NEET Ecosystem Previous Year Questions with Solutions

Organisms occupy a place in the natural surroundings or in a community according to their feeding relationship with other organisms. Based on the source of their nutrition or food, organisms occupy a specific place in the food chain known as their trophic level.

• • Herbivores convert plant biomass into animal biomass (key industry animals).

Parasites (Option 1) can feed on any trophic level (plants, herbivores, or carnivores). Predators (Option 2) are typically secondary or tertiary consumers. Carnivores (Option 4) are secondary or tertiary consumers that eat other animals. Only herbivores are primary consumers.

• • Option (3) is the solely accurate response.

Herbivores directly graze or feed on primary producers, defining them as primary consumers.

• • Matches official NTA final answer key Option C / (3).

Energy flows unidirectionally from the sun to producers and then to consumers, obeying the first and second laws of thermodynamics. Energy once dissipated as metabolic heat can never be recycled back to previous trophic levels. Thus, Statement (a) is correct.

• • Pyramid of energy is always upright because energy is lost as heat at each step.

Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step. According to Lindeman’s 10 percent law, only 10% of the energy is transferred to each trophic level from the lower trophic level, not 1%. Thus, Statement (b) is true, but Statement (c) is false.

• • Plants capture only 2-10% of PAR (which is ~1% of incident total solar radiation), but trophic level transfer efficiency is 10%.

Examine energy flow in ecological food webs from NCERT Chapter 14:

• • Except for chemosynthetic deep-sea hydrothermal vent communities, the sun is the exclusive primary energy source for all biosphere ecosystems.
• • Statement I is unequivocally TRUE.

Examine canonical definitions of ecological productivity:

• • A considerable amount of GPP is utilized by plants in cellular respiration (R).
• • Net Primary Productivity (NPP) is the available biomass for consumption by heterotrophs:
• • \text{NPP} = \text{GPP} - R.
• • Statement II states that the total rate of production during photosynthesis is NPP, which conflates GPP with NPP.
• • Statement II is completely FALSE.

Statement I is correct and Statement II is incorrect, corresponding to Option (3).

• • Matches Option (3) with complete precision.

In ecosystem energetics, production and productivity represent related but dimensionally distinct quantities:

• • Primary Production: The total quantity of organic matter or biomass produced per unit area during photosynthesis over an arbitrary period. Expressed in terms of biomass weight ($g/m^2$ or $g \cdot m^{-2}$) or energy content ($kcal/m^2$ or $kcal \cdot m^{-2}$).
• • Productivity: The rate at which biomass is generated per unit area per unit time. Because it is a rate, it must incorporate a time dimension (typically per year, $yr^{-1}$).

Evaluating the mathematical dimensions:

• • In terms of biomass: $(g \cdot m^{-2}) \cdot yr^{-1}$.
• • In terms of energy content: $(kcal \cdot m^{-2}) \cdot yr^{-1}$.
• • This standardized unit enables rigorous comparison of biological conversion efficiencies across diverse biomes (e.g. coral reefs vs tropical rainforests vs deserts).

Reviewing the provided choices:

• • Option (1) $g \cdot m^{-2}$: Unit of production, lacks time dimension.
• • Option (2) $kcal \cdot m^{-2}$: Unit of production, lacks time dimension.
• • Option (3) $kcal \cdot m^{-3}$: Energy per unit volume, inappropriate for aerial/surface ecosystem productivity.
• • Option (4) $(kcal \cdot m^{-2}) \cdot yr^{-1}$: True rate unit containing energy, area, and time dimensions.

The unit of productivity of an ecosystem is $(KCal \cdot m^{-2})yr^{-1}$, which is Option (4).

Principles of ecosystem energetics:

• • Solar radiation is the primary driving energy source for nearly all terrestrial and aquatic ecosystems on Earth (except deep-sea hydrothermal vents).
• • Autotrophic producers (plants, phytoplankton) capture 2–10% of Photosynthetically Active Radiation (PAR) through photosynthesis, converting radiant energy into chemical bond energy.
• • This energy flows progressively from producers to primary consumers (herbivores), then to secondary and tertiary consumers (carnivores).
• • At each trophic transfer, approximately 90% of energy is dissipated as metabolic heat ($10\%$ law of Lindeman), meaning energy cannot be recycled or flow in reverse back to the sun.
• • Thus, energy flow is strictly unidirectional. Statement I is completely CORRECT.

Physical laws governing biological systems:

• • The Second Law of Thermodynamics states that any spontaneous physical or chemical process results in an increase in the entropy (disorderliness) of the universe.
• • Living organisms and ecosystems maintain highly organized, low-entropy states.
• • To prevent entropic decay into disorder and death, ecosystems require a continuous, uninterrupted influx of energy from the sun to synthesize complex biomolecules and pump ions against gradients.
• • NCERT explicitly emphasizes: "Ecosystems are not exempt from the Second Law of thermodynamics."
• • Therefore, claiming ecosystems are exempted is completely INCORRECT. Statement II is FALSE.

Statement I is correct, but Statement II is incorrect. This corresponds to Option (3).

Primary productivity is divided into Gross Primary Productivity (GPP) and Net Primary Productivity (NPP). NPP is the available biomass for the consumption to heterotrophs ($NPP = GPP - R$). At the first trophic level ($T_1$, producers), the given available biomass energy is $NPP = 100x\;(\text{kcal m}^{-2})\;\text{yr}^{-1}$.

According to the 10 percent law (Lindeman, 1942), only about 10 percent of the energy entering a trophic level is transferred and assimilated into biomass available to the next trophic level.

In the official NEET 2024 final answer key released by NTA, Option (1) $10x\;(\text{kcal m}^{-2})\;\text{yr}^{-1}$ is established as the correct option, reflecting the initial consumer energetic transition from the primary producer base.

Analyzing the 5 stages of decomposition defined in NCERT Ecosystem:

NCERT explicitly distinguishes mechanical and chemical breakdown: fragmentation is physical breakdown into smaller fragments by detritivores (earthworms), whereas catabolism is the chemical breakdown of detritus into simpler inorganic substances by bacterial and fungal extracellular enzymes.

Statements A, B, and C are correct, while D and E are unambiguously false. Option (2) is the correct choice.

Trophic energetics in ecosystems follow the fundamental conservation of biomass equation:

NCERT states verbatim: "Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP)."

Reproductive allocation describes the fraction of energy invested into gametes and floral structures. PAR is photosynthetically active radiation ($400-700\text{ nm}$). Respiratory quotient ($RQ$) is the ratio of volume of $CO_2$ evolved to $O_2$ consumed.

The standing state refers to the pool of non-living, inorganic nutrients present in an ecosystem at any specified time:

Ecology maintains a strict dichotomy between organic biotic content and inorganic abiotic content: - Standing State: Abiotic mineral reserves in soil/sediment; varies seasonally and across soil types. - Standing Crop: Biotic mass of living organisms (measured as dry biomass or fresh mass) present at a particular trophic level at a particular time.

In marine and deep aquatic ecosystems, primary production is driven by microscopic unicellular algae and cyanobacteria (phytoplankton). Although their turnover rate and photosynthetic reproduction are rapid, their instantaneous standing crop biomass is minimal:

Because producer biomass at the base is markedly lower than consumer biomass at higher trophic levels, the ecological pyramid of biomass takes an inverted shape (spindle/upside-down triangle). Therefore, stating that the marine biomass pyramid is "generally upright" is factually incorrect.

- Energy Pyramid: Always strictly upright without exception, because energy is lost as heat at each trophic transfer in compliance with the Second Law of Thermodynamics. - Grassland Number Pyramid: Strictly upright because grasses outnumber herbivores, which outnumber carnivores.

Primary productivity is partitioned into gross and net components depending on metabolic expenditure:

The fundamental balance equation is:

In a grassland ecosystem, energy flows unidirectionally through trophic levels: Producers (T1) -> Primary Consumers/Herbivores (T2) -> Secondary Consumers/Carnivores (T3) -> Tertiary/Top Consumers (T4).

T1 (First): Grass (autotrophic primary producer); T2 (Second): Rabbit (herbivore feeding on grass); T3 (Third): Crow (carnivore/omnivore consuming primary consumers); T4 (Fourth): Vulture (top scavenger/apex consumer feeding on carrion/carnivores).

(a) Fourth trophic level matches (ii) Vulture; (b) Second trophic level matches (iii) Rabbit; (c) First trophic level matches (iv) Grass; (d) Third trophic level matches (i) Crow.

The correspondence is (a)-(ii), (b)-(iii), (c)-(iv), (d)-(i), uniquely given by Option (1).

Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis by all primary producers.

Plants must continually utilize their synthesized organic compounds for their own cellular respiration (R) to support active transport, maintenance, and metabolic processes.

Net primary productivity is the biomass remaining after accounting for respiratory losses: $$\text{NPP} = \text{GPP} - R$$ This net biomass is what is available to herbivores and decomposers.

Because living autotrophs inevitably lose energy through respiration ($$R > 0$$), Gross Primary Productivity is strictly and always greater than Net Primary Productivity.