every species has intrinsic value and we have a moral duty to protect it).