DAY – 15 : Insta 75 Days Revision Plan-2026 : ENVIRONMENT & ECOLOGY

Answer: (b)

Explanation

Statement I – Correct:
A population refers to a group of individuals belonging to the same species living in a defined geographic area, capable of interbreeding and sharing a common gene pool. Interaction among individuals (for mating, competition, etc.) is an essential feature.

Statement II – Incorrect:
A community consists only of biotic components, i.e., different populations of species interacting with each other in a given area.
The inclusion of abiotic components (such as soil, water, climate) makes it an ecosystem, not a community. Hence, the statement is conceptually incorrect.

Statement III – Correct:
The biosphere is the highest level of ecological organisation and represents the global sum of all ecosystems. It encompasses interactions among living organisms and the three major physical realms of Earth—lithosphere (land), hydrosphere (water), and atmosphere (air)—thereby sustaining life on a planetary scale.

Answer:(d)

Explanation:

Statement I – Correct:
An ecotype is a genetically distinct population within a species, evolved through natural selection to suit specific environmental conditions. These differences are heritable, not merely phenotypic.

Statement II – Correct:
A landscape is a higher-level ecological unit comprising a heterogeneous mosaic of ecosystems (patches) such as forests, grasslands, rivers, etc., often connected through ecological processes. It is used in landscape ecology to study spatial patterns and interactions.

Statement III – Correct:
A guild refers to a group of species (not necessarily closely related taxonomically) that exploit the same class of resources in a similar manner (e.g., nectar-feeding birds and insects). It is a functional classification.

Statement IV – Correct:
Ecads are phenotypic variants of a species produced due to environmental influences (e.g., light, water, nutrients), but they are genetically identical. These changes are non-heritable and reversible if conditions change.

Answer: (c)

Explanation:

Statement I: Correct
Tropical rainforests exhibit high Net Primary Productivity (NPP) due to favourable climatic conditions. However, marshes and estuaries can match or even exceed this productivity because of high nutrient availability and efficient recycling. Hence, the statement is factually balanced.

Statement II: Correct
Energy flow in an ecosystem is unidirectional and governed by the laws of thermodynamics. At each trophic transfer, a large proportion of energy (commonly approximated as ~90%) is lost as heat, leaving only a small fraction available to the next level. This explains energy transfer efficiency, not differences in productivity across ecosystems.

Statement III: Incorrect
Although decomposition is generally oxygen-consuming (aerobic), its rate is slower in detritus rich in lignin and chitin. These are complex, resistant compounds that hinder microbial breakdown. Hence, the statement incorrectly states that decomposition is faster.

Answer: (b)

Explanation

The concepts of fundamental niche and realized niche are based on the idea that a niche represents the range of environmental conditions and resources within which a species can survive, grow, and reproduce. This includes factors such as temperature, moisture, food availability, and habitat conditions.

• The fundamental niche refers to the full potential range of conditions and resources that a species can occupy in the absence of biotic interactions like competition, predation, or parasitism. It is primarily determined by the species’ physiological tolerance and resource requirements.
• The realized niche, on the other hand, is the actual range occupied by the species in nature. It is usually narrower because biotic factors such as competition, predation, and other species interactions restrict the species from utilizing its entire potential range.
• Thus, the realized niche is generally a subset of the fundamental niche, reflecting ecological constraints operating in natural conditions.

Answer: (b)

Explanation:

Statement I – Correct:
Secondary succession is generally faster than primary succession because it begins on a site where soil, nutrients, and often seed banks or surviving organisms are already present. This allows rapid re-establishment of vegetation.

Statement II – Correct and it explains Statement I
In secondary succession, the presence of pre-existing soil eliminates the need for initial soil formation by pioneer species (such as lichens and mosses), which is a time-consuming process in primary succession.

Statement III – Correct but does not explain Statement I:
As succession progresses toward a climax community:

• Net Community Productivity (NCP) declines because respiration approaches gross primary productivity (GPP ≈ R).
• Total biomass and structural complexity increase due to species diversification and ecosystem maturation.
  However, this describes ecosystem dynamics during succession, not the reason for faster secondary succession.

Answer: (b)

Explanation

Statement I is incorrect: Epiphytic growth and a multi-layered canopy are hallmarks of Tropical Rainforests, where high rainfall and humidity support dense, stratified vegetation. The Mediterranean biome, by contrast, is characterised by open woodlands and shrub-dominated landscapes due to seasonal drought, and does not support such complex vertical stratification.

Statement II is correct: Sclerophyllous vegetation with thick, leathery leaves is the defining adaptation of Mediterranean ecosystems. These leaves reduce water loss and withstand prolonged dry summers, making them well-suited to the region’s hot, arid seasonal conditions.

Statement III is incorrect: Permafrost subsoil and the dominance of lichens and mosses are typical of the Tundra biome, which exists in extremely cold regions with permanently frozen ground. The Mediterranean biome has a temperate climate with no permafrost conditions.

Statement IV is correct: Pyrophytic plants adapted to frequent seasonal fires are a key feature of Mediterranean ecosystems. Dry summers and accumulated biomass lead to recurrent fires, and vegetation has evolved traits such as fire-resistant bark, resprouting ability, and fire-triggered seed germination.

Answer: (b)

Explanation:

Statement I: Incorrect
Eutrophication leads to rapid nutrient enrichment, causing dense algal blooms in surface waters where sunlight is available. While photosynthesis may temporarily increase dissolved oxygen (DO) near the surface, bottom layers in stagnant water bodies experience oxygen depletion, not enrichment. This is because:

• Limited vertical mixing prevents oxygen transfer to deeper layers
• Sinking organic matter from algal blooms undergoes decomposition, consuming oxygen
  Thus, DO in bottom layers declines, often leading to hypoxic conditions.

Statement II: Correct
Following the collapse of algal blooms, large quantities of organic matter are decomposed by aerobic microorganisms. This process significantly increases oxygen consumption, leading to:

• Sharp decline in DO levels
• Formation of hypoxic or anoxic “dead zones”
  These conditions are detrimental to aquatic life, especially fish and benthic organisms.

Statement III: Incorrect
Biological Oxygen Demand (BOD) is a measure of the amount of oxygen required by microorganisms to decompose organic matter in water.

• High BOD → High organic pollution → Greater oxygen depletion
• Low BOD → Cleaner water with less organic load
  Thus, the statement incorrectly interprets BOD as indicating low pollution.

Answer: (a)

Explanation

Statement I is correct: Keystone species exert an influence on ecosystem structure that is not proportional to their numerical abundance. Their ecological role is critical in regulating species interactions, maintaining biodiversity, and ensuring stability within the community.

Statement II is correct: The removal of such species leads to cascading changes across multiple trophic levels, resulting in alterations in species composition, population dynamics, and ecosystem functioning.

The disproportionate influence is evidenced by the cascading effects. The ability of keystone species to regulate trophic interactions means that their removal disrupts ecological balance, thereby demonstrating their outsized impact relative to their abundance.

Answer: (b)

Explanation:

Statement 1 is incorrect. The Asian Giant Tortoise (Manouria emys) is a terrestrial tortoise inhabiting tropical and subtropical forests. It is not an aquatic or freshwater species. The classification between turtles and tortoises is ecological and morphological, and confusing the two is a common conceptual trap.

Statement 2 is correct. The species is herbivorous and contributes significantly to ecosystem functioning by dispersing seeds, breaking down leaf litter, and aiding nutrient recycling. Its movement and feeding behaviour help maintain forest floor dynamics.

Statement 3 is incorrect. The Asian Giant Tortoise is listed as Critically Endangered on the IUCN Red List, not Near Threatened. This reflects severe population decline due to habitat loss, hunting, and fragmentation across its range.

Answer: (a)

Explanation:

Statement 1 is correct. Dibru–Saikhowa National Park is located in eastern Assam and lies between the Brahmaputra and Dibru rivers, forming a riverine island ecosystem.

Statement 2 is incorrect. Dibru–Saikhowa is designated as a Biosphere Reserve under India’s national framework, but it is not included in UNESCO’s World Network of Biosphere Reserves under the Man and Biosphere (MAB) Programme.

Statement 3 is incorrect. The horses in the park are feral populations derived from domesticated animals abandoned during historical events, not a naturally evolved wild equid population. Hence, describing them as “naturally evolved wild population” is inaccurate.

Answer: (a)

Solution:
Let original price = x

After 20% increase → x × 1.2
After 20% decrease → 1.2x × 0.8 = 0.96x

So, 0.96x = 960
x = 960 / 0.96 = 1000

Answer: (b)

Solution:
Let income = 100
Rent = 40 → Savings = 60

New income = 125
New rent = 40 × 1.25 = 50

New savings = 125 − 50 = 75

Increase = 75 − 60 = 15
Percentage increase = (15/60) × 100 = 25%

Answer: (b)

Solution:
A = 3 × 12 = 36
B = 2 × 12 = 24
C = 2 × 8 = 16

Ratio = 36 : 24 : 16 = 9 : 6 : 4

Total parts = 19

C’s share = (4/19) × 7600 = 1600

Answer: (c)

Solution:
A = 10000 × 12 = 120000

B:
First 4 months → 15000 × 4 = 60000
Remaining capital = 10000 for 8 months → 80000

Total B = 140000

Ratio = 120000 : 140000 = 6 : 7

Answer: (c)

Solution:
The passage presents a balanced view of data-driven policymaking. It acknowledges that quantitative tools improve efficiency and precision but cautions against overdependence, as they may oversimplify complex human realities and ignore ethical dimensions. The author emphasizes that policymaking should not rely solely on measurable data but must also incorporate normative judgment and lived experiences. Thus, extreme options rejecting data or dismissing ethics are incorrect. The most logical inference is that effective policymaking requires a balance between empirical evidence and ethical considerations. Therefore, option (c) best captures the central idea and intended conclusion of the passage.