NCERT Solutions: Class 12 Biology Chapter 12 - Ecosystem

Plants fix carbon dioxide by photosynthesis, so they are producers. A tree ecosystem may support many insects and birds on a few large producers, giving an inverted number pyramid. In aquatic ecosystems light often limits productivity. Earthworms are common detritivores. Oceans are the major carbon reservoir.

1. a. Producers
2. b. Primary consumers
3. c. Secondary consumers
4. d. Decomposers

Decomposers such as bacteria and fungi are extremely numerous and act on dead organic matter across trophic levels, so they generally have the largest population.

1. a. Phytoplankton
2. b. Zooplankton
3. c. Benthos
4. d. Fishes

Phytoplankton are producers and form the first trophic level. Zooplankton feed on phytoplankton, so they are primary consumers at the second trophic level.

1. a. Herbivores
2. b. Producers
3. c. Carnivores
4. d. None of the above

Secondary productivity is the rate of formation of new organic matter by consumers. The organisms themselves are consumers, not 'secondary producers' in the producer sense.

1. a. 100%
2. b. 50 %
3. c. 1-5%
4. d. 2-10%

NCERT states that less than 50 per cent of incident solar radiation is photosynthetically active radiation. Among the options, 50% is the expected answer.

(a) Grazing food chain begins with living green plants; detritus food chain begins with dead organic matter. (b) Production forms organic matter; decomposition breaks complex organic matter into inorganic substances. (c) Upright pyramids have broad producer base; inverted pyramids have smaller producer number/biomass than consumers. (d) A food chain is a linear feeding sequence; a food web is a network of interconnected chains. (e) Litter is dead plant material such as leaves, bark and flowers; detritus includes dead plant and animal remains and fecal matter. (f) Primary productivity is biomass formation by producers; secondary productivity is new organic matter formation by consumers.

Abiotic components include water, air, soil, inorganic and organic substances, light and temperature. Biotic components include producers such as plants/phytoplankton, consumers such as herbivores and carnivores, and decomposers such as fungi and bacteria. In a pond, water and bottom soil are abiotic components; phytoplankton and aquatic plants are producers; zooplankton and other animals are consumers; bacteria and fungi are decomposers.

The base represents producers and the apex represents top consumers. Pyramid of numbers shows number of individuals at each trophic level; it is upright in grassland, but can be inverted in a tree ecosystem where one tree supports many insects. Pyramid of biomass shows living biomass at each trophic level; it is usually upright in terrestrial ecosystems but may be inverted in aquatic ecosystems because small phytoplankton support a larger standing crop of zooplankton/fish.

It depends on plant species in the area, environmental factors, availability of nutrients and photosynthetic capacity of plants. It varies across ecosystems; oceans have lower productivity despite large area because nutrients and light can be limiting.

Detritus is fragmented by detritivores such as earthworms. Leaching moves water-soluble nutrients into soil. Bacterial and fungal enzymes carry out catabolism. Humification forms dark, resistant humus that acts as a nutrient reservoir. Mineralisation releases inorganic nutrients from humus. Products include CO2, water and mineral nutrients.

Plants capture a small fraction of photosynthetically active radiation and convert it into chemical energy. Producers form the first trophic level, herbivores the second and carnivores higher levels. Only about 10% of energy is transferred from one trophic level to the next, so food chains are limited in length. When organisms die, their biomass enters the detritus food chain. Energy is not recycled, unlike nutrients.