BIOLOGY 5 Marks Questions

(i) Standing crop : Each trophic level has a certain mass of living material at a particular time called as the standing crop. The standing crop is measured as the mass of living organisms (biomass) or the number in the unit area.
(ii) All living beings existing on earth eventually die. Decomposers breakdown the dead body into its components. Earthworms are being referred to as the farmer's 'friend' because they help in the breakdown of complex organic matter as well as in loosening of the soil. Similarly, decomposers breakdown complex organic matter into inorganic substances like carbon dioxide, water and nutrients and the process is called decomposition. Dead plant remains such as leaves, bark, flowers and dead remains of animals including fecal matter, constitute detritus, which is the raw material for decomposition. The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralisation.
Detritivores (e.g. earthworm) breakdown detritus into smaller particles. This process is called fragmentation. By the process of leaching, water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts. Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called as catabolism.
It is important to note that all the above steps in decomposition operate simultaneously on the detritus. Humification and mineralisation occur during decomposition in the soil.
Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients. The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.
Decomposition is largely an oxygen-requiring process. The rate of decomposition is controlled by chemical composition of detritus and climatic factors. In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker, if detritus is rich in nitrogen and water soluble substances like sugars. Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes. Warm and moist environment favour decomposition whereas low temperature and anaerobiosis inhibit decomposition resulting in build up of organic materials. 

(i) Net primary productivity : A considerable amount of gross primary productivity is utilised by plants in respiration. Gross primary productivity minus respiration losses (R) is the net primary productivity (NPP).
GPP – R = NPP
where, R = Respiration loss.
(ii) The pyramid of energy clearly depicts the nature of the ecosystem as the whole. In the food chain, food passes from producers to consumers, that is, food keeps moving from one level to another or it can be said that energy flows from one level to another. It has been observed that when the energy stored from one trophic level goes to another trophic level, only 10 percent of the total energy is converted into the form of biomass. Therefore, energy is highest in producers and energy gradually decreases in primary, secondary and tertiary and highest consumers, hence when depicted on the basis of energy, pyramids are always formed straight. The base of the pyramid formed on the basis of energy is always big and the top is small. 

All the trophic levels are arranged in sequence wise one after another in a food chain in an ecosystem. If the first trophic level is considered as base which followed by successive trophic levels as represented in a diagram, then a shape of pyramid is formed, called ecological pyramid.
Pyramid of biomass : An ecological pyramid to show the food chain and the interrelationships of organisms at each trophic level in the ecosystem, is biomass pyramid. The dry weight of organisms in a unit area of an ecosystem is called biomass. If the inter-proportional relationships of successive trophic levels are graphically represented on the basis of biomass, then a pyramidal shape is formed. In the pyramid of biomass, the biomass at the producer level is usually maximum and at the other levels biomass gradually decreases. Pyramid of biomass is always upright for terresterial ecosystems e.g. Pyramid of biomass for grassland ecosystem in which biomass gradually decreases from producer to consumers in successive manner.

[Hint : For decomposition and its cycle, see Q. no. 10 of Textbook.]

self

[Hint : See point 12.4 in the Text.]