Shankar IAS Test: Functions Of Ecosystem - UPSC MCQ

Let's analyze each statement regarding the pyramid of numbers - upright:

1. The number of individuals decreases from lower level to higher trophic level.

   • This statement is correct. In an upright pyramid of numbers, the number of individuals typically decreases as you move from the base (producers) to the top (apex predators). This is because energy is lost at each trophic level, supporting fewer individuals at higher levels.

2. Grasses occupy the lowest trophic level because of their biomass.

   • This statement is incorrect. Grasses occupy the lowest trophic level because they are producers, not specifically because of their biomass. The trophic level is determined by the organism's role in the ecosystem (producers, primary consumers, etc.), not by biomass alone.

Given the analysis:

• Statement 1 is correct.
• Statement 2 is incorrect.

Therefore, the correct answer is:

2. 1 Onl

Both statements are correct.

1. In a pyramid of biomass, the weight (biomass) of all individuals in each trophic level is considered instead of counting the number of individuals. This gives a more accurate representation of the energy available in each trophic level, as larger organisms typically have more energy stored in their tissues.

2. The pyramid of biomass is structured with producers at the bottom, representing the largest amount of biomass in the ecosystem. Producers (e.g., plants) convert sunlight into energy through photosynthesis and form the base of the food chain. As energy transfers up the trophic levels through consumption, energy is lost in the form of waste and heat, resulting in a decrease in biomass as you move up the pyramid. This is why the biomass of producers is at the bottom of the pyramid, as they represent the largest amount of available energy in the ecosystem.

The flow of energy from producers to top consumers in an ecosystem is termed as "Energy flow." This unidirectional process involves the transfer of energy through trophic levels, with some energy being lost as unusable heat at each level. This concept is crucial in understanding how energy moves through an ecosystem to sustain life.

Let's analyze each statement:

Statement-I: "In a grazing food chain, the consumers that initiate the food chain by feeding on plants form the base, with subsequent trophic levels following."

• This statement is generally correct. In a grazing food chain, primary producers (plants) are consumed by herbivores (consumers), which are then consumed by secondary consumers, and so on. The primary producers indeed form the base of the food chain.

Statement-II: "Detritus food chains start with decaying organic matter, progressing through microorganisms, detritivores, and then other predators."

• This statement is also correct. Detritus food chains, also known as decomposer food chains, start with dead organic matter (detritus) being decomposed by microorganisms (like bacteria and fungi). Detritivores (organisms that feed on detritus) then consume these decomposed materials, and the energy flow continues through higher trophic levels.

From our analysis:

• Both Statement-I and Statement-II are correct based on their descriptions of grazing and detritus food chains, respectively.
• Statement-II (detritus food chain) does not directly explain Statement-I (grazing food chain) as they describe different types of food chains.

Therefore, the correct answer is: Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I.

Competition in an ecosystem involves both species being harmed by the interaction. When two species compete for the same limited resources, such as food or territory, both may suffer a shortage of resources compared to if they were alone. This competitive interaction can have negative effects on the populations of both species involved, highlighting the struggle for survival in ecological communities.

Statement 1 is correct. In most terrestrial ecosystems, the pyramid of biomass has a large base of primary producers with smaller trophic levels on top, reflecting the greater biomass of plants compared to herbivores and carnivores.

Statement 2 is correct. In many aquatic ecosystems, the pyramid of biomass can be inverted, meaning that the biomass of primary producers (phytoplankton) is less than that of the higher trophic levels (zooplankton and fish). This inversion occurs because the primary producers reproduce and are consumed rapidly, maintaining a lower biomass.

Statement 3 is incorrect. The energy pyramid always remains upright because energy flow in an ecosystem follows the laws of thermodynamics, with energy decreasing at each successive trophic level due to inefficiencies in energy transfer.

Therefore, the correct answer is Option B: 1 and 2 Only.

Statement-I correctly describes the typical structure of a pyramid of biomass in most land ecosystems, where there is a large base of primary producers supporting higher trophic levels. Statement-II accurately notes that an inverted pyramid of biomass is often seen in many aquatic ecosystems, where the biomass decreases as you move up the trophic levels, contrary to the typical terrestrial pyramid. Therefore, both statements are correct, and Statement-II explains the concept illustrated in Statement-I, making option (a) the correct answer.

1. Gaseous Cycle - Carbon Cycle: Correct - The Carbon Cycle is a gaseous cycle where carbon dioxide circulates between the atmosphere and living organisms.

2. Sedimentary Cycle - Nitrogen Cycle: Incorrect - The Nitrogen Cycle is primarily a gaseous cycle, involving the atmosphere as a major reservoir.

3. Gaseous Cycle - Water Cycle (Hydrologic Cycle): Correct - The Water Cycle is a gaseous cycle driven by solar energy, involving processes like evaporation and precipitation.

4. Sedimentary Cycle - Phosphorus Cycle: Correct - The Phosphorus Cycle is a sedimentary cycle, as phosphorus is primarily stored in the earth's crust.

Thus, pairs 1, 3, and 4 are correctly matched, while pair 2 is incorrectly matched. Hence, only two pairs are correctly matched.

The correct option is: Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I.

Statement-I correctly describes the carbon cycle, and Statement-II correctly describes the nitrogen cycle. However, Statement-II does not explain Statement-I as they pertain to different biogeochemical cycles.

1. Phosphorus Cycle : Weathering of rocks
This pair is correctly matched. Phosphorus enters the cycle through the weathering of rocks which releases phosphates into rivers and streams.

2. Sulphur Cycle : Release from organic matter decomposition
This pair is correctly matched. Sulphur is released into the ecosystem through the decomposition of organic matter among other processes.

3. Primary Succession : Occurs on a well-developed soil
This pair is incorrectly matched. Primary succession occurs on newly formed, barren substrates where no soil exists initially, such as a lava flow or a region left bare by a retreating glacier.

4. Secondary Succession : Occurs on newly formed substrate
This pair is incorrectly matched. Secondary succession occurs on pre-existing soil where a previous community has been disturbed or destroyed, such as abandoned farmland.

Correct pairs: 1 and 2