Ecosystem–Structure and Function Questions in English

(A) Climax communities represent the final,stable stage of ecological succession. They are characterized by high species diversity,complex food webs,and greater stability compared to pioneer communities,which are the initial,simple,and less stable stages of succession.

(A) In the process of primary succession in water (hydrosere),the sequence of stages is as follows: Phytoplankton stage $\rightarrow$ Submerged plant stage $\rightarrow$ Submerged free-floating plant stage $\rightarrow$ Reed-swamp stage $\rightarrow$ Marsh-meadow stage $\rightarrow$ Scrub stage $\rightarrow$ Forest stage. Therefore,the reed-swamp stage is followed by the marsh-meadow stage.

(B) Statement $(a)$ is incorrect because the atmosphere contains only about $1\;\%$ of the total global carbon.
Statement $(b)$ is correct; decomposers play a vital role in releasing $CO_2$ back into the atmosphere by decomposing organic matter.
Statement $(c)$ is correct; approximately $4 \times 10^{13}\; kg$ of carbon is fixed annually through photosynthesis.
Statement $(d)$ is incorrect; respiration by producers and consumers is a major source of $CO_2$ input into the atmosphere.
Statement $(e)$ is correct; carbon constitutes $49\;\%$ of the dry weight of organisms.
Therefore,statements $(b), (c),$ and $(e)$ are correct. Thus,the total number of correct statements is $3$.

(B) The phosphorus cycle is a sedimentary cycle,meaning it primarily involves the movement of phosphorus through the lithosphere,hydrosphere,and biosphere.
Unlike the carbon,oxygen,and nitrogen cycles,the phosphorus cycle does not have a significant gaseous phase in the atmosphere.
Therefore,the atmospheric input of phosphorus is negligible.

(A) An anthropogenic ecosystem is an ecosystem that is created and maintained by human activities.
Agriculture and aquaculture are prime examples of man-maintained (anthropogenic) ecosystems.
In these ecosystems,humans actively manage both biotic factors (e.g.,selection of crops,pest control,stocking of fish) and abiotic factors (e.g.,irrigation,soil fertilization,water quality management).
Since the human intervention is the primary reason for the maintenance and functioning of these ecosystems,the Reason correctly explains the Assertion.

(C) Stratification refers to the vertical distribution of different species occupying different levels in an ecosystem.
Assertion is correct because vertical layering allows plants to utilize different light intensities and resources,thus accommodating more species in the same area.
Reason is incorrect because stratification is the vertical zonation,not horizontal zonation.

(B) Eutrophic lakes or water bodies are characterized by high nutrient levels,particularly nitrogen and phosphorus. This nutrient enrichment promotes the rapid and dense growth of plankton and littoral vegetation. As these organisms die and decompose,microorganisms consume the dissolved oxygen,which can lead to the depletion of oxygen for other aquatic life.

(B) In deep ponds and lakes,the oxygen concentration at the bottom is very low primarily due to the limited penetration of sunlight. Sunlight is essential for photosynthesis by phytoplankton and aquatic plants,which produce oxygen. As sunlight cannot reach the deeper layers,photosynthesis is significantly reduced or absent,leading to lower oxygen levels at the floor of the water body.

(D) Grazing animals can be harmful as overgrazing leads to the destruction of vegetation and may cause desertification.
However,the possible beneficial aspect of grazing animals is the addition of their excreta (dung) into the soil.
This process acts as a natural fertilizer,which increases soil fertility by adding organic matter and essential nutrients back into the ecosystem.

(A) The amount of inorganic nutrients (such as carbon, nitrogen, phosphorus, and calcium) present in the soil or in the water phase at any given time is known as the $Standing \text{ } state$.
In contrast, the amount of living biomass (organic matter) present in an ecosystem at a given time is referred to as the $Standing \text{ } crop$.

(N/A) keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions. Without keystone species,the ecosystem would be dramatically different or cease to exist altogether. These species often have a disproportionately large effect on their environment relative to their abundance.

(B) The phosphorus cycle is a sedimentary cycle,meaning its primary reservoir is the Earth's crust.
Weathering of rocks is the natural process that releases phosphorus from phosphate minerals into the soil and water,making it available for uptake by plants.
This process is the primary driver that introduces phosphorus into the biological cycle,thereby accelerating the movement of phosphorus through the ecosystem.
Volcanic activity,rainfall,and burning of fossil fuels have a negligible or indirect effect on the phosphorus cycle compared to the weathering of rocks.

(B) Mosses,along with lichens,are pioneer species in xerarch succession (succession on bare rocks).
They help in the process of weathering of rocks and soil formation,which makes the habitat suitable for higher plants to grow.

(A) Photosynthetic organisms,such as plants,algae,and certain bacteria,are known as producers or autotrophs.
They synthesize their own food using solar energy through the process of photosynthesis.
In an ecosystem,producers occupy the base of the food chain and are classified as the $First$ trophic level $(T_1)$.

(A) The organisms that live at the bottom of a water body are collectively referred to as $Benthic$ organisms.
- $Benthic$ zone refers to the lowest ecological region of a body of water,including the sediment surface and some sub-surface layers.
- $Limnetic$ zone refers to the open water area away from the shore.
- $Littoral$ zone is the near-shore area where sunlight penetrates all the way to the sediment.
- $Euphotic$ zone is the upper layer of water where sunlight is sufficient for photosynthesis.

(D) An ecosystem is defined as the functional unit of nature where living organisms interact among themselves and with the surrounding physical environment.
It encompasses both biotic (living) and abiotic (non-living) components,which function as a single unit through nutrient cycling and energy flow.
Therefore,the ecosystem is considered the basic functional unit of nature.

(A) Ecosystems are broadly classified into two main categories based on their habitat:
$1$. Terrestrial Ecosystems: These are land-based ecosystems. Examples include $I-$ Forest,$II-$ Grassland,and $IV-$ Desert.
$2$. Aquatic Ecosystems: These are water-based ecosystems. Examples include $III-$ Pond,$V-$ Lake,$VI-$ Wetland,$VII-$ River,and $VIII-$ Estuary.
Therefore,the Terrestrial group consists of $I, II, IV$ and the Aquatic group consists of $III, V, VI, VII, VIII$.

(B) An artificial ecosystem is a man-made system where living organisms interact with their environment.
$1$. Forest: Natural ecosystem.
$2$. Estuary: Natural ecosystem.
$3$. Paddy field: Artificial (man-made) ecosystem.
$4$. Desert: Natural ecosystem.
$5$. Aquarium: Artificial (man-made) ecosystem.
Therefore,there are $2$ artificial ecosystems in the given list: Paddy field and Aquarium.

(D) The vertical distribution of different species occupying different levels in an ecosystem is known as $Stratification$.
For example, trees occupy the top vertical strata or layer, shrubs occupy the second, and herbs and grasses occupy the bottom layers.

(D) An ecosystem is a functional unit of nature where living organisms interact among themselves and with the surrounding physical environment.
The four basic functional aspects or components of an ecosystem are:
$1$. Productivity: The rate of biomass production.
$2$. Decomposition: The breakdown of complex organic matter into inorganic substances.
$3$. Energy flow: The unidirectional movement of energy through trophic levels.
$4$. Nutrient cycling: The movement of nutrient elements through the various components of an ecosystem.
Since all four of these components are essential for the functioning of any ecosystem,including a pond ecosystem,the correct answer is $4$.

(D) The function of an ecosystem is a dynamic process that involves the flow of energy and the cycling of nutrients.
$1$. The input of solar energy is the primary driver for all biological processes,including photosynthesis,which fuels the food chain.
$2$. Temperature cycles and day length (photoperiod) significantly influence the metabolic rates of organisms and the timing of biological events.
$3$. Climatic conditions (such as rainfall,humidity,and wind) determine the types of organisms that can survive and the overall productivity of the ecosystem.
Therefore,all these factors collectively regulate the rate of ecosystem functioning.

(A) In most ecosystems on Earth,the sun is the ultimate source of energy for all organisms. However,in deep sea hydrothermal vent ecosystems,sunlight does not reach the ocean floor. In these environments,organisms like chemosynthetic bacteria derive energy from the oxidation of inorganic chemicals (such as hydrogen sulfide) released from hydrothermal vents. Therefore,the sun is not the primary source of energy for these ecosystems.

(B) Photosynthetically Active Radiation $(PAR)$ is 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 has been observed that $PAR$ constitutes less than $50 \%$ of the total incident solar radiation reaching the Earth's surface.
Therefore,the correct option is $B$.

(D) The food chain that starts from dead organic matter is known as the detritus food chain $(DFC)$.
It begins with dead organic matter,which is decomposed by decomposers such as fungi and bacteria.
Unlike the grazing food chain,which starts with producers (green plants),the detritus food chain relies on the energy stored in dead organic remains.
Therefore,the correct answer is $D$.

(B) In an ecosystem,trophic levels are defined based on the position of an organism in a food chain:
$1$. The first trophic level $(T_1)$ consists of producers (autotrophs) like phytoplankton,grass,and trees $(P-III)$.
$2$. The second trophic level $(T_2)$ consists of primary consumers (herbivores) like zooplankton,grasshoppers,and cows $(Q-II)$.
$3$. The third trophic level $(T_3)$ consists of secondary consumers (carnivores) like birds,fishes,and wolves $(R-IV)$.
$4$. The fourth trophic level $(T_4)$ consists of tertiary consumers (top carnivores) like humans and lions $(S-I)$.
Therefore,the correct matching is $(P-III), (Q-II), (R-IV), (S-I)$.

(A) Each trophic level has a certain mass of living material at a particular time,which is known as the $Standing \ crop$.
This $Standing \ crop$ is measured as the mass of living organisms $(biomass)$ or the number in a unit area.
The biomass of a species is expressed in terms of fresh or dry weight.
Measurement in terms of dry weight is more accurate because it excludes the water content of the organism.

(C) In an ecosystem,trophic levels are organized as follows:
$T_1$: Producers (Plants)
$T_2$: Primary consumers (Herbivores)
$T_3$: Secondary consumers (Primary carnivores)
$T_4$: Tertiary consumers (Secondary carnivores)
Secondary carnivores occupy the $T_4$ trophic level and feed on organisms from the $T_3$ trophic level (primary carnivores).

(B) Primary succession is the process of ecological succession that occurs in an area where no community previously existed,such as on bare rocks.
Lichens are considered the pioneer species in this process.
They secrete acids to dissolve rock,helping in the process of weathering and soil formation,which eventually allows other plants to colonize the area.

(A) Primary succession occurs in areas where no living organisms ever existed,such as newly cooled lava $(III)$,bare rock $(IV)$,and newly created ponds or reservoirs $(VI)$.
Secondary succession occurs in areas where life previously existed but was lost due to disturbances like fire,cutting,or flooding,such as abandoned farmland $(I)$,burned or cut forests $(II)$,and flooded lands $(V)$.

(A) Ecological succession is the process by which the structure of a biological community evolves over time.
It involves a gradual and predictable change in the species composition of a given area,leading to the establishment of a stable climax community.
This process occurs in response to environmental changes or colonization of new habitats.

(A) In ecological succession,the entire sequence of communities that successively change in a given area is called a $Sere$.
Each individual transitional community in this sequence is known as a $Seral$ $stage$ or $Seral$ $community$.
Therefore,$P$ is $Sere$ and $Q$ is $Seral$ $stages$.

(B) Ecological succession is the gradual and predictable change in the species composition of a given area. During this process,the community becomes more complex. As succession progresses,the number of species (diversity),the total number of organisms,and the total biomass of the ecosystem generally increase until a stable climax community is reached.

(A) Ecological succession is the process by which the structure of a biological community evolves over time.
In hydrarch succession,which takes place in wet areas,the progression moves from hydric (aquatic) to mesic (moderate) conditions.
In xerarch succession,which takes place in dry areas,the progression moves from xeric (dry) to mesic (moderate) conditions.
Therefore,both types of successions eventually lead to a climax community that is in equilibrium with the environment,characterized by mesic or medium water conditions.

(B) In ecological succession, the final community that is in near equilibrium with the environment of that area is known as the $Climax$ $community$.
This community remains relatively stable as long as the climate and other environmental conditions remain unchanged.

(D) In an ecosystem,the amount of inorganic nutrients such as carbon,nitrogen,phosphorus,and calcium present in the soil or water at any given time is known as the $Standing \ state$.
This represents the nutrient status of the ecosystem at a specific point in time.
In contrast,the $Standing \ crop$ refers to the total biomass of living organisms in an ecosystem at a particular time.

(B) Biogeochemical cycles are classified into two types based on the reservoir of the nutrient:
$1$. Gaseous cycles: The reservoir is the atmosphere or hydrosphere. Examples include the Carbon cycle $(III)$ and Nitrogen cycle $(II)$.
$2$. Sedimentary cycles: The reservoir is the Earth's crust (rocks/soil). Examples include the Sulfur cycle $(I)$ and Phosphorus cycle $(IV)$.
Therefore,the gaseous cycles are $(II, III)$ and the sedimentary cycles are $(I, IV)$.

(B) The total quantity of global carbon is distributed across various reservoirs. According to the $NCERT$ biology textbook,approximately $71 \%$ of the total global carbon is found dissolved in the oceans. This reservoir regulates the amount of atmospheric $CO_2$.

(D) Carbon constitutes approximately $49 \%$ of the dry weight of organisms and is the second most abundant element in living organisms after oxygen. This is a fundamental fact regarding the elemental composition of biomass in ecosystems.

(A) According to the carbon cycle data provided in the $NCERT$ textbook,the total global carbon is distributed across various reservoirs.
Approximately $71 \%$ of the total global carbon is found dissolved in the oceans.
Only about $1 \%$ of the total global carbon is present in the atmosphere.
Therefore,the correct option is $A$.

(C) The natural reservoir of phosphorus is rock,which contains phosphorus in the form of phosphates. When rocks are weathered,minute amounts of these phosphates dissolve in soil solution and are absorbed by the roots of the plants. Unlike the carbon cycle,there is no significant respiratory release of phosphorus into the atmosphere. Therefore,the correct answer is $C$.

(C) The phosphorus cycle is a sedimentary cycle.
Respiration is a process associated with gaseous cycles,such as the carbon cycle,where $CO_2$ is released into the atmosphere.
Phosphorus is not released into the atmosphere in a gaseous form; therefore,the statement that organisms can release phosphorus into the atmosphere through respiration is incorrect.
All other statements are correct regarding the phosphorus cycle.

(C) Ecosystem services are defined as the products of ecosystem processes. These processes include the purification of air and water,mitigation of droughts and floods,cycling of nutrients,generation of fertile soils,provision of wildlife habitat,maintenance of biodiversity,pollination of crops,and provision of storage site for carbon. These services are essential for human survival and well-being.

(D) Robert Costanza and his colleagues have put very price tags on nature's life-support services. Researchers have put an average price tag of $33$ trillion $US$ dollars a year on these fundamental ecosystem services,which are largely taken for granted. This is nearly twice the value of the global Gross National Product $(GNP)$ which is $18$ trillion $US$ dollars.

(C) According to Robert Costanza and his colleagues, the global ecosystem services are valued at an average of $US \$ 33$ trillion a year.
Out of this total, soil formation accounts for about $50\%$ of the total cost.
Contributions of other services like recreation and nutrient cycling are less than $10\%$ each.
The value of climate regulation and habitat for wildlife is about $6\%$ each.
Therefore, $P = 50\%, Q = 10\%,$ and $R = 6\%$.
Thus, the correct option is $C$.

(B) $Keystone$ species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions.
Without $Keystone$ species,the ecosystem would be dramatically different or cease to exist altogether.
Even though their population density or biomass might be relatively low,their impact on the ecosystem is disproportionately large compared to their abundance.
Therefore,option $B$ is the correct definition.

(C) Hydrarch succession takes place in wetter areas and the successional series progress from hydric to the mesic conditions.
In hydrarch succession,the pioneer species are the small phytoplanktons (such as blue-green algae,green algae,and diatoms).
These organisms are the first to colonize the bare water body,eventually leading to the formation of more complex communities.

(A) The process of ecological succession begins in a bare area where no life previously existed.
$1$. The first group of organisms that colonize such a barren area are known as pioneer species.
$2$. Examples include lichens on bare rocks or phytoplankton in a new pond.
$3$. These species modify the environment,making it suitable for subsequent species to establish.
$4$. Therefore,the correct answer is Pioneer species.

(B) Ecological succession is the gradual and predictable change in the species composition of a given area.
Regardless of whether the succession begins in a dry area (xerarch) or a wet area (hydrarch),it eventually leads to a stable,mature community known as the climax community.
In both cases,the climax community is characterized by moderate moisture conditions,which is referred to as a $Mesic$ environment.
Therefore,all successions proceed to a $Mesic$ climax community.

(B) The term $\text{PAR}$ stands for $\text{Photosynthetically Active Radiation}$.
It represents the spectral range of solar radiation from $400 \text{ nm}$ to $700 \text{ nm}$ that photosynthetic organisms are able to use in the process of photosynthesis.
Out of the total incident solar radiation reaching the Earth's surface,less than $50 \%$ is $\text{PAR}$.
Therefore,the correct option is $B$.