RowQ
The Vault
RowQ
The Vault
CBSE Class 12 Biology · 11 questions · 26 marks
Biodiversity is not simply a long list of species — it is the variety of life at every level, from the alleles inside one species to the range of ecosystems in a landscape. This chapter asks three connected questions: how much diversity there is and how it is distributed, why it is disappearing so fast, and what actually works to protect it. Keep the species-area relationship, the four main causes of extinction, and the difference between in situ and ex situ methods at your fingertips, because they carry most of the marks.
In the species-area relationship log S = log C + Z log A, the value of the slope Z for small areas is found to lie in a narrow range regardless of the group of organisms studied. That range is about:
Answer
0.1 to 0.2 is correct — whether the survey covers plants, birds, freshwater fishes, or molluscs, the regression slope for small areas is remarkably constant in this range, which is one of the more striking regularities in ecology. The higher range of about 0.6 to 1.2 applies only when the analysis is carried out over very large areas such as entire continents.
Which of the following is the single largest cause of the present loss of biodiversity?
Answer
Habitat loss and fragmentation is correct — when a forest or wetland is cleared, drained, or broken into small isolated patches, whole communities lose the space and resources they need at once, and animals with large territory requirements or migratory habits suffer most. Over-exploitation, alien species, and co-extinction are genuine causes and complete the group of four, but none of them destroys diversity on the same scale.
Cryopreservation of gametes of a threatened deer and the maintenance of its seed-eating food plants in a seed bank are examples of:
Answer
Ex situ conservation is correct — the organism or its reproductive material is taken out of its natural habitat and kept under special protected care, as in zoological parks, botanical gardens, seed banks, gene banks, and cryopreservation at very low temperature. In situ conservation, which includes hotspots and sacred groves, works by protecting the species where it naturally lives.
The variation found among individuals of a single medicinal plant species growing in different valleys, differing in the concentration of the active compound they produce, is an example of:
Answer
Genetic diversity is correct — the variation lies within one species, among its populations, and shows in different alleles producing different amounts of the chemical. Species diversity refers to the variety of different species in a region, while ecological or ecosystem diversity refers to the variety of habitat types, such as forests, wetlands, mangroves, and grasslands, present in an area.
Assertion (A): Conservation effort is deliberately concentrated on a small number of regions identified as biodiversity hotspots. Reason (R): These regions combine an exceptionally high level of endemism with an exceptionally high level of threat, so protecting a small area saves a disproportionately large number of species that exist nowhere else.
Answer
Both A and R are true and R is the correct explanation of A — a hotspot is defined by exactly those two criteria together. Endemic species occur in that region and nowhere else on earth, so their loss there is a global loss, and the high level of threat means that loss is imminent. Since conservation money, staff, and legal protection are limited, directing them at these regions prevents the greatest number of extinctions per unit of area protected. India's share of these regions includes the Western Ghats and Sri Lanka, the Indo-Burma region, and the Himalaya.
Distinguish between in situ and ex situ conservation, giving one example of each.
Answer
In situ conservation means protecting a threatened species where it naturally occurs, by protecting the whole habitat and the community it belongs to. The species continues to live, breed, and evolve under natural selection in its own surroundings. National parks, wildlife sanctuaries, biosphere reserves, and the sacred groves protected by local communities are examples. Ex situ conservation means removing the threatened organism, or its gametes, seeds, or tissue, from its natural habitat and keeping it under special protected care elsewhere. Zoological parks, botanical gardens, wildlife safari parks, seed banks, and cryopreservation of gametes at very low temperature are examples. It is used when a population is too small or its habitat too damaged for in situ protection alone, and the stock so preserved can later be used for reintroduction.
Give one narrowly utilitarian, one broadly utilitarian, and one ethical argument for conserving biodiversity.
Answer
Narrowly utilitarian: human beings draw countless direct economic goods from nature — food grains and fruits, fibre, firewood, timber, industrial products such as resins, dyes, and tannins, and above all medicines, a large proportion of which were first obtained from plants. Bioprospecting, the search among species for new molecules of medicinal or industrial value, means that an unexamined species may hold a product of great worth, and destroying it destroys that possibility permanently. Broadly utilitarian: biodiversity delivers the services that make the biosphere habitable. Forests release the oxygen we breathe and absorb carbon dioxide, pollinators sustain the flowering plants on which food production depends, and intact ecosystems regulate water flow, control floods, and build soil. None of these services can be manufactured at a comparable cost. The aesthetic and cultural pleasure of a forest in bloom or a river full of birds belongs to the same category. Ethical: every species is the product of millions of years of evolution and has an intrinsic value that does not depend on its usefulness to us. We share this planet with these species, and we have a moral obligation to hand over to the coming generations a world as biologically rich as the one we ourselves inherited.
What is a sacred grove, and why is it regarded as an effective method of conservation?
Answer
A sacred grove is a patch of forest that a local community protects from felling and disturbance on religious and cultural grounds, treating the trees and the wildlife within it as sacred. Such groves are maintained by communities in several parts of India, including the Khasi and Jaintia hills of Meghalaya, the Aravalli hills, the Western Ghats region of Karnataka and Maharashtra, and parts of Bastar in Chhattisgarh. It is an effective form of in situ conservation because the whole habitat is preserved intact, so the species within it continue to live and reproduce in their natural community. Protection rests on voluntary community belief rather than on external policing, which makes it durable and inexpensive, and some of these groves shelter rare or endemic species that have been wiped out of the surrounding, heavily used landscape.
Explain the major causes of the present loss of biodiversity, with a named example for each, and state why the current rate of extinction is regarded as unusual.
Answer
Extinction has always occurred, but the current rate is estimated to be many hundreds of times faster than the natural background rate, and unlike earlier mass extinctions it is driven by a single species. That is why the present episode is described as a human-caused sixth extinction. Four causes account for most of it. Habitat loss and fragmentation: this is by far the largest cause. Tropical rainforests, once covering a substantial part of the land surface, have been cleared on a vast scale for agriculture, timber, mining, and settlement, and the Amazon has been reduced most dramatically of all. Wetlands are drained, mangroves cut, and grasslands ploughed. Even where the total area cleared is modest, cutting a large forest into small isolated patches is severely damaging, because animals that require large territories and those with migratory habits cannot survive in fragments, and small isolated populations lose genetic variation and are wiped out by a single local disturbance. Over-exploitation: when human use turns from need to greed, harvesting outstrips the ability of the population to replace itself. Marine fisheries in many parts of the world have collapsed after sustained heavy fishing, and several large birds and mammals hunted for meat, oil, skin, or horn have been driven to extinction within historical time. Species that are slow to reproduce, produce few offspring, or aggregate in predictable places are the most vulnerable. Alien species invasion: an organism introduced deliberately or accidentally into a new region may find no natural predators or diseases there and multiply unchecked, driving out native species by competition or predation. Introduced weeds have taken over large areas of grazing land in India, an aggressive aquatic weed has choked lakes and blocked waterways, and predatory fish introduced for aquaculture have damaged native fish communities in rivers and lakes. Co-extinction: species are bound together, so the loss of one drags down others. A plant that depends on a single pollinator dies out if that insect disappears; a host-specific parasite is lost when its host is lost; and in an obligate partnership such as a fig and its pollinating wasp, the extinction of one member ends the other. Why it matters: these causes act together and reinforce one another, so a fragmented habitat is also more easily invaded and more easily over-harvested. Since extinction is irreversible, and since ecosystem stability generally increases with species richness, the loss weakens the very systems that provide oxygen, pollination, soil, and water regulation.
Describe the patterns of biodiversity observed on earth, explain the species-area relationship, and outline the conservation strategies used to protect biodiversity.
Answer
Latitudinal gradient: species diversity is not spread evenly. It is greatest near the equator and decreases steadily towards the poles, so a given area of tropical rainforest contains far more species of trees, birds, and insects than an equivalent area of temperate woodland, and far more again than a polar region. Three explanations are usually offered. First, temperate and polar regions were repeatedly disturbed by glaciation, while tropical regions have remained relatively undisturbed for a very long period, giving their communities a much longer uninterrupted span for speciation. Second, the tropical environment is more constant and predictable through the year, which favours narrow niche specialisation and therefore allows more species to divide up the same resources. Third, the tropics receive more solar energy and have higher productivity, which supports a greater diversity of consumers. Species-area relationship: within a region, the number of species found rises as the area surveyed increases, but the rise is not linear — it slows down and the curve flattens, giving a rectangular hyperbola. On a logarithmic scale it becomes a straight line, log S = log C + Z log A, where S is species richness, A is area, C is the y-intercept, and Z is the regression coefficient or slope. Remarkably, Z lies between about 0.1 and 0.2 for small areas whatever the taxonomic group — plants, birds, freshwater fishes, or molluscs — but when the analysis is extended across very large areas such as entire continents the slope becomes much steeper, about 0.6 to 1.2. The practical importance of this relationship is that it predicts how many species will be lost when a habitat is reduced in area, which is why fragmentation is so damaging. In situ conservation: this protects species in their natural habitat, and because the entire community is conserved together it is the more economical and more effective approach. Since resources are finite, effort is concentrated on biodiversity hotspots — regions with exceptionally high endemism combined with exceptionally high threat — and India's share of these includes the Western Ghats and Sri Lanka, the Indo-Burma region, and the Himalaya. Legally protected areas take the form of national parks, wildlife sanctuaries, and biosphere reserves, while sacred groves protected by local communities on religious grounds preserve small but often exceptionally rich patches. Ex situ conservation: where a population has become too small, or its habitat too degraded, threatened animals and plants are taken out of their natural home and kept under special care. Zoological parks, botanical gardens, and wildlife safari parks maintain living collections; seed banks and gene banks preserve the genetic material of crop varieties and wild relatives; gametes can be preserved by cryopreservation at very low temperature, eggs can be fertilised in vitro, and plants can be propagated from tissue culture. Animals bred in captivity under such programmes can later be reintroduced into the wild. International effort: the Earth Summit held at Rio de Janeiro produced the Convention on Biological Diversity, calling on nations to conserve biodiversity and share its benefits fairly, and at the later World Summit on Sustainable Development at Johannesburg countries committed themselves to achieving a significant reduction in the rate of biodiversity loss.
A coastal district is surveyed by a state biodiversity board. The report notes that a large mangrove belt has been cleared and the rest broken into four small patches separated by new roads and ponds; that a fast-growing aquatic weed introduced years ago now covers most of the district's freshwater lakes and has displaced several native water plants; that a small frog found in the forested hills above the coast occurs nowhere else in the world and is now known from only two streams; and that a fig species in those hills is fruiting poorly because the tiny wasp that pollinates it has become scarce. (a) Name the four causes of biodiversity loss illustrated by this report, matching each to the observation. (b) Explain what is meant by calling the frog endemic, and why the region may qualify as a hotspot. (c) Using the species-area relationship, explain why breaking the mangrove belt into four small patches is more damaging than the total area lost would suggest. (d) Recommend one in situ and one ex situ measure for the frog, and justify each.
Answer
(a) Habitat loss and fragmentation is shown by the clearing of the mangrove belt and the cutting of the remainder into four isolated patches by roads and ponds. Alien species invasion is shown by the introduced aquatic weed that has spread through the lakes and displaced native water plants, since an introduced organism with no natural enemies in the new region multiplies unchecked. Co-extinction is shown by the fig fruiting poorly as its pollinating wasp declines, because in such an obligate partnership the decline of one member directly threatens the other. The frog surviving in only two streams reflects the same habitat loss acting on an already restricted species, and any collection of such a rare animal for trade would add over-exploitation, the fourth cause, in which harvesting outstrips the population's ability to replace itself. (b) The frog is endemic because it occurs naturally in this region and nowhere else on earth; its global population is entirely contained in these hills, so a local extinction would be a permanent global extinction. A region qualifies as a biodiversity hotspot when it combines exceptionally high endemism with exceptionally high threat, and both conditions are met here — a species found nowhere else, in a landscape being rapidly cleared and fragmented. That is why conservation resources are directed at such regions: protecting a small area saves a disproportionately large number of irreplaceable species. (c) The species-area relationship, S = CA^Z or log S = log C + Z log A, shows that the number of species a habitat supports depends on its area, so reducing area necessarily costs species. Fragmentation makes matters worse than the arithmetic of area alone suggests, because four small patches do not function as one large one. Animals needing large territories and those that migrate cannot complete their life cycles in a small patch; each fragment holds a small population that loses genetic variation and can be wiped out by a single storm, disease, or fire; and the roads and ponds between the patches prevent recolonisation from neighbouring fragments. The exposed edges of each patch also differ in light, salinity, and temperature from the interior, so the effective habitat is smaller still. (d) In situ: declare the two streams and their catchment a protected area — a sanctuary or a community-protected reserve — and stop clearing and water diversion within it. This is the preferred approach because it conserves the frog together with its whole community and lets it continue to live and evolve under natural conditions, at far lower cost than maintaining animals artificially. Ex situ: establish a captive breeding programme at a zoological park, supported by cryopreservation of gametes so that genetic material survives even if the wild population fails. This is justified because two streams are far too few to be safe against a single local disaster, and captive-bred individuals can later be reintroduced into the restored habitat once the in situ protection is working.
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