Ecology: Populations, interactions, niche and ecotone

NCERT describes the physical structure of an ecosystem through two features. Identifying and counting the species present gives its species composition, while the vertical layering of species at different heights is called stratification. In a forest, trees form the top stratum, shrubs the middle and herbs and grasses the lowest.

Only one, the phytoplankton, is biotic. NCERT uses the pond as its model ecosystem: the abiotic component is the water with its dissolved inorganic and organic substances and the rich soil at the bottom, while solar input, temperature and day length regulate the whole system. The autotrophs are phytoplankton, some algae and the floating, submerged and marginal plants; zooplankton and fish are consumers, and fungi, bacteria and flagellates are decomposers.

Statements 1 and 3 are correct. An ecotone is the junction zone between two or more diverse ecosystems; mangroves (between land and sea), grasslands (between forest and desert) and estuaries (between fresh and salt water) are standard examples. An ecotone may be narrow or wide and usually shows a gradual change, so statement 2 is wrong. It holds species of both adjoining communities plus some that are found mostly there, called edge species; this increase in variety and density is the edge effect.

This is Liebig's law of the minimum (1840), first framed for crop nutrients: yield is limited by the scarcest essential nutrient, like a barrel that holds water only up to its shortest stave. Shelford's law of tolerance (1913) widened the idea, holding that too much of a factor can limit an organism as well as too little. Allen's rule is about the shorter limbs and ears of mammals in colder climates.

Pairs 1 and 3 are correct. The Millennium Ecosystem Assessment groups services into four types. Provisioning services are products such as food, fresh water, fuelwood and fibre. Regulating services are benefits from regulating processes, including climate and flood regulation, water purification and pollination. Cultural services are non- material benefits such as spiritual, aesthetic and recreational value. Supporting services, such as soil formation, nutrient cycling and primary production, underpin all the others; so pairs 2 and 4 are wrong.

Costanza and colleagues put an average price tag of about US $33 trillion a year on nature's life-support services, nearly twice the global GNP of the time (about US $18 trillion). Of this total, soil formation accounts for about 50 per cent. Recreation and nutrient cycling each contribute less than 10 per cent, and climate regulation and habitat for wildlife about 6 per cent each.

Statements 1 and 2 are correct. The ecological niche is the functional role of a species in its community together with the physical space and conditions it uses; the habitat is only its 'address'. NCERT states Gause's competitive exclusion principle: two closely related species competing for the same resources cannot coexist indefinitely, and the inferior competitor is eventually eliminated. That is why species sharing a habitat usually divide resources, for example by feeding at different times or heights; they do not occupy identical niches, so statement 3 is wrong.

Statements 1 and 3 are correct. NCERT notes that a few organisms tolerate and thrive in a wide range of temperatures (eurythermal), but a vast majority are restricted to a narrow range (stenothermal). The same prefixes apply to salinity: wide tolerance is euryhaline and narrow tolerance is stenohaline, so statement 2 has the term reversed. Shelford's law of tolerance says that each organism has an ecological minimum and maximum for every factor, with an optimum range between them.