Shankar IAS Test: Ecology for UPSC Environment - Free MCQs with solutions

Species that are the first to colonise barren or disturbed environments during ecological succession

Pioneer species are the initial organisms to establish on sites where living communities were absent or greatly reduced; they initiate processes that make the environment more habitable for later arrivals by contributing to soil formation, adding organic matter, aiding weathering of substrate, and sometimes fixing atmospheric nitrogen.

Typical pioneers in terrestrial primary succession include lichens and mosses on bare rock; in disturbed sites with existing soil (secondary succession) pioneers often include fast-growing herbs and colonising plants such as grasses, weeds and fireweed. In aquatic contexts, early colonisers can include phytoplankton and microbial mats; cyanobacteria also act as pioneers in some systems.

Primary succession occurs on newly exposed substrates without soil (for example after lava flows or glacial retreat) and is typically led by organisms like lichens and cyanobacteria, while secondary succession follows disturbances that leave soil intact and is often initiated by herbaceous plants and fast-growing shrubs.

The other option statements are incorrect: species dominating the final, stable stage are part of the climax community; species introduced and spreading in new areas by humans are called invasive (or exotic) species; and species that move seasonally between regions are migratory species.

a relationship in which one species is inhibited while the other is unaffected.

This describes amensalism, an interspecific interaction in which one species is inhibited or harmed while the other is essentially unaffected. It is an asymmetric, negative interaction for only one party.

Common examples include the black walnut (Juglans spp.) releasing the allelopathic compound juglone into the soil, which suppresses growth of nearby plants such as tomatoes and potatoes while the walnut itself is not noticeably harmed. Another example is microorganisms that secrete antibiotics that inhibit or kill sensitive microbial species without being affected themselves.

Amensalism differs from competition (where both species suffer), from parasitism (where one benefits and the other is harmed), and from commensalism (where one benefits and the other is unaffected).

The tendency of an organism to maintain internal stability despite external environmental changes. This describes homeostasis, the active regulation of relatively constant internal conditions such as temperature, pH, osmolarity and blood glucose, even when the external environment varies.

Homeostasis is achieved mainly by negative feedback mechanisms. Examples include sweating and vasodilation to lose heat, shivering and vasoconstriction to conserve or generate heat, hormonal control by insulin and glucagon to regulate blood glucose, and renal adjustments for osmoregulation.

Regulation can be physiological or behavioral: animals use internal mechanisms as well as actions (seeking shade, basking, migrating) to keep conditions stable. Plants also exhibit homeostatic responses (for example, stomatal regulation and osmotic adjustments). In aquatic animals, some are osmoconformers (matching external osmolarity) while others are osmoregulators (actively maintaining internal osmolarity). Migration is an adaptive strategy that helps organisms remain in favorable conditions but is not the same as the immediate internal regulatory processes called homeostasis.

Both 1 and 2

In predation, a predator captures and usually kills its prey and consumes it; the death of the prey is the typical outcome.

In parasitism, a parasite lives on or in a larger host and obtains resources at the host's expense, generally without causing immediate death (chronic harm is typical), although severe infestations or certain parasitoids can ultimately kill the host.

The characterization of "strong feeding on weak" for predation and "weak feeding on strong" for parasitism reflects the common ecological pattern that predators are often larger or more powerful than their prey, while parasites are usually smaller than their hosts; this is a useful generalization though it simplifies some exceptions.

Example: the Glochidium larva attaching to fish fins is a case of parasitism, whereas birds capturing and eating fish illustrate predation.

Thus both statements are correct as general biological distinctions, with the caveat that exceptions (e.g., parasitoids or lethal heavy parasitism) exist.

C: In ecological succession, species diversity typically decreases over time.

This statement is incorrect. During succession, communities pass through a sequence of seral stages starting with colonization by pioneer species and progressing toward a more complex, stable assemblage. As succession proceeds, habitat complexity and the number of available niches usually increase, allowing more species to establish and coexist; therefore overall species diversity generally rises until it approaches the climax community stage.

By contrast, primary succession occurs on newly formed or exposed substrates (e.g., bare rock, lava, sand), while secondary succession follows disturbance of an existing community (e.g., floods, fires, agriculture). The climax community is relatively stable and in equilibrium with the regional climate. In some specific cases diversity may level off or change composition near climax, but the typical trend through succession is an increase in species diversity, not a decrease.

Ecotone - a transitional zone between two adjacent plant communities or ecosystems that contains species from both sides as well as species uniquely adapted to the intermediate conditions; such zones often show increased local species richness.

Ecotones can be sharp or gradual and occur naturally (for example, the boundary between mangrove forests and open water, or a forest-grassland boundary) or be created/altered by human activity. They frequently exhibit the edge effect, where overlap of species and habitat heterogeneity lead to higher biodiversity than in the adjacent communities alone.

This concept differs from ecological succession (which describes temporal changes in community composition over time), from ecological niche (which denotes the functional role and requirements of a species), and from a climax community (the relatively stable final stage of succession).

Gause's Hypothesis

Gause's Hypothesis, also known as the Competitive Exclusion Principle, states that two species with completely overlapping niches cannot stably coexist in the same environment; one species will competitively exclude the other.

This principle was demonstrated by Gause's laboratory experiments with Paramecium aurelia and Paramecium caudatum: each species persisted when cultured alone, but in mixed cultures P. aurelia outcompeted and drove P. caudatum to local extinction.

The rule applies under conditions of constant, homogeneous environment and limited resources. If niches are not fully overlapping, coexistence can occur through mechanisms such as resource partitioning, spatial or temporal heterogeneity, predation, or evolutionary changes like character displacement.

Therefore, near-complete niche overlap typically leads to competitive exclusion, with one species persisting and the other disappearing locally.

It is an aquatic species which can survive and adapt in a wide range of salinities.

This describes Euryhaline organisms: animals capable of maintaining internal physiological balance while experiencing large changes in external salinity.

They achieve this through active osmoregulation - for example, adjusting ion transport across gills or epithelia, changing urine production and concentration, and using behavioral strategies (moving between habitats) to control water and ion fluxes.

By contrast, Stenohaline organisms tolerate only a narrow range of salinity and can suffer severe osmotic stress if transferred outside that range. Typical euryhaline examples include molly (Poecilia), green crab (Carcinus maenas), and many anadromous fishes such as salmon, which migrate between fresh and marine waters.

Both 1 and 2

Edge effect describes the ecological conditions in an ecotone (the boundary between two different ecosystems) where species diversity and biological activity are often higher than in either adjoining interior because the area supports species from both communities plus edge-adapted species.

However, edges also have distinct environmental conditions (changed microclimate, light, and resource availability) that can favor generalists and invaders while disadvantaging interior specialists; so higher overall diversity at edges does not imply uniformly positive effects for all species.

Habitat fragmentation increases the amount of boundary relative to interior by breaking continuous habitat into smaller patches, thereby raising the edge-to-interior ratio. This amplifies edge-related processes across the landscape (e.g., altered microclimate, increased predation and invasion, and greater exposure to human impacts), often magnifying the ecological consequences of edges.

Therefore, both statements are correct.

1, 2 and 3

Statement 1 is correct. A population is a group of individuals of the same species that occupy a defined area, interact (for example by mating, competing, or cooperating) and share a common gene pool.

Statement 2 is correct. A community is the assemblage of populations of different species living together in a particular area and interacting through relationships such as predation, competition, and mutualism; it represents the biotic component of an ecosystem.

Statement 3 is correct. A biome is a large geographic region characterized by a distinctive climate and dominant vegetation (for example tropical rainforest, desert, or grassland) and contains many similar ecosystems with comparable structure and life-forms.

Therefore, all three statements are correct and consistent with standard NCERT/CBSE ecological definitions.

1. Caraka-Samhita – Classification of animals based on habitat
   Correct. Ancient Ayurvedic texts, including the Caraka Samhita, classify animals on the basis of habitat such as land, water, and amphibious types.

2. Abiotic component – Plants
   Incorrect. Abiotic components are non-living factors like air, water, soil, light, and temperature. Plants are living and belong to the biotic component.

3. Major Community – Relatively independent, large-sized
   Correct. A major community is large, self-sustaining, and relatively independent.

4. Minor Community – Dependent on neighboring communities
   Correct. Minor communities are smaller and depend on nearby major communities.

Thus, three pairs are correctly matched.

Correct answer: c) Only three pairs

• Structure of a community: In a community, the number of species and size of their population vary greatly. A community may have one or several species. The environmental factors determine the characteristic of the community, as well as the pattern of organisation of the members in the community.

• The characteristic pattern of the community is termed as the structure which is reflected in the roles played by various population, their range, the type of area they inhabit, the diversity of species in the community and the spectrum of interactions between them.

1. Precipitation that falls near the equator is most likely to fall as rain into the ocean. This statement is partially true because while a significant amount of precipitation near the equator does fall into the ocean, a considerable amount also falls on land, particularly in tropical rainforest regions like the Amazon Basin and the Congo Basin. However, since the statement emphasizes the likelihood of rain falling into the ocean without negating the substantial rainfall on land, its accuracy can be contextually accepted given the vast expanse of oceans around the equatorial belt.

2. Water vapor is a greenhouse gas that helps to regulate the temperature of the earth's atmosphere. This statement is correct. Water vapor is indeed a significant greenhouse gas, contributing to the natural greenhouse effect that helps regulate the Earth's temperature. Without this effect, the planet would be much colder and less hospitable to life as we know it.

3. Globally, precipitation far exceeds evaporation. This statement is incorrect. The global water cycle is balanced over time, meaning that the amount of water that evaporates from the Earth's surface (including both land and oceans) approximately equals the amount of precipitation that falls back to Earth. If precipitation far exceeded evaporation globally and consistently, we would see a continuous increase in the volume of water in the Earth's surface reservoirs, which is not the case.

Therefore, the correct answer is: 1 and 2 only

1. Herbivores : Primary consumers

Correct. Herbivores feed mainly on plants and are primary consumers. Examples include cows and rabbits.

2. Wolves : Secondary consumers

Correct. Wolves feed on primary consumers and are considered secondary consumers.

3. Algae : Primary producers

Correct. Algae, like green plants, perform photosynthesis and are primary producers in aquatic ecosystems.

4. Earthworm : Macro consumers

Incorrect. Earthworms are not macro consumers; they are detritivores, which helps in the decomposition of organic matter. They are considered micro consumers as they decompose dead organic substances.

Thus, three pairs are correctly matched.

- Statement-I correctly defines abiotic components as non-living parts like soil, water, air, and light energy that impact ecosystems through physical processes.
- Statement-II correctly explains how biotic components (plants and animals) depend on abiotic factors such as soil nutrients and sunlight for growth and reproduction.
- Both statements are accurate, and Statement-II provides a clear illustration of how abiotic components support life, making it explanatory of Statement-I.

Thus, option A is correct: Both statements are correct, and Statement-II explains Statement-I.