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NCERT Solutions · Class 12 Biology Biodiversity and Conservation

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Exercises 13.1–13.10

  1. Exercise 13.1

    Name the three important components of biodiversity.

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    The three important components are genetic diversity, species diversity and ecological (ecosystem) diversity — the three levels of biological organisation at which conservation efforts are aimed.
    Genetic diversity — the variation a single species shows over its distributional range. The medicinal plant Rauwolfia vomitoria growing in different Himalayan ranges differs in the potency and concentration of reserpine; India has more than $\displaystyle 50,000$ genetically different strains of rice and $\displaystyle 1,000$ varieties of mango.
    Species diversity — diversity at the species level. The Western Ghats have a greater amphibian species diversity than the Eastern Ghats.
    Ecological diversity — diversity at the ecosystem level. India, with its deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows, has a greater ecosystem diversity than a Scandinavian country like Norway.

  2. Exercise 13.2

    How do ecologists estimate the total number of species present in the

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    Ecologists do not count them; they extrapolate statistically from a well-studied group to all the others.
    The problem: species inventories are far more complete in temperate than in tropical countries, yet an overwhelmingly large proportion of the species still waiting to be discovered are in the tropics.
    The method: take an exhaustively studied group of insects, make a statistical comparison of its temperate–tropical species richness, and apply that ratio to other groups of animals and plants to arrive at a gross estimate of the total number of species on earth.
    The results differ widely — extreme estimates run from $\displaystyle 20$ to $\displaystyle 50$ million, while the more conservative and scientifically sound estimate by Robert May places global species diversity at about $\displaystyle 7$ million.
    Against this, only slightly more than $\displaystyle 1.5$ million species have actually been described so far (IUCN, $\displaystyle 2004$) — i.e. about $\displaystyle 22$ per cent of the total.
    These estimates give no figure for prokaryotes, because conventional taxonomic methods cannot identify microbial species and many are simply not culturable in the laboratory.

  3. Exercise 13.3

    Give three hypotheses for explaining why tropics show greatest levels of species richness.

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    (a) More evolutionary time. Speciation is generally a function of time. Unlike temperate regions, which were subjected to frequent glaciations in the past, tropical latitudes have remained relatively undisturbed for millions of years and so had a long evolutionary time for species diversification.
    (b) A constant, predictable environment. Tropical environments are less seasonal and relatively more constant than temperate ones. Such constant environments promote niche specialisation and lead to greater species diversity.
    (c) More solar energy. The tropics receive more solar energy, which contributes to higher productivity; this in turn may contribute indirectly to greater diversity.

  4. Exercise 13.4

    What is the significance of the slope of regression in a species - area relationship?

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    The slope \(\displaystyle Z\) (the regression coefficient) measures how steeply species richness rises as the area explored increases.
    The species–area relationship is a rectangular hyperbola, which on a logarithmic scale becomes the straight line \(\displaystyle \log S = \log C + Z \log A\), where \(\displaystyle S\) = species richness, \(\displaystyle A\) = area, \(\displaystyle C\) = Y-intercept and \(\displaystyle Z\) = slope.
    Within a region the slope is remarkably constant, \(\displaystyle Z = 0.1\) to \(\displaystyle 0.2\), regardless of the taxonomic group or the region — plants in Britain, birds in California and molluscs in New York state all give amazingly similar slopes.
    Across very large areas, such as entire continents, the slope is much steeper, \(\displaystyle Z = 0.6\) to \(\displaystyle 1.2\) — for frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents it is 1.15.
    So the value of \(\displaystyle Z\) tells us the scale at which the relationship is being examined: shallow and near-universal within a region, steep between continents.
    The chapter puts the further interpretation of those steeper continental slopes to the reader as an open question rather than answering it.

  5. Exercise 13.5

    What are the major causes of species losses in a geographical region?

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    Four major causes, collectively nicknamed 'The Evil Quartet', all traceable to human activity.
    (i) Habitat loss and fragmentation — the most important cause. Tropical rain forests once covered more than $\displaystyle 14$ per cent of the earth's land surface and now cover no more than $\displaystyle 6$ per cent; the Amazon is being cut and cleared for soya beans and beef cattle. Pollution degrades habitats further, and when large habitats break into small fragments, mammals and birds needing large territories and animals with migratory habits suffer population declines.
    (ii) Over-exploitation — when 'need' turns to 'greed'. Steller's sea cow and the passenger pigeon were lost this way, and many marine fish populations are over-harvested today.
    (iii) Alien species invasions — introduced species that turn invasive cause decline or extinction of indigenous species. The Nile perch in Lake Victoria wiped out more than $\displaystyle 200$ species of cichlid fish; carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia) threaten our native species, as does the illegally introduced African catfish Clarias gariepinus.
    (iv) Co-extinctions — when a species becomes extinct, species obligately associated with it go too: a host fish takes its unique assemblage of parasites with it, and in a coevolved plant–pollinator mutualism the extinction of one invariably leads to the extinction of the other.

  6. Exercise 13.6

    How is biodiversity important for ecosystem functioning?

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    Rich biodiversity makes a community more stable and more productive, and is essential for ecosystem health.
    A stable community is defined by three things: it does not show too much variation in productivity from year to year, it is resistant or resilient to occasional disturbances (natural or man-made), and it resists invasion by alien species.
    David Tilman's long-term ecosystem experiments in outdoor plots gave the evidence: plots with more species showed less year-to-year variation in total biomass, and increased diversity contributed to higher productivity.
    The 'rivet popper hypothesis' of Paul Ehrlich makes the point as an analogy — in an aeroplane (the ecosystem) the parts are held together by thousands of rivets (species). Popping one rivet may not affect flight safety at first, but as more are removed the plane becomes dangerously weak. Which rivet goes also matters: losing rivets on the wings (key species that drive major ecosystem functions) is far more serious than losing a few from the seats or windows.
    Conversely, loss of biodiversity in a region leads to (a) decline in plant production, (b) lowered resistance to environmental perturbations such as drought, and (c) increased variability in ecosystem processes such as plant productivity, water use, and pest and disease cycles.

  7. Exercise 13.7

    What are sacred groves? What is their role in conservation?

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    Sacred groves are tracts of forest set aside by religious and cultural tradition, in which all the trees and wildlife are venerated and given total protection.
    In India they are found in the Khasi and Jaintia Hills of Meghalaya, the Aravalli Hills of Rajasthan, the Western Ghat regions of Karnataka and Maharashtra, and the Sarguja, Chanda and Bastar areas of Madhya Pradesh.
    Role in conservation — they are a form of in situ conservation: because the whole tract is protected, the entire ecosystem and its biodiversity at all levels are protected with it.
    In Meghalaya the sacred groves are the last refuges for a large number of rare and threatened plants.
    They stand alongside India's legally protected areas — $\displaystyle 14$ biosphere reserves, $\displaystyle 90$ national parks and $\displaystyle 448$ wildlife sanctuaries — as part of the country's in situ conservation effort.

  8. Exercise 13.8

    Among the ecosystem services are control of floods and soil erosion. How is this achieved by the biotic components of the ecosystem?

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    The plants of an ecosystem do it, chiefly forests and the vegetation of a catchment.
    Roots hold the soil. A dense root mesh binds soil particles together, so running water cannot carry the topsoil away.
    The canopy breaks the rain. Leaves and branches intercept raindrops, so water reaches the ground gently instead of striking bare soil and dislodging it.
    Litter keeps the soil open. Fallen leaves and the humus formed from them keep the surface porous, so rain soaks in rather than running off.
    Slower runoff means no flood peak. Water that infiltrates is released slowly to streams, so the river rises gradually instead of in a sudden flood.
    This is one of the indirect benefits biodiversity provides, alongside pollination, pest control and climate moderation — the broadly utilitarian argument for conserving biodiversity.

  9. Exercise 13.9

    The species diversity of plants ($\displaystyle 22$ per cent) is much less than that of animals ($\displaystyle 72$ per cent). What could be the explanations to how animals achieved greater diversification?

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    The gap is almost entirely an insect gap: insects make up more than $\displaystyle 70$ per cent of all animal species, so $\displaystyle 7$ of every $\displaystyle 10$ animals on the planet is an insect. Explain the insects and most of the $\displaystyle 72$-versus-$\displaystyle 22$ difference is explained.
    Niche specialisation is the principle that accounts for it: a setting in which organisms can specialise finely ends up holding more species. This is exactly the reasoning that explains why the tropics are richer than temperate regions — constant, less seasonal, predictable environments promote niche specialisation and lead to greater species diversity.
    Animals can specialise to a much finer grain than plants can, because an animal can specialise on a single other species. A host fish carries its own unique assemblage of parasites, and coevolved plant–pollinator mutualisms are so tight that the extinction of one partner invariably ends the other. Every plant species can therefore support its own specialist animals, so animal diversity is layered on top of plant diversity and multiplies it rather than competing with it.
    Nutrition divides animals many ways and plants only one. Every organism takes its place in an ecosystem by its source of nutrition; green plants are all producers doing the single job of fixing carbon dioxide, while animals fill the consumer levels — primary, secondary and beyond — and live on detritus as well. More distinct ways of feeding means more distinct places to occupy.
    Energy supplies the extra species: more solar energy means higher productivity, and higher productivity contributes indirectly to greater diversity. The insect wealth sits precisely where both are greatest — scientists estimate at least two million insect species still waiting to be discovered and named in the tropical rain forests alone.
    Time favoured them as well: speciation is a function of time, and the tropical latitudes escaped the frequent glaciations that repeatedly disturbed temperate regions, leaving a long uninterrupted stretch in which this fine specialisation could accumulate.
    Keep the two figures honest when comparing them: the $\displaystyle 22$ per cent counted as plants already includes algae, fungi, bryophytes, gymnosperms and angiosperms — and fungi alone outnumber all the fishes, amphibians, reptiles and mammals put together.

  10. Exercise 13.10

    Can you think of a situation where we deliberately want to make a species extinct? How would you justify it? NOTES

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    Yes — the usual candidates are disease-causing organisms, such as the smallpox virus (already eradicated), the polio virus, and the malarial parasite and its vector.
    Justification offered: they cause immense human suffering and death, and eliminating them saves lives. The chapter does not endorse this argument.
    A second, chapter-grounded case is the invasive alien. The chapter names the Nile perch, which wiped out more than $\displaystyle 200$ species of cichlid fish in Lake Victoria, and carrot grass (Parthenium), Lantana, water hyacinth (Eichhornia) and the African catfish Clarias gariepinus as threats to our indigenous species. Removing these protects native biodiversity.
    But note the honest distinction: what is wanted there is eradication from a region, not global extinction — the species survives in its native range, where it is a normal part of its own ecosystem.
    The chapter's own counter-argument is the ethical one: every species has an intrinsic value even if it is of no current economic value to us, we have a moral duty to care for the well-being of the species we share this planet with, and we must pass on our biological legacy in good order to future generations.
    The 'rivet popper hypothesis' adds the practical warning: we cannot tell in advance which rivet we are pulling, and removing a species that turns out to drive a major ecosystem function weakens the whole system.
    A defensible closing position: deliberate extinction can be justified only for an organism whose removal saves human life, and even then only with great caution, since we rarely know the full ecological role of what we destroy.