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

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

Organisms that cannot synthesize 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 series of organisms where one is eaten by the next and energy flows from producers to consumers is called a food chain.

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.

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.

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

The circular movement of nutrients between living organisms and the physical environment of an ecosystem is called biogeochemical cycle.

The gradual and predictable changes in the species composition of a given area are called ecological succession.

Succession initiating in ponds, pools, lakes or elsewhere in water is called hydrosere (hydrarch).

Succession beginning on the bare rock or other places where there is an extreme deficiency of water is called xerosere (xerarch).

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.

1. An ecosystem includes all living (biotic) and non-living (abiotic) components in an area that interact with each other.
2. Producers like green plants and certain bacteria make food through photosynthesis, forming the base of the food chain.
3. Consumers depend on producers for food and are classified as herbivores, carnivores, omnivores, and parasites.
4. Decomposers such as bacteria, fungi, and insects break down dead matter and waste, recycling nutrients and maintaining soil fertility.
5. Even dirty or unnoticed organisms (like caterpillars, termites, and dung insects) play a vital role in cleaning the environment and supporting the ecosystem.

• A food chain is a linear sequence of organisms in which food energy is transferred from producers to consumers.
• Every food chain begins with producers (green plants) and usually ends with top consumers that are not preyed upon.
• Energy decreases at each trophic level, so food chains are usually short (not more than five levels).
• Shorter food chains are more stable and efficient, as less energy is lost at each step.

• Trophic levels are the feeding positions of organisms in a food chain, with each step representing one level.
• Producers form the first trophic level, followed by herbivores and then carnivores at higher levels.
• A single species can occupy more than one trophic level depending on its food habits.
• Energy decreases at each successive trophic level because some energy is lost as heat during transfer.

• A food web is a network of interconnected food chains operating within an ecosystem.
• It shows multiple feeding relationships, as organisms may have more than one food source.
• Food webs represent the flow of energy and nutrients among producers and consumers.
• The complexity of a food web depends on the diversity of organisms and the number of alternative food choices.

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 at which producers fix solar energy into organic matter through photosynthesis or chemosynthesis.
• Gross Primary Productivity (GPP) is the total rate of photosynthesis, while Net Primary Productivity (NPP) is the stored energy after respiration (NPP = GPP – R).
• Net Community Productivity refers to the organic matter stored in an ecosystem after subtracting heterotrophic consumption.
• Secondary Productivity is the rate of organic matter formation at consumer levels and depends on primary productivity.
• Productivity varies among ecosystems and depends on plant species, nutrients, climate, and photosynthetic efficiency.

• Carbon is an essential element of all organic compounds, and atmospheric carbon dioxide is the main source of carbon for living organisms.
• Photosynthesis fixes carbon dioxide into organic compounds, which pass through food chains from producers to consumers.
• Respiration, decomposition, combustion of fossil fuels, and volcanic activity return carbon dioxide back to the atmosphere.
• Some carbon is stored long-term in fossil fuels and carbonate rocks, and is released slowly through burning, weathering, or acid rain.
• Human activities increase atmospheric CO₂, enhancing the greenhouse effect and leading to global warming and climate change.

• Phosphorus is an essential element of nucleic acids, ATP, proteins, bones, and teeth, and often acts as a limiting nutrient in ecosystems.
• The main reservoir of phosphorus is phosphate-containing rocks, from which phosphates are released slowly by weathering.
• Plants absorb phosphorus from soil as phosphate ions, which then pass through food chains to animals.
• Phosphorus returns to the environment through excretion, decomposition, leaching, and sedimentation, but has no gaseous phase.
• Human activities such as fertilizer use and sewage disposal increase phosphate levels, causing eutrophication in aquatic ecosystems.

• Biotic succession is the orderly, gradual, and predictable change in community structure at a given place over time, resulting from interactions among organisms and their environment.
• Succession begins with pioneer species, progresses through a series of seral stages (sere), and ends in a climax community.
• During succession, communities modify their environment, making it less suitable for themselves and more suitable for succeeding communities.
• The climax community is a stable, self-maintaining community controlled mainly by climate and is also called the climatic or prevailing climax.

1. Production of food, oxygen, and climate regulation: Plants and phytoplanktons produce food and oxygen through photosynthesis, remove carbon dioxide, regulate climate, and generate rainfall.
2. Pollination services: Bees, butterflies, birds, and other animals pollinate flowering plants, supporting biodiversity and about 35% of global crop production.
3. Seed dispersal: Animals such as birds, mammals, and humans help disperse seeds and fruits, aiding plant reproduction and ecosystem stability.
4. Nutrient cycling and waste recycling: Microorganisms decompose organic matter, recycle nutrients, fix atmospheric nitrogen, and cleanse wastes.
5. Genetic, medical, and industrial benefits: Living organisms provide genes, medicines, enzymes, and technologies like PCR, bioleaching, and phytoextraction.