Class 8 Science · Chapter 12 NotesHow Nature Works in Harmony

A habitat is the place where an organism lives, and it provides the conditions needed for survival. Every habitat has two kinds of components. Living things such as plants, animals and microorganisms are the biotic components. Non-living things such as air, water, soil, sunlight and temperature are the abiotic components. A pond and a forest are different habitats because they offer different living conditions and contain different organisms. A fish gets food from small plants and animals and oxygen from water, while a snake active at night and a rodent active during the day share a forest but face different conditions. This shows how organisms coexist in the same habitat by using resources in different ways.

• Habitatplace where an organism lives and grows.
• Biotic componentsplants, animals, microorganisms.
• Abiotic componentsair, water, soil, sunlight, temperature.

A population is a group of organisms of the same kind living together in a habitat at a given time, such as many fish of one kind in a pond. A community is made up of different populations sharing the same habitat, including plants, animals and microorganisms that interact and depend on one another. When the biotic community interacts with the abiotic components of an area, the whole system forms an ecosystem. Ecosystems may be aquatic (ponds, rivers, lakes) or terrestrial (forests, grasslands, farms). A large banyan tree can itself be an ecosystem, so ecosystems can be small or large. Farmland is a human-made ecosystem, and ecosystems can also overlap, as when a river flows through forests and fields.

• Populationsame kind of organisms in one habitat at one time.
• Communitydifferent populations living together in a habitat.
• Ecosystembiotic community plus abiotic components interacting together.

Green plants make their own food by photosynthesis, so they are called producers or autotrophs (auto means self, troph means food). Animals cannot make their own food and depend on other organisms, so they are consumers or heterotrophs. Consumers that eat only plants are herbivores, such as deer and hare. Those that eat only animals are carnivores, such as leopards. Those that eat both plants and animals are omnivores, such as crows, foxes and mice. Decomposers, also called saprotrophs, are organisms like fungi and bacteria that break down dead plants and animals into simpler substances. This returns nutrients to the soil, so in nature nothing is wasted and everything is reused.

• Producers (autotrophs)make their own food by photosynthesis.
• Consumers (heterotrophs)herbivores, carnivores, omnivores.
• Decomposers (saprotrophs)break down dead matter and recycle nutrients.

A food chain is a simple sequence showing who eats whom in an ecosystem, for example grass, then grasshopper, then frog, then snake, then eagle. Each organism has a fixed position called a trophic level. Producers such as green plants are at the first trophic level, herbivores at the second, small carnivores at the third, and large carnivores at the next level. In a real ecosystem, food chains are interlinked because one organism may be eaten by two or more kinds of organisms. This network of interconnected food chains is called a food web. Food chains and food webs show how energy and matter pass from one organism to another through feeding relationships.

• Food chainlinear sequence of who eats whom.
• Food webseveral food chains interlinked in an ecosystem.
• Trophic levelposition of an organism in a food chain.

Organisms interact in many ways besides feeding. They compete for common resources such as food, water, space and sunlight. Competition controls population size and keeps the ecosystem balanced; without it, one species could multiply too much. Some organisms live in close relationships. In mutualism both benefit, as when honeybees get nectar and flowers get pollinated. In commensalism one benefits and the other is unaffected, as when an orchid gets support from a tree branch. In parasitism one benefits and the other is harmed, as when a tick feeds on a dog's blood and causes skin irritation. These interactions form a complex web of life that keeps populations and resources stable.

• Competitionorganisms struggle for the same resources.
• Mutualismboth organisms benefit.
• Commensalismone benefits, the other is unaffected.
• Parasitismone benefits, the other is harmed.

Ecosystems stay in balance through interactions, but this balance is dynamic and can be disturbed by natural or human-made changes. One change can lead to many others. For example, if pond plants die due to pollution, less oxygen is produced, fish numbers fall, insects increase, and farmers may use more pesticides. Human activities such as deforestation, pollution, overuse of resources, invasive species and unsustainable land use threaten forests, rivers, wetlands and coastal areas. The Sundarbans, the world's largest mangrove forest, is a World Heritage Site but is threatened by cutting of mangroves, illegal hunting and pollution. Protected areas such as national parks, wildlife sanctuaries and biosphere reserves help conserve habitats and wildlife for future generations.

• Ecosystem balance is dynamic and can be disrupted.
• Threatsdeforestation, pollution, overuse of resources, invasive species.
• Conservationprotected areas like national parks and sanctuaries.

Ecosystems support human life by providing fresh air, fertile soil, food, fibres, timber, medicines, water and climate regulation, and they also have aesthetic and recreational value. Farming is a major livelihood in India, but some modern practices are unsustainable. The Green Revolution increased food production using tractors, machines, synthetic fertilisers and pesticides, but overuse of chemicals, excessive groundwater extraction and monoculture can degrade soil. Overuse of synthetic fertilisers may reduce friendly soil microorganisms and humus, making soil prone to erosion, and can reduce natural predators of pests. Organic and natural farming methods aim to reduce synthetic inputs and support sustainable farming with minimal interference in natural ecosystems.

• Ecosystem servicesair, water, food, medicine, climate regulation.
• Green Revolutionuse of machines, fertilisers and pesticides to raise production.
• Sustainable farmingorganic and natural methods that protect soil and biodiversity.