CBSE Questions for Class 12 Medical Biology Ecosystem Quiz 2 - MCQExams.com

• Disclimax is the ecological community that displaces the natural climax community due to interruptions by human activities and consists of organisms that are foreign to that region.
• Plagioclimax is the stage when succession doesn't reach the climax and stops due to various interruptions such as destruction of vegetation.

• A climax community is an ecological community in which populations of plants or animals remain stable and exist in balance with each other and their environment.
• It is the final stage(last and stable) of succession, remaining relatively unchanged until destroyed by an event such as fire or human interference. For example, The Daintree Rainforest in Queensland, Australia is an example of a climax forest ecosystem.
• Hence the last and stable community is an area is called Climax community.
• So, the correct answer is 'Climax community'.

• Crystalline rocks are the natural source of phosphorus. It is not found free in nature, but it is widely distributed in many minerals, mainly phosphates.
• Inorganic phosphate rock, which is partially made of apatite (an impure tri-calcium phosphate mineral), is today the chief commercial source of this element. 
• It is an essential mineral nutrient for all plants and animals.
• It acts as  building blocks of certain parts like bones and teeth in humans and other animals. 

• The term biogeochemical cycle is used for the exchange or circulation of biogenetic nutrients between living and nonliving components of the biosphere.
• The nutrient cycle can be a gaseous cycle in which the materials involved in circulation between living and nonliving components are gases or vapors, e.g., carbon, hydrogen, oxygen.
• In sedimentary cycles of matter; the materials involved in circulation between biotic and abiotic components of the biosphere are non-gaseous e.g., phosphorous, calcium, magnesium.
• The reservoir of phosphorous is phosphates present in rocks.
  
  Thus, the correct answer is option B.

• Lichens stage is the first stage in Lithosere (or succession on bare rock). Bare rock is invaded first by crustose lichens e.g: Graphis, Rhizocarpon. They corrode the rock at places causing foliose lichen e. g: Parmelia,
• Dermatocarpon to invade and eliminate crustose lichens and create conditions for invasion by mosses. In the tropics, blue-green algae are pioneers instead of lichens in lithosere.
• So, the correct answer is option C.

• The pyramid of biomass is more fundamental, they represent the quantitative relationships of the standing crops. In this pyramid, there is a gradual decrease in the biomass from the producers to the higher trophic levels. 
• In an ocean, as the producers are small organisms, their biomass is the least, and this value gradually shows an increase towards the apex of the pyramid, thus making the pyramid inverted in shape.

• For the sedimentary cycle, the major reservoir is the lithosphere which releases the elements through weathering. 
• The mineral elements required by living organisms are obtained from inorganic sources like salts dissolved in water or from rocks. 
• The sedimentary cycles are represented by phosphorus, sulphur, and iodine cycles.

• In the phosphorus cycle, weathering makes phosphate available to the soil. 
• Phosphorus is a critical element. It is obtained from the soil as orthophosphate. 
• Plants or producers absorb phosphorus and use it to produce several biological molecules like nucleic acids, nucleotides, etc. 
• From plants, the organic phosphorous transfers to other organisms, and organic phosphorous circulate in nature from plants to animals. 
• Phosphate is released by decomposers back to the soil. 

Solution:

• Correct answer is option D.
• Explanation for correct option:
• Xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions.
• Pioneer species in xerarch succession are usually lichens.
• Lichen secretes acid to dissolve rock, helping in weathering and soil formation.
• The correct order of succession in xerarch is:

• Most of the energy of the sun that falls on earth is reflected and scattered. 
• Plants absorb only a small fraction of the energy and out of that also lost a lot of energy as radiation and actually convert only a small fraction of the absorbed energy (2 %) into organic molecules or biomass. 

• Nitrogen is the mineral element required by plants in the largest amount.
• Plants cannot absorb nitrogen directly from the atmosphere but absorb most of the nitrogen as nitrates or nitrites from the soil. 
• The soil microbes fix the nitrogen in form of nitrates and nitrites which can be easily utilised by the plant.

• The term biogeochemical cycle is used for exchange or circulation of biogenetic nutrients between living and nonliving components of a biosphere. 
• The Biogeochemical or nutrient cycle can be- 

• The pyramid of energy is the ecological pyramid that gives a graphic representation of the amount of energy trapped by different trophic levels per unit area. 
• It is always upright because during the transfer of energy from one trophic level to the next a lot of wastage occurs in feeding, digestion, assimilation, and respiration i.e metabolism at each trophic level. 
• Hence organisms of each trophic level have more energy than each succeeding trophic level and less energy than organisms at the previous trophic level.
• So, the correct answer is option B.

• The climax community is stable, self-perpetuating and final community appearing in an area at the end of biotic succession, which is in perfect harmony with the climate of the area. 
• The vegetation in that particular area has been reached to a stable condition after ecological succession.
  
  

The succession which thus takes place through the process of living, growing and interactions of organisms with  the effect of the environment  within an area is referred as  biotic succession.  

Geographical succession is the directional change in the plant and animal community over a period of time in a specific area.

Physiographic succession is the directional change in the plant and animal community over a period of time in a specific area due to physical factors.

Climate succession is the directional change in the living community in a particular area over a period of time due to prevailing climatic conditions.

So, the correct answer is option B.

Solution

Correct answer is option C
Explanation for correct option:

• Biotic or ecological succession is the successive development of different biotic communities at the same site till a climax community develops there.
• Primary Succession is the ecological succession on a primary base area like rock, or outcrop. It takes a long time of 1000 years or more. 
• Secondary succession is biotic succession on the secondarily bare areas e.g., an area devastated by forest fire or deforestation. It takes 50 to 100 years for grassland and 100 to 200 years for the forest. Ferns are generally the first to grow after forest fire because of their underground rhizomes. 
• Secondary succession is not completed if invaded by weeds like Sphagnum, Lantana, Eichhornia, Eupatorium. 

 A) Correct option (A)
B) Explanation of the correct option 

• When the ecosystem in an area changes, it is called ecological succession. New species come into existence at this place. 
• Apart from this, ecological succession is of 2 types. One is primary succession, in which a barren land with no species affiliation shows the development of a new species. 
• Another is secondary succession where the species develop again in a land that was previously occupied by different organisms and was destroyed due to certain reasons. 

• Ecological succession is the gradual process by which ecosystems change and develop over time. 
• With the habitats, constantly changing, succession is of two types- primary and secondary.
• Succession develops through increasing complexity until it becomes stable or self-perpetuating as a climax community where the ecosystem is balanced and there is less risk of any interfering event or change to the environment. 

• A climax community is a stable, self-perpetuating and final community appearing in an area at the end of biotic succession which is in perfect harmony with the climate of the area. 
• Both xerach and hydrarch succession generally end in mesophytes. 
• But the type of vegetation or types of plants inhabiting is decided by the climate of the region. For example, two places A and B both may have mesophytes but exactly which plants are growing in those locations is decided by the local climate. 

• An ecological pyramid is a graphical representation of each trophic level in a given ecosystem. 
• The pyramid of numbers is a graphical representation of the number of organisms in a food chain in a particular ecosystem. 
• In a grassland ecosystem, the producers which are grasses are the maximum and are followed by the primary consumers-herbivores. The secondary consumers-snakes, lizards lesser in number and the final consumers the hawks and other birds are least in number. 
• Therefore, the grassland pyramid is always upright and erect.