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 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.

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.

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

• 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 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.

• Meaning → Movement of nutrient elements through the various components of an ecosystem is called Nutrient Cycling or Biogeochemical Cycles.
• Two Types → Nutrient cycles are of two types — Gaseous and Sedimentary.
• Gaseous Cycle → Reservoir is the atmosphere. Examples: Nitrogen cycle and Carbon cycle.
• Sedimentary Cycle → Reservoir is the Earth's crust. Examples: Sulphur cycle and Phosphorus cycle.
• Importance → Nutrient cycling ensures the continuous recycling of nutrients between the biotic (living) and abiotic (non-living) components of an ecosystem, unlike energy, which flows unidirectionally.

• Carbon is a key element of living organisms and is found in air, water, soil, rocks, and oceans (major reservoirs).
• The carbon cycle is the movement of carbon between the atmosphere, organisms, and the Earth through natural processes.
• Photosynthesis fixes CO₂ into organic matter, which passes through food chains.
• CO₂ returns to the atmosphere through respiration, decomposition, and combustion.
• Carbon sinks & humans: Fossil fuels and rocks store carbon, but human activities increase CO₂, causing global warming.

• Phosphorus is an essential element of DNA, ATP, proteins, bones, and teeth, and often acts as a limiting nutrient in ecosystems.
• The main reservoir of phosphorus is rocks, from which phosphates are released slowly by weathering.
• Plants absorb phosphates from soil, and phosphorus moves through the food chain to animals.
• Phosphorus returns to the environment through decomposition, excretion, runoff, and sedimentation, mainly with the help of microbes.
• The phosphorus cycle has no gaseous phase, and excess phosphates from human activities can cause eutrophication in water bodies.

• Ecological (biotic) succession is the gradual and predictable change in species composition and community structure over time, ending in a stable climax community.
• Succession starts with pioneer species, passes through a series of seral stages (sere), and finally reaches a climax community.
• The process follows steps like nudation, invasion, ecesis, aggregation, competition, reaction, and stabilisation.
• During succession, communities modify the environment, making it less suitable for themselves and more suitable for new species.
• Types of succession: Primary succession occurs on bare areas without life and is slow, while secondary succession occurs in previously inhabited areas and is faster.

• 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.