Revision: Ecology >> Ecosystem Biology Science (English Medium) Class 12 CBSE

A consumer is an organism that obtains energy by feeding on other organisms. As it cannot produce its own food, consumers rely on plants or other animals for nourishment, which are a vital part of the food chain. Consumers are further classified into primary consumers, secondary consumers, and tertiary consumers.

The rate at which biomass is synthesised by a trophic level per unit area per unit time is called its productivity.

The dead remains of plants, animals and faecal matter that serve as raw material for decomposition is called detritus.

The enzymatic degradation of detritus into simpler inorganic substances by bacteria and fungi is called catabolism.

The formation and accumulation of a dark-coloured, amorphous, resistant substance during decomposition is called humification.

The conversion of humus into inorganic nutrients by microbial action is called mineralisation.

The dark-coloured, colloidal, nutrient-rich substance formed during decomposition that decomposes very slowly is called humus.

Decomposition is the process by which decomposers break down complex organic matter into simple inorganic substances like carbon dioxide, water, and nutrients.

Decomposition is the process that involves the breakdown of complex organic matter or biomass from the bodies of dead plants and animals with the help of decomposers into inorganic raw materials such as carbon dioxide, water, and other nutrients.

An ecological pyramid is a graphical representation of various environmental parameters, such as the number of individuals present at each trophic level, the amount of energy, or the biomass present at each trophic level. Ecological pyramids represent producers at the base, while the apex represents the top-level consumers present in the ecosystem.

Net Primary Productivity (NPP) = Gross Primary Productivity (GPP) - Respiratory loss by plants (R)

• Two Structural Features → Species Composition (identifying species) + Spatial Pattern (distribution of biotic/abiotic components).
• Two Spatial Patterns → Stratification (vertical, e.g., trees→shrubs→herbs) + Zonation (horizontal, e.g., inter-tidal, littoral zones).
• Types → Terrestrial (forest, grassland, desert) + Aquatic (lakes, rivers, seas, oceans).
• Classification → Natural (self-sustainable) + Artificial (needs human input, e.g., farmland, fish tank).
• Components → Biotic (living) + Abiotic (non-living).
• 4 Functions → Productivity + Decomposition + Energy Flow + Nutrient Cycling (PDEN).

• Productivity is the rate of formation of biomass (organic matter) at any trophic level per unit area over time, mainly through photosynthesis or chemosynthesis. Unit: g m⁻² yr⁻¹ or g m⁻² day⁻¹.
• Primary productivity is the amount of biomass produced by plants; it includes Gross Primary Productivity (total production) and Net Primary Productivity (usable biomass left after plant respiration).
• Net Primary Productivity (NPP) is important because it represents the energy available to herbivores and other consumers in the ecosystem.
• Secondary productivity is the rate of formation of organic matter by consumers (heterotrophs) and depends on primary productivity.
• Productivity varies across ecosystems depending on factors like plant species, nutrient availability, climate, and photosynthetic efficiency; globally, oceans contribute a significant share.

• Energy flow in an ecosystem is strictly unidirectional, moving from the sun through producers to consumers and decomposers.
• Unlike physical matter, energy within an ecosystem is never recycled and is progressively lost as heat at every trophic step.
• Less than 50% of incident solar radiation is Photosynthetically Active Radiation (PAR), and plants capture merely 2–10% of this PAR to sustain the living world.
• Grazing Food Chains (GFC) originate with living plants and dominate aquatic ecosystems, while Detritus Food Chains (DFC) originate with dead organic matter and dominate terrestrial ecosystems.
• Interconnected food chains create complex food webs that provide ecosystems with essential stability, resilience, and alternative energy pathways.
• Within a complex food web, a single organism possesses the ecological flexibility to occupy more than one trophic level simultaneously.
• Organisms are structured into sequential trophic levels, from producers (T1) to apex predators (T4), with available energy drastically decreasing at each successive tier.
• Decomposers act systematically across all trophic levels to break down dead organic matter, forming a critical biological link for continuous nutrient recycling.

• Ecological pyramids graphically display trophic levels, placing producers at the broad base and top consumers at the apex.
• The pyramid of numbers is typically upright, though it can become inverted in a single-tree ecosystem that supports many insects and parasites.
• The pyramid of biomass is generally upright, except in oceanic ecosystems where it is inverted (small phytoplankton supporting larger zooplankton).
• The energy pyramid is always upright because only 10% of energy is successfully transferred upward to the next trophic level.
• As a general rule in standard terrestrial ecosystems, producers naturally exceed consumers in both population count and total biomass.
• These models have major limitations: they assume simple linear food chains, ignore complex food webs, exclude decomposers, and cannot handle species that occupy multiple trophic levels.