An ecosystem is a self-sustaining unit comprising all living organisms (biotic) and non-living factors (abiotic) in an area, interacting with each other. Abiotic components: sunlight, temperature, water, soil, air, minerals. Biotic components: Producers (autotrophs — plants, algae), Consumers (heterotrophs — herbivores, carnivores, omnivores), Decomposers (fungi, bacteria — break down dead organic matter, recycle nutrients). Energy flows unidirectionally: Sun → Producers → Consumers → Decomposers. Nutrients cycle between biotic and abiotic components. Examples of ecosystems: forest, pond, grassland, desert, ocean.

Food chain: linear sequence showing transfer of energy and nutrients from one organism to another. Example: Grass → Grasshopper → Frog → Snake → Eagle. Each level is a trophic level. Producers = 1st trophic level; Primary consumers = 2nd; Secondary consumers = 3rd; Tertiary consumers = 4th. 10% Law (Lindeman's Law): only 10% of energy is transferred from one trophic level to the next — 90% is lost as heat, in metabolic processes, etc. Food web: interconnected food chains in an ecosystem, giving it stability. Biological magnification (biomagnification): concentration of harmful chemicals (e.g., DDT, pesticides) increases at higher trophic levels — humans (top consumers) most affected.

Ozone layer: in the stratosphere (~15–35 km); absorbs harmful UV radiation from the sun. Ozone depletion: caused by CFCs (chlorofluorocarbons) from refrigerants, aerosols. Each CFC molecule destroys thousands of ozone molecules. The ozone hole is most prominent over Antarctica. UV radiation causes: skin cancer, cataracts, reduced immunity, harm to marine ecosystems. Solutions: Montreal Protocol (1987) — international agreement to phase out CFCs. Alternatives: HFCs, natural refrigerants. Waste management: biodegradable waste (food, paper, plant matter) is broken down by microbes; non-biodegradable waste (plastics, metals, pesticides) persists in environment. Reduce, Reuse, Recycle — the 3R approach for waste reduction.

Air pollution: caused by burning fossil fuels, vehicles, industries; leads to smog, acid rain, respiratory diseases. Water pollution: sewage, industrial effluents, agricultural runoff (fertilisers, pesticides); causes eutrophication — algal bloom depletes oxygen, kills aquatic life. Soil pollution: pesticides, plastic waste, industrial waste. Noise pollution: heavy machinery, vehicles, loud speakers — causes hearing loss, stress. Global warming: greenhouse gases (CO₂, CH₄, N₂O, CFCs) trap heat — leads to climate change, rising sea levels, extreme weather. Solutions: use renewable energy, plant trees, reduce fossil fuel use, proper waste disposal, use of electric vehicles, sustainable agriculture.