Ecosystem — NCERT Solutions

An ecosystem has two main components:

• Abiotic components: Non-living factors such as air, water, soil, light, temperature, and nutrients.
• Biotic components: Living organisms, grouped as producers, consumers, and decomposers.

These components interact to give each ecosystem its characteristic structure and function.

Ecological pyramids are graphical representations of the relationship between organisms at different trophic levels in terms of number, biomass, or energy.

Pyramid of number:

• Shows the number of organisms at each trophic level.
• Example: In a grassland ecosystem, producers (grasses) are most numerous, herbivores are fewer, and top carnivores are least, so the pyramid is upright.
• In a tree ecosystem, the pyramid of number can be inverted because one tree may support many insects and birds.

Pyramid of biomass:

• Shows the total biomass at each trophic level.
• In most terrestrial ecosystems, it is upright because biomass decreases from producers to top consumers.
• In the sea, it is generally inverted because the biomass of fishes may exceed that of phytoplankton.

Primary productivity is the amount of biomass or organic matter produced per unit area per unit time by plants during photosynthesis. It is expressed as $g\,m^{-2}\,yr^{-1}$ or $kcal\,m^{-2}\,yr^{-1}$.

Factors affecting it:

• Plant species present in the area
• Availability of nutrients
• Environmental factors such as light, temperature, and water
• Photosynthetic capacity of plants

It is higher where these conditions are favourable and lower where they are not.

Decomposition is the breakdown of complex organic matter in dead plant and animal remains into simpler inorganic substances such as carbon dioxide, water, and nutrients by decomposers.

Processes involved:

1. Fragmentation – detritivores like earthworms break detritus into small particles.
2. Leaching – water-soluble nutrients go down into the soil and become unavailable salts.
3. Catabolism – bacterial and fungal enzymes break detritus into simpler substances.
4. Humification – formation of dark, amorphous humus.
5. Mineralisation – further breakdown of humus releasing inorganic nutrients.

Products: Humus and inorganic nutrients (plus carbon dioxide and water).

Energy flow in an ecosystem is unidirectional.

• The sun is the ultimate source of energy for almost all ecosystems.
• Producers capture a small fraction of solar energy and convert it into chemical energy.
• Energy then passes from producers to consumers through food chains and food webs.
• In a detritus food chain, energy also passes to decomposers from dead organic matter.
• At each trophic level, some energy is lost as heat, so the amount of energy decreases successively.
• Only about 10% of energy is transferred from one trophic level to the next.

Thus, energy flows in one direction from the sun through organisms and is never recycled like nutrients.