Ecosystem–Structure and Function Questions in English

(D) In an ecosystem, particularly in a forest, the vertical distribution of different species occupying different levels is called $Stratification$.
Plants are arranged in layers based on their height and their requirement for sunlight.
For example, trees occupy the top vertical strata or layer, followed by shrubs in the second layer, and herbs and grasses in the bottom layers.
This vertical layering helps in the efficient utilization of resources like sunlight and space.

(D) The structure of an ecosystem is characterized by the organization of its biotic and abiotic components. Key structural features include species composition,species diversity,stratification (vertical distribution of different species),and dominance.
Decomposition is a functional aspect of an ecosystem,not a structural one. It represents the process of breaking down complex organic matter into simpler inorganic substances,which is a key part of energy flow and nutrient cycling.

(A) In an ecosystem,food chains,food webs,and nutrient cycles are fundamental processes that ensure the survival and stability of the system.
$1$. Energy Flow: Energy enters the ecosystem from the sun and moves through trophic levels via food chains and webs. This flow is essential for the metabolic activities of all organisms.
$2$. Nutrient Cycling: Nutrient cycles (such as the carbon,nitrogen,and phosphorus cycles) ensure that essential elements are recycled from the environment to organisms and back to the environment,preventing the depletion of resources.
$3$. Stability: These interactions maintain the balance of populations and ensure that energy and matter are continuously available,which is necessary for the ecosystem to function effectively.

(B) The first law of thermodynamics states that energy can neither be created nor destroyed,only transformed from one form to another. This law is fully applicable to ecosystems,as energy enters the ecosystem (usually as solar energy) and is converted into chemical energy by producers,then transferred through trophic levels. Therefore,the statement that the first law of thermodynamics is not applicable to ecosystems is incorrect.

(B) $1$. Stratification refers to the vertical distribution of different species occupying different levels in an ecosystem.
$2$. Zonation refers to the horizontal distribution of different species in an ecosystem.
$3$. Productivity is defined as the rate of generation of biomass in an ecosystem.
$4$. Decomposition is the process of breakdown of complex organic matter (detritus) into simpler inorganic substances.
Therefore,the correct matching is $1-d, 2-a, 3-b, 4-c$.

(A) Hydrarch succession occurs in water bodies and progresses through several seral stages. The sequence is as follows:
$1$. Phytoplankton stage
$2$. Rooted submerged plant stage
$3$. Free-floating plant stage
$4$. Reed swamp stage
$5$. Marsh-meadow stage
$6$. Scrub stage
$7$. Forest stage
Since the reed swamp stage follows the free-floating plant stage,the correct answer is $A$.

(B) In most ecosystems on Earth,the Sun is the primary source of energy,which is captured by producers (plants,algae,or cyanobacteria) through photosynthesis.
However,in deep sea hydrothermal vent ecosystems,sunlight cannot penetrate the water.
In these environments,organisms rely on chemosynthesis,where bacteria derive energy from chemical reactions involving inorganic substances like hydrogen sulfide $(H_2S)$ rather than sunlight.

(B) Photosynthetically Active Radiation $(PAR)$ refers to the spectral range of solar radiation from $400$ to $700$ $nm$ that photosynthetic organisms are able to use in the process of photosynthesis.
Plants are not able to capture all the available $PAR$ for photosynthesis.
It is estimated that plants capture only about $2-10\%$ of the $PAR$ that falls on them to perform photosynthesis and produce organic matter.

(C) In an aquatic ecosystem like a lake,the trophic levels are organized as follows:
$1$. The first trophic level consists of producers,which are phytoplankton,algae,and submerged plants.
$2$. The second trophic level consists of primary consumers,which feed on the producers. In a lake,these are zooplankton.
$3$. The third trophic level consists of secondary consumers,such as small fishes.
$4$. Therefore,the second trophic level is represented by zooplankton.

(B) The Detritus Food Chain $(DFC)$ begins with dead organic matter. It is made up of decomposers which are heterotrophic organisms,mainly fungi and bacteria. These organisms meet their energy and nutrient requirements by degrading dead organic matter or detritus. These are also known as saprotrophs ($SAPRO$ = to decompose).

(C) The incident solar radiation consists of various wavelengths of light.
Photosynthetically active radiation $(PAR)$ refers to the spectral range of solar radiation from $400 \ nm$ to $700 \ nm$ that photosynthetic organisms are able to use in the process of photosynthesis.
It is observed that $PAR$ constitutes less than $50$ % of the total incident solar radiation reaching the Earth's surface.
Therefore, the correct option is $C$.

(B) In an aquatic ecosystem,phytoplankton are the primary producers that synthesize food through photosynthesis. Zooplanktons feed on these phytoplankton. Since they consume the primary producers,zooplanktons are classified as primary consumers,occupying the second trophic level $(T_2)$.

(D) $2-10\%$ of photosynthetically active radiation $(PAR)$ is captured by plants.
$PAR$ refers to the light spectrum with wavelengths between $400-700 \ nm$, which is utilized by plants for the process of photosynthesis.
Out of the total incident solar radiation reaching the Earth, less than $50\%$ constitutes $PAR$.
Plants capture only a small fraction $(2-10\%)$ of this $PAR$ to synthesize organic compounds.