Revision: Ecosystems and Energy Flow Biology HSC Science (General) 12th Standard Board Exam Maharashtra State Board

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.

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.

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.

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.

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.

• 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.

• 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.

• 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.

• 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.