Revision: Ecology and Environment >> Ecosystem Biology (Theory) ISC (Science) ISC Class 12 CISCE

Organisms that manufacture their own food from simple inorganic substances using sunlight through photosynthesis are called producers.

The living organisms of an ecosystem, including producers, consumers and decomposers, which interact with one another and maintain ecological balance, are called biotic components.

Organisms that cannot synthesise their own food and depend directly or indirectly on producers for nourishment are called consumers.

Microorganisms like bacteria and fungi that break down dead plants and animals into simpler substances and recycle nutrients back into the environment are called decomposers.

A food chain is the sequential sequence of living organisms in an ecosystem where one organism consumes another to transfer food energy.

A sequence of energy transfer that begins with dead organic matter (decaying plants and animals), flows to microorganisms and detritivores, and then to their predators, relying on accumulated organic matter rather than direct solar energy.

A sequence of energy transfer that begins with living green plants (producers), flows to grazing herbivores (primary consumers), and then to carnivores, relying directly on an influx of solar energy.

Each step or level in a food chain where organisms occupy a specific position in the flow of energy is called a trophic level.

The phenomenon in which harmful non-degradable chemicals (like pesticides) accumulate progressively at each trophic level of the food chain is known as biological magnification.

A network of interconnected food chains showing how various organisms are related through feeding relationships is called a food web.

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.

The non-living, physical, and chemical factors of an environment that interact with biotic components to shape and sustain an ecosystem.

The portion of sunlight (roughly 400–700 nm wavelength) that plants can actually use for photosynthesis.

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

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

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

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

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

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

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

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.

Standing crop is the biomass or the amount of living matter (biotic components) present in an ecosystem at a given time. It represents the mass or number of organisms in a particular area at that moment.

Ecosystem services are the economic, environmental, medical, and aesthetic benefits that humans obtain from healthy, functioning ecosystems. The Millennium Ecosystem Assessment (MEA) report of 2005 categorises these into four main groups.

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

Only about 10% of the energy available at one trophic level is transferred to the next; the remaining 90% is lost as heat during respiration, movement, and other metabolic activities.

• Explains the pyramid shape of energy in ecosystems (energy pyramids are always upright).
• Limits food chains to typically 3–4 trophic levels, since energy becomes insufficient beyond that.

• An ecosystem (a term coined by A.G. Tansley in 1935) is a self-sustaining unit where living (biotic) and non-living (abiotic) components continuously interact and exchange energy.
• It is structurally composed of abiotic factors (physical/chemical elements like sunlight, water, and soil) and biotic factors (all living organisms).
• The living organisms are divided by their roles into producers (synthesise food), consumers (feed on others), and decomposers (recycle nutrients from dead matter).
• Ecosystems are classified by their origin into natural (forests, oceans) or artificial/man-made (crop fields, aquariums) and by habitat into terrestrial or aquatic.
• Every ecosystem operates through four core functions: productivity (biomass generation), decomposition, a unidirectional energy flow, and continuous nutrient cycling.

• Biotic components encompass all living organisms in an ecosystem – namely producers, consumers, and decomposers – that interact to sustain ecological balance.
• Producers are organisms capable of manufacturing their own food from simple inorganic substances, serving as the foundational energy source for the ecosystem.
• Terrestrial producers (such as trees, shrubs, and grasses) drive land-based ecosystems by synthesising food using sunlight through photosynthesis.
• Aquatic producers (such as phytoplankton, algae, and aquatic plants like Hydrilla) perform the same photosynthetic role within water-based ecosystems.
• Chemosynthetic producers (such as sulphur bacteria) operate in extreme habitats like deep-sea vents, uniquely utilising chemical energy instead of sunlight to produce food.

• Consumers are organisms that cannot synthesise their own food and must depend entirely on producers, either directly or indirectly, for nourishment.
• Based on their feeding habits and position in the food chain, consumers are classified into four distinct trophic levels.
• Primary consumers (2nd trophic level) are herbivores that feed directly on producers, such as deer, cows, insects, and zooplankton.
• Secondary consumers (3rd trophic level) are carnivores that feed on primary consumers, including foxes, crows, and small carnivorous fish.
• Tertiary consumers (4th trophic level) are larger carnivores that prey on secondary consumers, such as wolves and large predatory fish.
• Quaternary or top consumers (5th/apex trophic level) are apex predators that are rarely or never preyed upon by other animals, including lions, tigers, and sharks.

• Decomposers are microorganisms, primarily bacteria and fungi, that break down dead organic matter to recycle essential nutrients back into the environment.
• They operate by colonizing dead tissue and secreting extracellular enzymes that convert complex organic molecules into simpler inorganic substances.
• These released inorganic nutrients, such as nitrates and phosphates, enrich the soil and water, becoming readily available for primary producers to absorb.
• Key examples include soil bacteria (Bacillus, Pseudomonas), fungi that degrade tough plant materials like cellulose (Rhizopus, Aspergillus), and actinomycetes (Streptomyces) that aid in humus formation.
• Ecologically, they are crucial for preventing the accumulation of dead matter, improving soil fertility, and maintaining the continuous flow of global biogeochemical cycles.

• A food chain is a linear sequence of living organisms in an ecosystem where food energy is transferred as one organism consumes another.
• The structural flow typically moves from primary producers to successive levels of consumers, such as from grass to insects, then to a thrush, and finally to a sparrow hawk.
• According to the principle of continuity, a complete food chain requires a continuous external energy supply, always beginning with photosynthesis and ending with decomposition.
• The principle of efficiency dictates that shorter food chains are more stable and energy-efficient because less energy is wasted across fewer transfers.
• Under the 10% law, only about 10% of the available energy is passed to the next trophic level, while the remainder is lost as heat through respiration.

• A trophic level is defined as a specific step or position within a food chain that represents the sequential flow of energy among organisms.
• The first trophic level consists of producers (autotrophs like green plants) that fix solar energy, which is then transferred to primary consumers (herbivores like grasshoppers) at the second level.
• The third and fourth trophic levels are respectively occupied by secondary consumers (primary carnivores) and tertiary consumers (secondary carnivores) that feed on the organisms directly below them.
• Decomposers, such as bacteria and fungi, do not occupy a single numbered level; rather, they function across all levels to break down dead organic matter.
• Biological magnification is the phenomenon where harmful, non-degradable chemicals (such as pesticides) progressively accumulate in higher concentrations at each successive trophic level.

• A food web is a complex network of interconnected food chains that illustrates the broader pattern of energy and nutrient flow within an ecosystem.
• The complexity of a food web increases with the length of its food chains and the availability of alternative dietary choices for consumers.
• Energy flow in an ecosystem is strictly unidirectional, meaning energy captured by autotrophs never reverts back to the solar input or to previous trophic levels.
• As energy transfers progressively through successive trophic levels, the total amount of available energy consistently diminishes due to energy loss at each stage.
• Biological magnification occurs when non-degradable, harmful chemicals enter the food chain and accumulate progressively at each consecutive trophic level.
• Because human beings occupy the apex or top level in food chains, the maximum concentration of these toxic chemicals ultimately accumulates within our bodies.

• 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.

• A pond ecosystem is self-sufficient and self-regulating, combining abiotic and biotic components.
• Abiotic components include soil, water, air, nutrients, sunlight, temperature, and climatic factors.
• Biotic components are classified into producers, consumers, and decomposers.
• Producers include submerged, floating, and amphibious plants plus phytoplankton/algae.
• Fish can occupy multiple trophic levels depending on their diet.
• Decomposers are chiefly fungi and bacteria that recycle nutrients.
• Four core ecosystem functions: productivity, decomposition, energy flow, and nutrient cycling.

1. Solar energy is the ultimate source of energy for all ecosystems, but only a very small fraction reaches and is used by living organisms.
2. Green plants (producers) trap about 1% of solar energy through photosynthesis and convert light energy into chemical energy of food.
3. Energy flow is unidirectional (non-cyclic): it moves from the sun → producers → consumers → decomposers and is finally lost as heat.
4. According to the laws of thermodynamics, energy cannot be created or destroyed, and with every transfer some energy is lost as heat.
5. The Ten Percent Law states that only about 10% of energy at one trophic level is transferred to the next, limiting food chains to few levels.

• Productivity is the rate of biomass (organic matter) formation at any trophic level per unit area over time, primarily 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 at which organic matter is formed 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.

• Ecosystem services are the benefits humans obtain from ecosystems, supporting life and maintaining environmental balance.
• These services are grouped into supporting, provisioning, regulating, and cultural services, including nutrient cycling, food, climate regulation, and recreation.
• Ecosystems provide essential functions like oxygen production, carbon dioxide removal, rainfall generation, and climate control.
• Pollination, seed dispersal, decomposition, and nutrient cycling by organisms help maintain soil fertility, crop production, and waste recycling.
• Ecosystem services have economic, ecological, and survival value, but human activities can disturb them, making conservation of biodiversity important.