NEET Organisms and Populations Previous Year Questions with Solutions

In population ecology, the intrinsic rate of natural increase (r) is determined by subtracting the per capita death rate (d) from the per capita birth rate (b).

• • r is the vital statistic measuring inherent capacity of a population to grow.

The population size after t time periods is given by the integral form of exponential growth. When r = 0, e^(rt) = e^0 = 1, meaning the population remains completely stationary.

• • Stationary population exhibiting Zero Population Growth (ZPG).

Resource availability is obviously essential for the unimpeded growth of a population. In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible (carrying capacity K). Thus, the logistic model is considered more realistic than exponential growth.

• • Assertion (A) is completely true.

In the real world, food and space are never unlimited. Competition for these limited resources inevitably occurs among individuals, eventually resulting in the survival and reproduction of the fittest, establishing an upper asymptote at carrying capacity K. Thus, Reason (R) is true and is the precise scientific rationale for (A).

• • Both are true, and (R) correctly explains (A).

Competition is best defined as a process in which the fitness of one species is significantly lower in the presence of another species. A classic ecological example documented in NCERT is the Abingdon tortoise in Galapagos Islands, which became extinct within a decade after goats were introduced on the island.

• • Assertion (A) is completely true.

Goats are agile, fast-eating herbivores with greater browsing efficiency than the slow-moving, ectothermic Abingdon tortoises. Goats outcompeted the tortoises for scarce island vegetation, driving them to extinction. Thus, Reason (R) is true and perfectly explains Assertion (A).

• • Illustrates Gause’s Principle of Competitive Exclusion in nature.

Biological interactions between two species are formally designated using positive (+), negative (-), and neutral (0) signs based on fitness outcomes.

• • Amensalism is (-, 0); Parasitism is (+, -).

A matches III, B matches I, C matches II, D matches IV. This corresponds uniquely to Option (4).

• • No other option correctly pairs all four interactions.

Commensalism is a positive interspecific interaction in which one population is benefited while the other remains unaffected:

• • The benefiting partner is the commensal; the neutral partner is the host.

Evaluate the ecological exchange between epiphyte and mango branch:

• • Epiphytes are non-parasitic space-dwellers; they synthesize their own food via photosynthesis and absorb atmospheric moisture via hygroscopic velamen roots.
• • They do not produce haustoria or draw sap from the mango tree.

NCERT explicitly cites two benchmark examples of commensalism:

• • Thus the interaction is unequivocally Commensalism, matching Option (1).

The opening biographical profile of Unit X (Ecology) in the NCERT textbook honors Professor Ramdeo Misra:

• • Revered as the Father of Ecology in India.
• • Established ecology as a foundational biological discipline in India at Banaras Hindu University (BHU).
• • Conducted pioneering ecological research on tropical communities, primary productivity, and nutrient cycling.

Distinguishing the historical figures in Indian botany:

• • S. R. Kashyap (Shiv Ram Kashyap): Revered as the "Father of Indian Bryology".
• • Ram Udar: Renowned Indian bryologist, student of S.R. Kashyap.
• • Birbal Sahni: Revered as the "Father of Indian Palaeobotany" (established the Birbal Sahni Institute of Palaeosciences in Lucknow).

Professor Ramdeo Misra is unequivocally recognized as the Father of Ecology in India.

Addressing the popular myth vs biological reality of fig fruits:

• • Botanically, a fruit is a mature, ripened ovary (or inflorescence axis, syconus in *Ficus*) produced exclusively by angiosperm plants.
• • There is no scientific or botanical taxonomic category called a "non-vegetarian fruit".
• • While a female agaonid pollinator wasp may enter the ostiole of the inflorescence and die after oviposition/pollination, the plant secretes a proteolytic enzyme called ficin that completely digests the insect into amino acids and elemental nutrients absorbed by the developing fruit tissue.
• • Asserting that "Fig fruit is a non-vegetarian fruit" is biologically erroneous. Hence, Statement I is INCORRECT.

Scrutinizing the precise morphological stage of pollination and life cycle completion:

• • Pollination is strictly defined as the transfer of pollen grains from the anther to the receptive stigma of a flower.
• • In the genus *Ficus*, the pollinator wasp enters the specialized urn-shaped inflorescence (hypanthodium), which houses hundreds of tiny male and female flowers along its inner cavity wall.
• • The wasp pollinates the female flowers while searching for suitable egg-laying sites in the ovaries of gall flowers.
• • A fruit (*syconus*) is the post-fertilization developmental product. A fruit does not possess stigmas or ovules and CANNOT "get pollinated".
• • Therefore, claiming that the "fig fruit gets pollinated by fig wasp" is a glaring botanical contradiction. Hence, Statement II is INCORRECT.

Because both Statement I and Statement II contain fundamental factual and botanical errors, the correct verdict is that both statements are incorrect, matching Option (2).

In nature, resource availability is finite and limited. No population can sustain geometric/exponential growth indefinitely:

• • A habitat has finite resources to support a maximum possible number of individuals, beyond which no further growth is possible. This limit is termed the carrying capacity ($K$).
• • A population growing in a habitat with limited resources shows an initial lag phase, followed by phases of acceleration and deceleration, and finally reaches an asymptote as $N \rightarrow K$.
• • A plot of population density ($N$) in relation to time ($t$) yields a sigmoid ($S$-shaped) logistic curve, designated as Verhulst-Pearl Logistic Growth.

Deriving the differential equation from first principles:

• • Unconstrained intrinsic growth rate: $\frac{dN}{dt} = rN$.
• • Correction for carrying capacity (environmental resistance): As $N$ approaches $K$, the fraction of unutilized carrying capacity is $\frac{K - N}{K}$.
• • When $N \ll K$, $\frac{K - N}{K} \approx 1$, and growth is exponential ($rN$).
• • When $N = K$, $\frac{K - N}{K} = 0$, and growth stops ($\frac{dN}{dt} = 0$, reaching the asymptote).
• • Combining these yields: $\frac{dN}{dt} = rN \left( \frac{K - N}{K} \right)$.

The canonical equation for Verhulst-Pearl Logistic Growth is $\frac{dN}{dt} = rN \left( \frac{K - N}{K} \right)$, matching Option (2).

No population of any organism in nature has at its disposal unlimited resources to permit exponential growth. This leads to competition between individuals for limited resources. Eventually, the fittest individual survives and reproduces.

Verhulst-Pearl Logistic Growth is described by: $$\frac{dN}{dt} = rN\left(\frac{K - N}{K}\right)$$ where $N$ is population density at time $t$, $r$ is the intrinsic rate of natural increase, and $K$ represents carrying capacity—the upper ecological limit of population size supported by the habitat without degradation.

NCERT defines Gause’s 'Competitive Exclusion Principle' as follows: "two closely related species competing for the SAME resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually." Statement I says "competing for different resources", which completely subverts the definition. Thus, Statement I is false.

This competitive exclusion may be true if resources are limiting, but not otherwise. More recent studies do not support such gross generalisation about competition; they point out that species facing competition might evolve mechanisms that promote co-existence rather than exclusion (e.g., resource partitioning). However, as stated under Gause's core principle, under limiting resources the inferior is eliminated. Thus, Statement II is true.

Statement I is false but Statement II is true, matching Option (2).

Biotic interactions between two interacting species (A and B) are formalized via positive (+), negative (-), and neutral (0) fitness effects:

Examples include: Lichens/Mycorrhizae (Mutualism); Orchid on mango tree / Barnacles on whale (Commensalism); Penicillium inhibiting bacteria / Grazing cattle trampling grass (Amensalism); Cuscuta on host hedge (Parasitism).

A maps to IV (eliminates Option 1). B maps to I (eliminates Options 2 and 4). This uniquely leaves Option (3) as the only correct match.

Gause's principle describes resource competition under limiting conditions:

Plants and herbivores compete intensely for limited forage, sunlight, and soil nutrients. Field ecological studies summarized in NCERT demonstrate that herbivores experience greater competitive displacement than carnivores:

NCERT states verbatim: - Statement I: "Gause's 'Competitive Exclusion Principle' states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually." - Statement II: "In general, herbivores and plants appear to be more adversely affected by competition than carnivores."

Analyzing the ecological dynamics of each species pair:

NCERT states under Population Interactions: - "Competition: In South American lakes, visiting flamingos and resident fishes compete for their common food... Leopard and lion compete for same prey." - "Brood parasitism in birds is a fascinating example... cuckoo (koel) and the crow." - "Mycorrhizae are associations between fungi and the roots of higher plants (Mutualism)." - "The cattle egret and grazing cattle in close association... a classic example of commensalism."

A-I, B-II, C-III, D-IV corresponds directly to Option (2).

Analyzing the population growth curves described in NCERT:

Analyzing the age distribution profiles of human populations:

Matching A-II, B-I, C-III, D-IV perfectly corresponds to Option (2).

Amensalism is an interspecific ecological interaction wherein one species suffers negative effects (inhibition, injury, or death) while the second species remains completely unaffected:

Classic textbook manifestations include: - Antibiosis: *Penicillium notatum* secretes the antibiotic penicillin, destroying surrounding bacterial colonies while *Penicillium* itself experiences no nutritional or reproductive change ($-\,, 0$). - Allelopathy: Roots of Black Walnut (*Juglans nigra*) release juglone, inhibiting germination of nearby herbaceous seedlings without direct benefit to the tree.

Gause's Competitive Exclusion Principle dictates that two species competing for the exact same limiting resource cannot coexist at constant population values; the inferior competitor is ultimately eliminated.

To circumvent competitive exclusion, natural selection favors behavioral and morphological divergence:

MacArthur demonstrated that five closely related species of warblers living on the same spruce tree were able to coexist without competitive exclusion by foraging at different heights, exploring different zones of the foliage, and adopting different hunting rhythms.

When resources in the habitat are unlimited, the rate of increase of population density is given by the differential equation:

Integrating between limits $N_0$ and $N_t$ over time duration $t$ yields the exponential form:

The NCERT textbook explicitly lists the legend for this formula: - $N_t$ = Population density after time $t$ - $N_0$ = Population density at time zero - $r$ = Intrinsic rate of natural increase - $e$ = The base of natural logarithms ($2.71828$)

When an unacclimatized human ascends rapidly to high altitude (such as Rohtang Pass near Manali, altitude >3,500 m), they experience acute mountain sickness: - Pathophysiology: Total barometric atmospheric pressure ($P_{\text{atm}}$) decreases exponentially with altitude. Although the fractional concentration of oxygen remains ~21%, its partial pressure ($pO_2 = 0.21 \times P_{\text{atm}}$) drops drastically. - Symptom Manifestations: Inadequate alveolar $pO_2$ causes arterial hypoxia (tissue oxygen deprivation). The respiratory center in the brainstem stimulates rapid hyperventilation (breathing difficulty), while the sympathetic nervous system triggers tachycardia (heart palpitations) to increase cardiac output and compensate for diminished oxygen transport.

Gradually, the body acclimatizes to high altitude by: 1. Increasing red blood cell production (erythropoietin release from kidneys stimulating bone marrow erythropoiesis). 2. Increasing breathing rate. 3. Decreasing the binding affinity of haemoglobin for oxygen (shifting curve to right via elevated 2,3-BPG) to facilitate easier unloading of $O_2$ in systemic tissues.

- (a) Allen’s Rule: Formulated by Joel Asaph Allen: Mammals from colder climates generally possess shorter ears, limbs, and extremities to minimize surface area-to-volume ratio and reduce conductive heat loss. In polar seas, aquatic mammals like the polar seal exhibit shortened flippers and an insulating subcutaneous fat layer (blubber). - (b) Physiological Adaptation: The Kangaroo rat (*Dipodomys*) of North American deserts is capable of living without drinking liquid water: it fulfills 90% of its water requirements via the endogenous physiological oxidation of internal storage fat (producing metabolic water) and possesses elongated loops of Henle that concentrate its urine to hyperosmolar levels. - (c) Behavioural Adaptation: Desert lizards lack the internal physiological capacity for autonomic thermoregulation (homeothermy). Instead, they manage body temperature behaviourally: they bask in the morning sun to absorb solar radiation when body temperature drops below comfort zone, and scurry into subterranean burrows or shaded crevices when ambient temperatures climb. - (d) Biochemical Adaptation: Organisms living in extreme marine abyssal depths (where hydrostatic pressure exceeds 100 atmospheres) possess biochemical adaptations, including piezolyte pressure-protecting osmolytes (TMAO) and specialized membrane unsaturations that keep membranes fluid and enzymes functional under immense hydrostatic crush forces.

Sequence of matches: (a)-(iv), (b)-(i), (c)-(ii), (d)-(iii). This corresponds uniquely to Option (3).

In ecology, a population is defined as a group of individuals belonging to the same species that inhabit a defined geographic area, share or compete for similar resources, and potentially interbreed. While individual organisms are born, die, and have a male or female sex, a population exhibits aggregate demographic attributes: - Natality (Birth Rate): Number of births per unit of population per unit time. - Mortality (Death Rate): Number of deaths per unit of population per unit time. - Sex Ratio: Ratio of males to females within the population. - Age Distribution / Pyramids: Proportion of individuals in pre-reproductive, reproductive, and post-reproductive age groups.

Species interactions (such as mutualism, competition, predation, parasitism, commensalism, and amensalism) occur between populations of two or more distinct species coexisting within a habitat. Therefore, species interactions operate at the biotic community level, not as an intrinsic demographic attribute of a single-species population.

Secondary metabolites (alkaloids, tannins, terpenoids, resins, cardiac glycosides) are organic compounds not directly involved in the primary processes of plant growth, development, or reproduction.

Because sessile plants cannot flee from predators (herbivores and browsers), they have evolved morphological defenses (thorns, spines) and chemical defenses (toxic secondary metabolites) to deter grazing.

NCERT explicitly highlights that a wide variety of chemical substances that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium) are produced by them actually as defenses against grazers and browsers.

These toxic alkaloids have zero nutritive value for the plant, do not regulate growth (auxins/gibberellins do), and are not reproductive attractants.