CBSE Questions for Class 12 Medical Biology Ecosystem Quiz 13 - MCQExams.com

The population of a particular region depends on the climatic condition prevailing in that particular region. A population's growth rate is strongly influenced by the proportions of individuals of particular ages. In an age pyramid, the population is sequentially arranged according to the increasing age. The young ones forms the base of the pyramid.  For example, the population of a country with rapid growth has a triangle-shaped age structure with a greater proportion of younger individuals who are at or close to reproductive age. This pattern typically occurs where fewer people live to old age because of sub-optimal living standards, such as occurs in underdeveloped countries.

• The entire sequence of communities, that successively change in a given area is called sere. 
• The individual transitional communities are termed seral stages or seral communities. 
• In the successive seral stages, there is a change in the diversity of species of organisms, an increase in the number of species and organisms as well as an increase in the total biomass. 
• Thus, the correct answer is option C.

• Xerarch succession of ecological communities originated in extremely dry situation such as sand deserts, sand dunes, salt deserts, rock deserts etc. 
• Stages in correct order in xerarch succession are bare rocks, crustose lichen stage, foliose lichen stage, moss stage, annual herb stage, perennial herb stage, shrub stage, tree forest and finally climax stage. 

Correct Option: C
Explanation:  

• Secondary succession is a process started by an event (e.g., forest fire, harvesting, hurricane) that reduces an already established ecosystem (e.g., a forest or a wheat field) to a smaller population of species and as such secondary succession occurs on preexisting soil. 
  
• Secondary biotic succession occurs in abandoned farmlands, burned or cut forests and lands that have been flooded. It is not similar to primary succession and neither it follows primary succession. It does not begin on bare rock as well. 

• Gross primary production (GPP) is the amount of production of chemical energy as biomass by primary producers in a given length of time. 
• It is the rate at which photosynthesis and fixed energy are used by primary producers for cellular respiration and maintenance of existing tissues. 
• The remaining fixed energy (i.e., the mass of photosynthate) is referred to as net primary production (NPP). NPP = GPP - respiration (by plants). Whereas standing crop is the total dried biomass in a given environment

$$\bullet$$ These organisms help in the weathering of the region which makes room for the development or growth of small plants.

$$\bullet$$ Mosses and other bryophytes are able to enter the land area.

$$\bullet$$ As a stable forest community starts to form, the region becomes available for the larger plants and hence herbs enter the region followed by shrubs.

$$\bullet$$ With the environment, the climax community of shrubs remains stable, maintaining the forest ecosystem.

$$\textbf{Correct answer: C}$$ 

$$\textbf{Option (A):}$$ 

$$\bullet$$ The pyramid of numbers can be inverted if it depicts a tree ecosystem.

$$\bullet$$ In a tree ecosystem, the number of producers is extremely low as compared to primary consumers.

$$\bullet$$ This is why the pyramid of numbers may not always be upright. 

$$\textbf{Option (B):}$$ 

$$\bullet$$ An inverted pyramid of biomass is seen in the aquatic ecosystem.

$$\bullet$$ Here the phytoplankton is the producer and hence the biomass is lower at the level of the producers. Organisms present at the higher trophic levels have higher biomass. 

$$\textbf{Option (C):}$$ 

$$\bullet$$ The pyramid of energy is always upright and cannot be inverted.

$$\bullet$$ This is because of the 10% law of energy flow. Every trophic level has 10% of the energy available to them from the previous trophic level.

$$\bullet$$ This is why the energy at the producers will always be the highest and it will keep decreasing with every trophic level. 

$$\textbf{Option (D):}$$ 

$$\bullet$$ In an ecosystem, the number of species forming the producers can be lower than the herbivores present in the region.

$$\bullet$$ This completely affects the habitat and this is why such pyramids can be inverted.

Hence, the correct answer is option (C).

Detrivores are heterotrophs that obtain nutrients by consuming detritus or decomposing plant and animal parts as well as faeces. Earthworm is a detrivore.
In succession, pioneer species like lichen, algae, and fungi, as well as other abiotic factors like wind and water start to 'normalize' the habitat.
Pollination is a type of ecosystem service which enhances the service provided by insects/birds through behavioral interaction.
Natality is the crude birth rate or specific birth rate and is the greatest influence on a population's increase.
Thus, the correct answer is option D.

• 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 little soil leads to the growth of bryophytes (mosses). The mosses speed up the process of soil accumulation by trapping wind-blown particles. 
• Lichen moss carpet provides a suitable substratum for the germination of seeds of herbaceous plants. 
• Gradually more soil is accumulated and herbaceous species make way for the invasion of shrubs followed by trees. 
• The climax community is generally dominated by trees. 
• So, the correct answer is option C.