Ecological Niche, Ecotone and Keystone Species: GK Notes

The correct answer is B, living organisms and physical factors. An ecosystem is a working unit in which a community of living organisms interacts with the non-living surroundings in a given area, so it must contain both halves. The living or biotic half has producers such as green plants, consumers such as herbivores and carnivores, and decomposers such as bacteria and fungi. The non-living or abiotic half has sunlight, temperature, water, air, soil, minerals and pH. Option A is wrong because organisms alone, without energy and nutrients from the physical surroundings, form only a community and not an ecosystem. Option D is wrong for the opposite reason: physical factors alone make a habitat, with no life to cycle the nutrients. Option C is far too narrow, since insects are only one group of consumers inside a much larger web. Remember the short definition: biotic plus abiotic, interacting, equals ecosystem.

The correct answer is B, soil. Abiotic components are the non-living parts of an ecosystem, and soil belongs squarely to that list along with sunlight, temperature, water, air, minerals and pH. Soil supplies anchorage, water and nutrients, and its texture and chemistry decide which plants can grow, which in turn decides the animals that live there. Option A, fungi, is biotic: fungi are the chief decomposers that break down dead matter and return nutrients to the soil. Option C, microorganisms, is also biotic, since bacteria, protozoa and other microbes are living organisms acting as decomposers or as agents of disease and nitrogen fixation. Option D, insects, is biotic too, as insects are consumers and pollinators. The trap in such questions is soil, because students link it with living things; keep in mind that soil is a non-living medium even though it teems with life.

The correct answer is C, gardens. An artificial or man-made ecosystem is one that people create and keep going by their own effort, deciding which species live in it and supplying water, nutrients and protection. A garden is the standard example, along with crop fields, orchards, aquariums and the reservoir behind a dam; left alone, such a system is quickly invaded by other species and loses its planned form. Options A, B and D are natural ecosystems, which develop on their own through succession and are self-regulating and self-sustaining. A forest builds its own canopy, litter and nutrient cycle, while lakes and ponds are natural water bodies whose plants, fish and plankton establish themselves without human planning. Note the distinction examiners test: the artificial ecosystem has low species diversity, is simple in structure and depends on constant human input, whereas a natural ecosystem is diverse, complex and stable by itself.

The correct answer is C, the ocean ecosystem. Oceans cover close to seventy-one per cent of the earth's surface and hold about ninety-seven per cent of its water, which makes the marine ecosystem the largest on the planet by both area and volume. It runs from sunlit coastal shallows down to deep trenches where no light reaches, and its producers are mainly phytoplankton, whose photosynthesis supplies a very large share of the oxygen in the atmosphere. Option D, forests, form the largest terrestrial ecosystem but cover only about a third of the land surface, which is far less than the oceans. Option B, grasslands, occupy still less and are themselves often a transition between forest and desert. Option A, deserts, are large but have low productivity and sparse life because water is the limiting factor. A useful hierarchy to memorise: ocean, then forest, then grassland, then desert.

The correct answer is A, ecotone. An ecotone is the zone where two adjoining ecosystems meet and blend, such as the mangrove belt between land and sea, an estuary between a river and the ocean, or the strip where grassland gives way to forest. Its conditions are intermediate, it is usually linear and narrow, and it holds species from both neighbours together with edge species of its own, which produces the higher species density called the edge effect. Option B, ecocline, is the close relative and the common trap: an ecocline is a gradual environmental gradient with change spread out smoothly, whereas an ecotone marks a comparatively sharp boundary. Option C, ecad, means a population whose form has been moulded by local habitat conditions rather than by heredity. Option D, niche, is the functional role of a species in its community, not a place at all. Fix the pair in memory as sharp boundary equals ecotone, gradual gradient equals ecocline.

The correct answer is B, Robert T. Paine. Paine introduced the term in 1969 after an experiment on the rocky Pacific shore in which he removed the starfish Pisaster ochraceus from study plots. The starfish had been keeping mussel numbers down; once it was gone the mussels crowded out other organisms and the number of species on the shore fell sharply. From this he argued that a few species hold the structure of a community in place out of all proportion to their own numbers. Option A, Charles Elton, is remembered for the functional or Eltonian niche and for pyramids of numbers, not for this term. Option C, G. F. Gause, gave the competitive exclusion principle, which says that two species cannot occupy the same niche indefinitely. Option D, Norman Myers, proposed the concept of biodiversity hotspots in 1988. Keep the four names and their four ideas separate, because papers mix them deliberately.

The correct answer is A, air pollution. A lichen is a partnership of an alga and a fungus that takes its water and minerals straight from the air, having no protective cuticle and no roots. It therefore absorbs dissolved gases and particles without any filter, and sulphur dioxide in particular destroys its chlorophyll. Because of this, lichens vanish from industrial towns and crowded city centres and flourish in clean country air, and the barren belt around a smoke source is called a lichen desert. Option B is wrong because sound is a physical disturbance that does not act on lichen tissue. Option C, thermal pollution, concerns warm water discharged into rivers and lakes, where fish and aquatic invertebrates are the indicators. Option D, soil pollution, is better indicated by earthworms and soil microbes. Learn the set together: lichens for air, mayfly and stonefly larvae for clean water, Tubifex worms for organic waste, and coliform bacteria for sewage.

The correct answer is B, the competitive exclusion principle, also called Gause's principle after G. F. Gause, who demonstrated it with two species of Paramecium grown in the same culture. Where conditions stay constant and the resource is limiting, the better competitor increases while the weaker one dies out or shifts its niche. Species escape this fate by resource partitioning, dividing the resource by space, time or prey size, as in MacArthur's warblers feeding at different heights of one spruce tree. Option A, Allen's rule, says that animals of colder regions have shorter extremities, which is a thermoregulation rule and not about competition. Option C, the edge effect, is the higher density and diversity of species found in an ecotone. Option D, Liebig's law of the minimum, states that growth is limited by the resource in shortest supply. Only option B deals with two species and a shared niche, which is the giveaway in the question stem.

The correct answer is B, keystone species. A keystone species holds the structure of its community together out of all proportion to its own numbers, exactly as the keystone at the top of an arch holds the stones in place. Sea otters eat sea urchins; when otters were hunted out, urchin numbers exploded, the urchins grazed the kelp down to bare rock, and the fish and invertebrates that sheltered in the kelp disappeared with it. Option A, a flagship species, is a charismatic animal used to win public support and funding, such as the giant panda on the WWF emblem; the role is about publicity, not ecological function. Option C, an indicator species, signals the state of the environment, as lichens signal sulphur dioxide in the air. Option D, an invasive species, is one introduced outside its native range that spreads and harms local species, like water hyacinth. The chain of collapse described in the stem is the mark of a keystone species.

The correct answer is A, the realised niche. G. Evelyn Hutchinson distinguished the fundamental niche, which is the full range of conditions and resources a species could use if nothing interfered, from the realised niche, which is the narrower part left to it once competitors, predators, parasites and disease are at work. The realised niche is therefore always equal to or smaller than the fundamental niche, and it can widen again if a competitor is removed, a phenomenon called competitive release. Option B, the Grinnellian niche, is Joseph Grinnell's version of the concept, built on the physical conditions a species requires, and it is not defined by competition. Option C, a microhabitat, is a small distinct pocket inside a habitat, such as the underside of a log or the soil of a termite mound. Option D, ecological equivalents, are unrelated species that fill similar niches in different regions, such as the kangaroo of Australia and the deer of Asia.