NCERT Solutions for Class 12 Biology Chapter 13: Biodiversity and Conservation

Biodiversity, a term popularised by the sociobiologist Edward Wilson, describes the combined diversity existing at all levels of biological organisation in the biosphere — not just at the species level, but ranging all the way from macromolecules within cells to entire biomes. Of these many levels, three are considered the most important components of biodiversity:

1. Genetic diversity. This is the diversity shown by a single species at the genetic level across its distributional range — different populations or individuals of the same species may carry different genetic variants. The chapter illustrates this with the medicinal plant Rauwolfia vomitoria, which growing in different ranges of the Himalayas shows genetic variation in the potency and concentration of the active chemical reserpine that it produces. India itself is a striking example of genetic diversity, having more than 50,000 genetically different strains of rice and 1,000 varieties of mango.
2. Species diversity. This is diversity measured at the level of species themselves — essentially, how many different species are present, and how they are distributed. The chapter’s own example: the Western Ghats have a greater amphibian species diversity than the Eastern Ghats, illustrating that even within the same country, different regions can differ sharply in the number of species they support.
3. Ecological diversity. This is diversity at the level of entire ecosystems within a region — how many distinct types of ecosystems a geographical area contains. The chapter contrasts India, which possesses deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows, with a country like Norway, concluding that India has far greater ecosystem (ecological) diversity than Norway, despite Norway’s own natural richness.

Conclusion: the three important components of biodiversity are genetic diversity (variation within a species), species diversity (variation among species), and ecological diversity (variation among ecosystems) — together capturing the full hierarchy of biological variation that conservation efforts must address.

Because there are published records of every species discovered and formally named, ecologists know with reasonable confidence how many species have been recorded so far — according to the IUCN (2004), the total number of described plant and animal species is slightly more than 1.5 million. However, this recorded figure is very different from the true total number of species actually existing on Earth, since an unknown (and likely very large) number remain undiscovered, especially in under-explored regions.

1. The core difficulty. Species inventories are far more complete in temperate countries than in tropical countries, yet an overwhelmingly large proportion of the species still waiting to be discovered are believed to be concentrated in the tropics — meaning a simple global count of recorded species badly underestimates true diversity, and badly undersamples exactly the regions richest in undiscovered life.
2. The extrapolation method. To work around this, biologists take an exhaustively studied group of insects for which both temperate and tropical species counts are reliably known, and calculate the statistical ratio of tropical species richness to temperate species richness within that one well-studied group. This temperate-to-tropical ratio is then extrapolated — applied as an assumed proportion — to other groups of animals and plants that have not been as thoroughly surveyed, in order to arrive at a gross, estimated total for global species diversity.
3. Resulting estimates. Using such extrapolation methods, estimates vary widely: some extreme estimates range as high as 20 to 50 million species, but a more conservative and scientifically grounded estimate, made by the ecologist Robert May, places global species diversity at approximately 7 million — a figure generally regarded by the scientific community as the more credible estimate.

Conclusion: because tropical species remain far less completely catalogued than temperate ones, ecologists estimate global species diversity indirectly — by extrapolating the known temperate-to-tropical species ratio of a well-studied insect group onto other taxa — arriving at a conservative estimate of about 7 million species (Robert May), against roughly 1.5 million species actually recorded so far.

Species diversity is not evenly distributed across the globe — it follows a well-known latitudinal gradient, generally decreasing as one moves away from the equator towards the poles, so that tropical regions (the latitudinal band 23.5°N to 23.5°S) harbour far more species than temperate or polar regions (e.g., Colombia, near the equator, has nearly 1,400 bird species, compared to only 105 in New York at 41°N and just 56 in Greenland at 71°N). Ecologists and evolutionary biologists have proposed several hypotheses to explain why tropics are so species-rich:

1. Speciation as a function of time. Speciation is generally understood to be a function of time — the longer an evolutionary lineage has had to diversify without major disruption, the more species it tends to produce. Temperate regions were subjected to repeated glaciations in the past, which periodically disrupted and reset species diversification. Tropical latitudes, by contrast, have remained relatively undisturbed for millions of years, giving species in these regions a much longer uninterrupted evolutionary time over which to diversify.
2. Environmental constancy and niche specialisation. Tropical environments are less seasonal and relatively more constant and predictable than temperate ones, which experience dramatic seasonal swings. Such stable, predictable conditions allow species to evolve highly specialised ecological niches (narrow, specific roles) without the risk of those niches disappearing with the seasons — and this fine-grained niche specialisation permits many more species to coexist by minimising direct competition, leading to greater overall species diversity.
3. Greater solar energy and productivity. The tropics receive substantially more solar energy than higher latitudes throughout the year. This greater energy input contributes to higher productivity (more biomass/organic matter generated by producers), and this higher productivity, in turn, is believed to indirectly support and sustain a greater diversity of consumer species up the food chain.

Conclusion: the tropics’ exceptional species richness is explained by three complementary hypotheses — a longer, glaciation-free evolutionary time for speciation, a stable and predictable environment favouring fine niche specialisation, and greater solar energy driving higher productivity that can support more species.

The German naturalist and geographer Alexander von Humboldt, during his explorations of South American jungles, observed that within a region, species richness increased with increasing explored area, but only up to a limit. For a wide variety of taxa (angiosperm plants, birds, bats, freshwater fishes), this relationship between species richness and area turns out to be a rectangular hyperbola. When plotted on a logarithmic scale, however, this relationship becomes a straight line, described by the equation:

log S = log C + Z log A

where S = species richness, A = area, Z = the slope of the line (the regression coefficient), and C = the Y-intercept.

The significance of the slope (Z):

1. At normal/regional scales, Z is remarkably consistent. Ecologists have found that the value of Z typically lies in the range of 0.1 to 0.2, and strikingly, this holds true regardless of the taxonomic group or the region being studied — whether it is plants in Britain, birds in California, or molluscs in New York State, the slopes of their regression lines are remarkably similar. This consistency suggests a fairly universal, predictable relationship between local species richness and sampled area across very different taxa and places.
2. At very large (continental) scales, Z becomes much steeper. However, when the species–area relationship is analysed across very large areas, such as entire continents, the slope of the line is found to be much steeper, with Z values in the range of 0.6 to 1.2. For example, for frugivorous (fruit-eating) birds and mammals compared across the tropical forests of different continents, the slope works out to Z = 1.15.
3. What a steeper slope signifies. A steeper slope at the continental scale means that species richness increases much more rapidly with area than it does at the smaller, regional scale — because at continental scales, you are no longer just sampling more individuals of the same regional species pool, but crossing into genuinely different biogeographic regions and species pools altogether (different evolutionary histories, different climate zones, different barriers to dispersal). The slope, in other words, is a signature of the scale at which ecological vs biogeographic/evolutionary processes dominate the species–area pattern.

Conclusion: the slope (Z) of the species–area regression line reveals the rate at which species richness accumulates with area — a remarkably consistent, shallow slope of 0.1–0.2 across taxa and regions at normal scales, but a much steeper slope of 0.6–1.2 at continental scales, signifying that species richness grows far faster once sampling crosses into genuinely different biogeographic regions.

The accelerated rate of species extinctions the world faces today is largely driven by human activities, and ecologists have grouped the four major causes of this into a single, memorable sobriquet: ‘The Evil Quartet’.

1. Habitat loss and fragmentation. This is described as the single most important cause driving animals and plants towards extinction. The most dramatic examples come from tropical rain forests, which once covered more than 14% of the Earth’s land surface but now cover no more than 6% — and are still being destroyed at a staggering pace (roughly 1,000 more hectares lost in the time it takes to read this chapter). The Amazon rain forest, often called the ‘lungs of the planet’ for the sheer volume of oxygen it produces, is being cut and cleared for soybean cultivation or converted into grassland for raising beef cattle. Beyond total habitat loss, the gradual degradation of habitats through pollution also threatens species survival, while breaking large habitats into small, isolated fragments badly affects mammals and birds that need large territories, as well as species with migratory habits, leading to population declines.
2. Over-exploitation. Humans have always depended on nature for food and shelter, but when ‘need’ turns into ‘greed’, natural resources become over-exploited. Many species extinctions of the last 500 years — such as Steller’s sea cow and the passenger pigeon — were caused directly by human overexploitation. In the present day, many marine fish populations around the world are being over-harvested, putting the continued existence of several commercially important species at risk.
3. Alien species invasions. When alien (non-native) species are introduced into an ecosystem, whether unintentionally or deliberately, some of them turn invasive and cause the decline or outright extinction of indigenous species. The chapter’s central example is the Nile perch, introduced into Lake Victoria in East Africa, which eventually drove to extinction an ecologically unique assemblage of more than 200 species of native cichlid fish in that single lake. Other well-known invasive threats include weed species like carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia), and the recently, illegally introduced African catfish (Clarias gariepinus), which now threatens indigenous catfish species in Indian rivers.
4. Co-extinctions. When a species becomes extinct, other species that are obligately associated with it — dependent on it in a way they cannot survive without — also become extinct alongside it. For example, when a host fish species goes extinct, its entire unique assemblage of host-specific parasites is lost with it. Similarly, in a coevolved plant–pollinator mutualism, the extinction of either partner (the plant or its specific pollinator) invariably leads to the extinction of the other as well.

Conclusion: the major causes of species loss in a region are collectively known as the Evil Quartet — habitat loss and fragmentation (the most important), over-exploitation, alien species invasions, and co-extinctions — each driven substantially by human activity and each illustrated by well-documented real-world extinctions.

Whether the number of species in a community genuinely matters to how well an ecosystem functions is, as the chapter candidly notes, a question ecologists have not been able to answer with complete certainty — but the available evidence strongly suggests that biodiversity plays a vital, stabilising role.

1. The stability hypothesis. For many decades, ecologists have believed that communities with more species generally tend to be more stable than communities with fewer species. A stable biological community is defined by three properties: it should not show excessive year-to-year variation in productivity; it must be either resistant or resilient to occasional natural or man-made disturbances; and it must also be resistant to invasions by alien species.
2. David Tilman’s experimental evidence. Although the precise links between species richness and these stability attributes are not fully understood, ecologist David Tilman’s long-term ecosystem experiments, using outdoor experimental plots, provide tentative but valuable answers. Tilman found that plots containing more species showed less year-to-year variation in total biomass, and further demonstrated that increased diversity directly contributed to higher productivity in his experiments.
3. The ‘rivet popper hypothesis’. Since we cannot fully explain, in mechanistic detail, how species richness translates into ecosystem well-being, Stanford ecologist Paul Ehrlich offered a powerful analogy to help develop the right perspective: imagine an airplane (representing an ecosystem), held together by thousands of rivets (representing species). If every passenger starts popping out a rivet to take home as a souvenir (each rivet-pop representing a species going extinct), the plane may not show any immediate problem with flight safety — but as more and more rivets are removed, the structure becomes dangerously weak over time. Crucially, which rivet is removed also matters enormously: losing rivets on the wings (representing key species that drive major ecosystem functions) is a far more serious threat to flight safety than losing a few rivets from the seats or windows inside the cabin (representing less functionally critical species).

Conclusion: biodiversity is important for ecosystem functioning because richer communities tend to be more stable — showing less productivity variation and higher productivity, as Tilman’s experiments demonstrate — and because, as Ehrlich’s rivet popper hypothesis illustrates, every species lost weakens the ecosystem a little further, with the loss of key ‘wing rivet’ species posing a far graver threat than the loss of minor ones.

Sacred groves are tracts of forest that have been set aside by local communities, out of long-standing religious and cultural tradition, in which all the trees and wildlife within are venerated and given total protection. India has a particularly rich history of such religious and cultural traditions that emphasised the protection of nature, and sacred groves are a living expression of that heritage.

1. Where they are found. Sacred groves are found in several regions across India, including the Khasi and Jaintia Hills of Meghalaya, the Aravalli Hills of Rajasthan, the Western Ghats regions of Karnataka and Maharashtra, and the Sarguja, Chanda and Bastar areas of Madhya Pradesh.
2. Their role in conservation. Because an entire tract of forest, along with everything living within it, is given total protection by community/religious sanction, sacred groves function as a traditional, community-driven form of in situ conservation — protecting whole ecosystems rather than individual species in isolation. In Meghalaya specifically, the chapter notes that sacred groves serve as the last refuges for a large number of rare and threatened plant species, meaning that without these community-protected groves, some of these plants might already have been lost entirely from the wild.

Conclusion: sacred groves are community-protected forest tracts, preserved through religious and cultural tradition across regions such as Meghalaya, Rajasthan, Karnataka/Maharashtra and Madhya Pradesh, and they play a vital in situ conservation role by protecting whole ecosystems, serving in Meghalaya as the last refuge for several rare and threatened plant species.

This chapter identifies flood control as one of the valuable ecosystem services that biodiversity provides, alongside pollination, climate moderation and pest control. The biotic (living) components of a healthy, biodiverse ecosystem — chiefly its vegetation — achieve flood and soil-erosion control through several interlinked mechanisms:

1. Canopy interception of rainfall. A dense cover of vegetation — especially forest canopy, but also grasses and shrubs — physically intercepts falling rain before it reaches bare ground. This breaks the force of heavy rainfall and slows the speed at which water reaches the soil surface, instead of letting it strike the ground directly at full force.
2. Root systems binding and stabilising soil. The extensive, interlocking root systems of trees, shrubs and grasses physically bind soil particles together, holding the soil firmly in place. Without this root network, loose topsoil is far more easily dislodged and swept away by flowing water or wind, which is precisely why deforested or degraded land shows dramatically higher rates of soil erosion than vegetated land.
3. Increased infiltration and reduced surface runoff. Plant roots create channels in the soil and improve its porosity and structure, which increases the rate at which rainwater can infiltrate (soak) into the ground rather than running off rapidly over the surface. By converting a large share of incoming rainfall into slow, gradual groundwater infiltration instead of fast surface runoff, vegetation directly reduces the volume and speed of water that would otherwise rush into rivers and streams, blunting flood peaks downstream.
4. Organic litter layer as a buffer. The layer of leaf litter and decomposing organic matter that accumulates on a forested or vegetated floor acts like a natural sponge, absorbing and temporarily holding moisture, which further slows down water movement and gives it more time to soak into the soil rather than running off.
5. Wetland vegetation as natural flood buffers. In wetland ecosystems specifically, dense aquatic and marginal vegetation can absorb large volumes of excess water during periods of heavy rainfall, acting as a natural reservoir, and then release this stored water slowly afterward, smoothing out what would otherwise be a sudden flood surge.

Conclusion: the biotic components of an ecosystem control floods and soil erosion mainly through vegetation — canopy and litter intercepting and slowing rainfall, root systems binding soil in place, and improved infiltration converting fast surface runoff into slow groundwater recharge — which together reduce both the erosive power of flowing water and the severity of flood surges.

Of the species recorded on Earth so far, more than 70% are animals (of which, in turn, more than 70% are insects alone), while plants — including algae, fungi, bryophytes, gymnosperms and angiosperms — make up no more than about 22% of the total. This is a discussion-type question without one single textbook-stated answer, but several well-reasoned, biologically grounded explanations can account for this striking imbalance:

1. Mobility opens up far more ecological niches. Unlike plants, which are fixed in place once rooted, most animals can actively move — allowing them to colonise and exploit a vastly wider range of microhabitats, food sources and conditions than a stationary organism ever could. This mobility alone multiplies the number of distinct ecological niches available for animal species to specialise into.
2. The sheer diversity of feeding strategies and life styles. Animals have evolved an enormous range of ways to obtain food — herbivory, carnivory, parasitism, filter-feeding, scavenging, and more — each opening the door to further specialisation and, consequently, further speciation. Insects in particular, being the single largest animal group, show extraordinary diversification partly because so many of them have become specialised to feed on just one or a few particular plant species, multiplying insect diversity roughly in step with plant diversity.
3. Co-evolution with plants itself drives animal diversification. Many animal lineages, especially insects, have co-evolved closely with flowering plants — as pollinators, herbivores, or seed dispersers — and this intimate, reciprocal relationship has repeatedly driven both partners to specialise and diversify together, but with a disproportionately large diversifying effect on the animal (particularly insect) side, since a single plant species can often support several specialised insect associates.
4. Shorter generation times and higher reproductive rates. Many animal groups, again especially insects, have much shorter generation times and produce far more offspring per generation than most plants. Faster generational turnover gives natural selection more opportunities to act and more chances for new species to arise within a given span of evolutionary time.
5. Greater structural/behavioural flexibility for niche specialisation. The animal body plan — particularly the jointed exoskeleton and modular body segments of arthropods such as insects — is remarkably adaptable, allowing relatively small genetic changes to produce large functional differences in mouthparts, limbs, or sensory structures, fine-tuning a species to a very narrow ecological role and thereby promoting rapid, fine-grained diversification.

Conclusion: animals, and insects especially, likely achieved far greater diversification than plants because their mobility, varied feeding strategies, close co-evolution with plants, shorter generation times and structurally flexible body plans together opened up many more fine-grained ecological niches for new species to specialise into.

Yes — the deliberate, complete eradication of the smallpox virus (Variola) is a real, historically documented example of humans intentionally driving a species to extinction (at least in the wild), and it remains one of the most celebrated achievements in the history of public health.

1. What happened. Through a sustained, worldwide vaccination campaign coordinated by the World Health Organization, the smallpox virus was deliberately and systematically eliminated from the human population. The World Health Organization officially declared smallpox eradicated in 1980 — meaning the virus survives today, if at all, only in a small number of high-security research laboratories, and no longer circulates anywhere in the wild or among the general population.
2. Why this deliberate extinction is justifiable. Several factors distinguish this case from the ordinary, harmful loss of biodiversity this chapter otherwise warns against:
   • Severe, direct harm with no redeeming ecological role. Smallpox was a lethal human pathogen responsible for enormous suffering and millions of deaths throughout history, and unlike most species in a natural ecosystem, it played no known beneficial or essential ecological role — it was not a keystone species, nor part of any food web or nutrient cycle that other organisms depended upon.
   • No demonstrated ecological disruption from its removal. In the decades since its eradication, there has been no evidence of any ecosystem being destabilised or any other species being harmed by the disappearance of the smallpox virus — strongly suggesting its removal carried essentially none of the ecological costs associated with losing, say, a keystone predator or a key pollinator.
   • Overwhelming, unambiguous human benefit. The eradication of smallpox has saved an estimated hundreds of millions of lives in the decades since, and ended a disease that caused immense, direct human suffering — an unambiguous and enormous benefit that vastly outweighs any ethical concern about eliminating a single pathogenic species.

Conclusion: the deliberate, globally coordinated eradication of the smallpox virus, declared complete by the WHO in 1980, is a real and justifiable case of intentionally making a species extinct — justified because the virus caused severe, direct harm to humans, played no beneficial ecological role, and its removal produced an overwhelming, unambiguous human benefit with no observed ecological cost.