Class 10 Science · Chapter 13 NotesOur Environment

All living organisms in an area interact with each other and with the non-living surroundings, maintaining a balance in nature. This interacting system is called an ecosystem. An ecosystem has two parts: biotic components, which include all living organisms such as plants, animals, microorganisms and human beings; and abiotic components, which include physical factors like temperature, rainfall, wind, soil and minerals. A garden, forest, pond or lake is an ecosystem. Forests, ponds and lakes are natural ecosystems, while gardens and crop-fields are human-made or artificial ecosystems. In an aquarium, if we add aquatic plants and animals along with fish, it can become a self-sustaining system, but it still needs cleaning once in a while.

• Ecosystem = interacting organisms + non-living surroundings
• Biotic componentsliving organisms
• Abiotic componentstemperature, rainfall, wind, soil, minerals
• Natural ecosystemsforests, ponds, lakes
• Human-made ecosystemsgardens, crop-fields, aquarium

Organisms are grouped by how they obtain their food. Producers are green plants and certain bacteria that make organic compounds like sugar and starch from inorganic substances using the radiant energy of the Sun in the presence of chlorophyll. Consumers depend on producers directly or indirectly. Herbivores eat plants directly, carnivores eat other animals, omnivores eat both plants and animals, and parasites live on or inside other organisms. Decomposers, mainly bacteria and fungi, break down the dead remains and waste products of organisms. They convert complex organic substances into simple inorganic substances that go into the soil and are reused by plants. Without decomposers, garbage and dead organisms would pile up and the natural replenishment of soil would not take place.

• Producersgreen plants and certain bacteria that photosynthesise
• Consumersherbivores, carnivores, omnivores, parasites
• Decomposersbacteria and fungi that break down dead remains
• Decomposers return simple inorganic substances to the soil

A series of organisms feeding on one another forms a food chain. Each step or level of a food chain is called a trophic level. Producers or autotrophs are at the first trophic level; they fix solar energy and make it available to consumers. Herbivores or primary consumers are at the second trophic level, small carnivores or secondary consumers at the third, and larger carnivores or tertiary consumers at the fourth. In nature, one organism is usually eaten by two or more kinds of organisms, so food chains branch and interconnect to form a food web. Food chains generally have only three or four steps because a large amount of energy is lost at each step.

• Food chainseries of organisms feeding on one another
• Trophic leveleach step of a food chain
• First trophic levelproducers
• Second trophic levelherbivores (primary consumers)
• Third trophic levelsmall carnivores (secondary consumers)
• Fourth trophic levellarger carnivores (tertiary consumers)
• Food webinterconnected food chains

Energy flows from the Sun to producers and then to consumers and decomposers. Green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight falling on their leaves and convert it into food energy. When primary consumers eat plants, a great deal of energy is lost as heat, some is used in digestion and work, and only about 10% of the food eaten is turned into the consumer's own body and made available to the next level. This 10% can be taken as the average value for organic matter reaching the next trophic level. The flow of energy is unidirectional: energy captured by autotrophs does not revert to the Sun, and energy passed to herbivores does not return to autotrophs. Harmful chemicals like pesticides are non-degradable and get accumulated progressively at each trophic level; since humans occupy the top level in many food chains, the maximum concentration accumulates in our bodies. This phenomenon is called biological magnification.

• Green plants capture about 1% of sunlight energy falling on leaves
• About 10% of food eaten is turned into body mass at each level
• Energy flow is unidirectional
• Energy available diminishes at each trophic level
• Biological magnificationprogressive accumulation of non-degradable chemicals at each trophic level

Ozone (O₃) is a molecule formed by three atoms of oxygen. While oxygen (O₂) is essential for aerobic life, ozone is a deadly poison. However, at higher levels of the atmosphere, ozone shields the Earth's surface from harmful ultraviolet (UV) radiation from the Sun, which can cause skin cancer in human beings. Ozone is formed when high-energy UV radiation splits molecular oxygen (O₂) into free oxygen atoms, which then combine with molecular oxygen to form ozone. The amount of ozone in the atmosphere began to drop sharply in the 1980s. This decrease has been linked to synthetic chemicals like chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers. In 1987, the United Nations Environment Programme (UNEP) succeeded in forging an agreement to freeze CFC production at 1986 levels. It is now mandatory for all manufacturing companies to make CFC-free refrigerators throughout the world.

• Ozone (O₃)three atoms of oxygen
• Ozone layer shields Earth from UV radiation
• UV radiation splits O₂ into free oxygen atoms; they combine with O₂ to form O₃
• CFCs used in refrigerants and fire extinguishers deplete ozone
• 1987 UNEP agreement froze CFC production at 1986 levels

Our daily activities generate a lot of waste material. Improvements in lifestyle have increased the amount of waste, and changes in packaging have made much of it non-biodegradable. Substances that are broken down by biological processes are biodegradable, such as kitchen waste, paper and cotton. Substances that are not broken down by biological processes are non-biodegradable, such as plastics. Non-biodegradable substances may be inert and persist in the environment for a long time, or they may harm various members of the ecosystem. Enzymes are specific in their action, so human-made materials like plastics are not broken down by bacteria or other saprophytes. They may be acted upon by physical processes like heat and pressure, but under ambient conditions they persist for a long time. Proper segregation and disposal of biodegradable and non-biodegradable waste is essential to reduce environmental problems.

• Biodegradablebroken down by biological processes (e.g., kitchen waste, paper)
• Non-biodegradablenot broken down by biological processes (e.g., plastics)
• Non-biodegradable waste persists in the environment for a long time
• Enzymes are specific in action, so plastics are not easily decomposed
• Segregation of waste helps in proper disposal