Ecology – Major Abiotic Factors: NEET MCQs with Answers

Explanation: Curve (i) stays flat, i.e. the internal level remains constant irrespective of the external level — a regulator (e.g. birds, mammals, which maintain homeostasis physiologically). Curve (ii) rises continuously with the external level — a conformer (99% of animals and nearly all plants, whose internal state changes with the environment). Curve (iii) rises initially and then flattens — a partial regulator, able to regulate only over a limited range of environmental conditions.

Explanation: Tropical rain forests show the clearest vertical stratification — emergent trees, canopy, understorey, shrub and herb layers — because light decreases sharply from top to bottom.

Explanation: The tropical rain forest has a tall, dense, multi-layered canopy that offers abundant food and shelter above the ground, so most of its animals are arboreal.

Explanation: Every winter, thousands of migratory birds arrive at Keoladeo National Park (Bharatpur, Rajasthan) from Siberia and other cold northern regions — an example of escaping unfavourable conditions in space.

Explanation: The dark colour of the surface soil is due to the accumulation of humus — leached organic substances and decomposed organic matter. Humus acts as a reservoir of nutrients and water, reduces compaction and surface crusting, and increases infiltration, which is why humus-rich topsoil is the most fertile layer.

Explanation: Soil texture is defined by the relative proportion of particles of different size classes — gravel (2–75 mm), sand (0.05–2 mm), silt (0.002–0.05 mm) and clay (< 0.002 mm). Texture in turn influences percolation, aeration and water-holding capacity, but the primary attribute directly determined by particle size is the texture itself.

Explanation: Loamy soil, containing roughly 20% clay, 40% silt and 40% sand, provides the best combination of aeration, drainage, nutrient content and water-retaining capacity, and is therefore the most suitable for plant growth. Pure clay is poorly aerated and waterlogged, while sandy and gravelly soils drain too fast and retain neither water nor nutrients.

Explanation: Stenohaline organisms tolerate only a narrow range of salinity, whereas euryhaline organisms can withstand a wide range. Anadromous fishes (e.g. salmon) live in the sea but migrate to freshwater to spawn, while catadromous fishes (e.g. eel) live in freshwater and return to the sea to spawn.

Explanation: Submerged hydrophytes remain completely inside water, so there is no transpiration and no need for stomatal gaseous exchange — gases diffuse directly across the thin cuticle-free epidermis. Hence such plants (e.g. Utricularia, Ceratophyllum, Hydrilla) lack stomata. Floating hydrophytes such as lotus, in contrast, bear stomata on the upper surface.

Explanation: Xerophytes possess sunken stomata that are generally restricted to the lower surface of the leaf and lie in pits below the epidermis, often shielded by epidermal hairs. The still, humid air trapped in these pits reduces the rate of transpiration and thus conserves water.

Explanation: Most desert plants are succulent or fleshy xerophytes that store water in thickened stems or leaves, e.g. Opuntia, Bryophyllum, Euphorbia and Mesembryanthemum (ice plant). Vivipary is characteristic of mangroves and heterophylly of some aquatic plants.

Explanation: Benthic organisms live at the bottom of a water body, either attached to the substratum or resting on and burrowing into the bottom sediments. They include filter feeders such as clams and deposit feeders such as snails. Free-floating forms are plankton and actively swimming forms are nekton.

Explanation: Allen's rule states that mammals of colder climates have shorter ears, limbs, tail and other extremities, which reduces the surface area available for heat loss. Bergmann's rule relates increasing body size to colder climates/higher latitude, Jordan's rule states that vertebral number increases with latitude, and Gloger's rule relates darker skin pigmentation to humid environments.

Explanation: Island communities are species-poor and evolved in isolation without mainland competitors, predators and diseases, so introduced alien species establish easily and often cause extinctions, as NCERT notes for alien species invasions.

Explanation: Tropical thorn forests (thorn shrub/scrub forests) occur where moisture conditions are intermediate between those of deserts and savannas. Acacia and Prosopis are non-succulent perennial plants and Capparis is a xerophytic shrub — all typical constituents of tropical thorn forests.