The principle that two species competing for the same limiting resource cannot coexist indefinitely is known as:

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.