Ecosystem notes

An ecosystem is a functional unit of nature where living organisms interact with each other and with physical factors such as light, temperature, water and soil. NCERT treats ecosystem through four major processes: productivity, decomposition, energy flow and nutrient cycling. For NEET, the most repeated ideas are ecosystem components, producers-consumers-decomposers, GPP, NPP, decomposition steps, 10% law, trophic levels and ecological pyramids. Energy flow is always unidirectional, while nutrients are recycled. Ecological pyramids graphically show number, biomass or energy at successive trophic levels. This chapter connects ecology with real examples such as ponds, forests, grasslands and detritus-based systems.

Ecosystem structure means the arrangement of abiotic and biotic components and their feeding relationships. Abiotic components include light, temperature, water, soil, minerals, pH and climate. Biotic components include producers, consumers and decomposers. Producers such as green plants and algae convert solar energy into chemical energy. Consumers obtain food directly or indirectly from producers, while decomposers such as bacteria and fungi break down dead organic matter. Food chains show a single linear feeding pathway, whereas food webs show interconnected food chains and provide stability. NCERT emphasizes that structure and function are linked: the type and number of organisms determine productivity, decomposition and energy flow.

Productivity is the rate of biomass production in an ecosystem and is usually expressed as dry weight or energy per unit area per unit time. Primary productivity refers to the rate at which producers form organic matter through photosynthesis. Gross primary productivity, or GPP, is the total organic matter produced by photosynthesis. Net primary productivity, or NPP, is what remains after respiratory losses and is available to herbivores and decomposers. Secondary productivity is the rate of formation of new organic matter by consumers. Productivity varies greatly among ecosystems and depends on sunlight, temperature, water, nutrients and photosynthetic capacity of producers.

Decomposition is the breakdown of complex dead organic matter, called detritus, into simpler inorganic substances such as carbon dioxide, water and nutrients. It is mainly performed by decomposers like bacteria and fungi, with detritivores assisting by fragmentation. NCERT describes decomposition through fragmentation, leaching, catabolism, humification and mineralisation. Fragmentation breaks detritus into smaller particles; leaching dissolves soluble nutrients into soil water; catabolism enzymatically degrades complex molecules; humification forms dark, resistant humus; mineralisation releases inorganic nutrients. Decomposition is faster in warm, moist, oxygen-rich conditions and slower when detritus contains lignin and chitin.

Energy flow explains how solar energy captured by producers moves through trophic levels. It is always unidirectional: sun to producers to consumers to decomposers, with heat loss at every transfer. Producers occupy the first trophic level, herbivores the second, carnivores the third and higher levels. Lindeman's 10% law states that only about 10% of energy at one trophic level is transferred to the next. Energy flow occurs through grazing food chains, which begin with living green plants, and detritus food chains, which begin with dead organic matter. Because energy decreases sharply, food chains are usually short and energy pyramids are always upright.

Ecological pyramids are graphical representations of trophic structure in an ecosystem. They may show number of organisms, biomass or energy at successive trophic levels. The base represents producers and upper levels represent consumers. Pyramid of numbers may be upright, as in grassland, or inverted, as in a tree ecosystem with many insects and birds supported by one tree. Pyramid of biomass may be upright in forests but inverted in aquatic ecosystems because phytoplankton have low standing biomass but high turnover. Pyramid of energy is always upright because energy is lost at every trophic transfer. NCERT highlights that pyramids usually ignore decomposers and do not account for species occupying multiple trophic levels.