Our Environment — CBSE Class 10 Science Important Questions

Answer: (a) Primary consumer - the deer is a herbivore that feeds directly on the producer (grass), so it forms the second trophic level (primary consumer).

Answer: (a) Vegetable peels, paper, cotton cloth - these are broken down by decomposers (microorganisms), whereas plastic, glass, DDT and nylon are non-biodegradable.

Answer: (a) 100 J - producers (10000 J) -> primary consumers (1000 J, i.e. 10%) -> secondary consumers (100 J, i.e. 10% of 1000 J).

Answer: (a) Human beings (the top consumer) - because of biomagnification the concentration of non-biodegradable chemicals increases at each trophic level and is highest at the top of the food chain.

Answer: (a) Both A and R are true and R is the correct explanation of A - since only about 10% of the energy is transferred to the next level, very little usable energy is left after three or four steps to support further trophic levels.

Decomposers are microorganisms (mainly bacteria and fungi) that break down the dead remains and waste products of plants and animals into simpler inorganic substances. Their importance: they recycle nutrients back into the soil and atmosphere for reuse by producers, and they keep the environment clean by removing dead organic matter.

Energy captured by producers flows from producers to herbivores to carnivores and never in the reverse direction. At each trophic level a large part of the energy is lost as heat to the surroundings and used up in life processes, so it cannot be returned to the previous level or reused. Hence the flow of energy is unidirectional (one-way).

(a) A food web is a network of many interconnected food chains in an ecosystem, in which an organism may be eaten by several others and may feed at more than one trophic level. A food chain is a single, straight-line sequence of who-eats-whom, whereas a food web is many such chains linked together.

(b) A grassland food chain of four trophic levels: Grass -> Grasshopper -> Frog -> Snake (producer -> primary consumer -> secondary consumer -> tertiary consumer).

(a) Using the ten per cent law: Plants (20000 J) -> Rats (2000 J) -> Snakes (200 J) -> Hawks (20 J).
(b) This is based on the ten per cent law of energy flow (only about 10% of the energy passes to the next trophic level).
(c) Because the usable energy falls to about one-tenth at each step, very little energy is left after a few levels, so food chains are usually limited to only three or four trophic levels.

(a) The ozone layer is a layer of ozone gas ($O_3$O₃) present in the upper part of the atmosphere (the stratosphere).
(b) It absorbs most of the harmful ultraviolet (UV) radiation coming from the Sun and prevents it from reaching the Earth's surface, thus protecting living organisms.
(c) It is mainly depleted by chlorofluorocarbons (CFCs). Increased UV radiation reaching the Earth can cause skin cancer (also cataracts and damage to the immune system).

(a) An ecosystem is a self-contained unit in which living organisms interact with one another and with their non-living surroundings. Its two components are biotic (living - e.g. plants, animals, microorganisms) and abiotic (non-living - e.g. air, water, soil, sunlight, temperature).
(b) Decomposers break down dead organic matter into simple inorganic substances and return nutrients to the soil and air, enabling the recycling of materials; without them dead matter would pile up and nutrient cycles would stop.
(c) Pond food chain: Phytoplankton -> Zooplankton -> Small fish -> Big fish (producer -> primary -> secondary -> tertiary consumer), as shown.
(d) Because only about 10% of the energy is transferred to the next level, the energy remaining after three or four levels is too little to support any further trophic level.

(a) Biological magnification (biomagnification) is the progressive increase in the concentration of harmful non-biodegradable chemicals (such as the pesticide DDT) at each successive trophic level of a food chain. Since these chemicals are neither broken down nor excreted, they accumulate; each higher level eats many organisms of the level below, so the top consumer (e.g. human beings) ends up with the highest concentration - hence the effect increases up the levels.
(b) Non-biodegradable substances are more harmful because (i) they are not decomposed by microorganisms, so they persist and accumulate in the environment for a long time, and (ii) they enter food chains and undergo biomagnification, poisoning organisms and polluting soil and water.
(c) (i) Follow the 3 Rs - Reduce, Reuse and Recycle (segregate and recycle plastics, metals and e-waste). (ii) Use eco-friendly, biodegradable alternatives such as cloth or jute bags in place of plastic bags. (any two)

(i) About 10% of the energy is transferred to the next trophic level (the ten per cent law).
(ii) Producers (5000 J) -> primary consumers (500 J) -> secondary consumers (50 J) -> tertiary consumers (5 J).
(iii) The pyramid of energy is always upright because energy decreases at every successive trophic level: only about 10% passes on to the next level, and the rest is lost, mostly as heat, never flowing back. Each level therefore has less energy than the one below it, so the pyramid narrows upward.
(iv) Photosynthesis.