CBSE Class 7 Science Advanced Biology Our Ecosystem Notes Set 01

Our Ecosystem

What is an Ecosystem?

An ecosystem is a natural unit made of all the living and non-living things of one area, which keep interacting with each other. It may be tiny, like the ground under a tree, or huge, like an ocean. The word was first used by Arthur Tansley. The science that studies ecosystems is ecology, a name given by Ernst Haeckel in 1866 from the Greek words oikos (house) and logos (study).

The three great parts of the earth that make up the setting of life are the atmosphere (air), the hydrosphere (water) and the lithosphere (land and rocks).

Components of an Ecosystem

Every ecosystem has biotic (living) and abiotic (non-living) components. Biotic components can reproduce and include microbes, fungi, plants, animals, birds and humans. Abiotic components are the non-living physical and chemical factors that come from soil, water and air, such as water, minerals, air, sunlight and gases. A plant and the water it absorbs are an example of the two interacting.

Types of Ecosystem

All ecosystems fall into two big groups. Terrestrial ecosystems are on land: forest, grassland, tundra, desert and mountain. Aquatic ecosystems are in water, and depending on the salt in the water they are freshwater or marine.

Forest Ecosystem

A forest is a large stretch of land with big trees, animals and microbes. Different organisms live in different layers of the forest. Its main components are:

1. Producers make food, for example plants.
2. Consumers depend directly or indirectly on plants for food, for example deer, rabbit and lion.
3. Decomposers break down dead and decaying matter, for example fungi and bacteria.

Forests are of three kinds: temperate forest (Canada, Europe, USA, China, Japan), tropical rainforest (South-East Asia, West Africa, North America) and taiga or boreal forest (across North America, Europe and Asia).

Temperate Forest

1. Found in both the northern and southern hemispheres, between the tropics and the polar regions.
2. Found where the climate changes a lot through the year.
3. Trees are evergreen (green all year) or deciduous (shed leaves in winter), or a mixture of both.
4. Example: the forests of California, USA.

An epiphyte is a plant that grows on another plant and takes moisture and nutrients from the air and water around it.

Tropical Rainforest

1. Located in the tropics, around the equator.
2. Home to a huge variety of organisms; trees are very tall and densely packed.
3. Heavy rainfall, very little temperature change and warm weather most of the year.
4. Typical plants: orchids, epiphytes and mosses.
5. Largest rainforests: the Amazon basin (South America), the Congo basin (western Africa) and much of south-east Asia.

Taiga or Boreal Forest

1. Circles a large part of the northern hemisphere and is the largest land biome.
2. Called the snow forest; mostly conifers like pine and spruce.
3. Found in the subarctic region: Alaska, Canada, Scandinavia and Siberia.
4. Lies between the tundra (north) and the temperate forests (south).

Grassland Ecosystem

Grasslands are dominated by grasses and get rainfall between that of a desert and a forest, enough for grass but not for many trees. About one-fourth of the earth is grassland. Their fertile soil suits farming and rearing animals. The two types are tropical (warm all year, taller grass because of warmth and steady rain) and temperate (warm summer, very cold winter).

Prairies

1. Flat, fertile grass lands with very little soil erosion.
2. Home to insects, reptiles, amphibians and grazing animals.
3. Temperate grasslands of North America (USA, Canada, Mexico).

Savannah

1. A mixture of woodland and grassland, with a rainy summer and dry winter.
2. Elephants, wildebeest, giraffes and ostriches graze here, hunted by lions, cheetahs, leopards and wild dogs.
3. Found in South Africa, India and Australia; a tropical grassland.

Steppes

1. Dry grassy plains with only enough rain for short grass.
2. Many have been turned into farmland by irrigation.
3. Cattle, horses, goats and sheep are seen; temperate grasslands of Russia, Europe and Asia.

Pampas

1. Temperate climate; the region is called the Patagonia highland.
2. Mostly grasses and herbs; large trees are few.
3. Pampas deer, guinea pig and dwarf mara live here; the puma is the biggest predator, followed by pampas fox, maned wolf and Geoffroy's cat.
4. Found in Argentina and Brazil in Latin America.

Desert Ecosystem

A desert is dry land with very little rain. The Sahara in Africa is the largest desert. Plants such as cactus and thorny weeds have long roots that draw water from deep in the ground. Small animals such as rodents and reptiles hide in burrows by day and come out at night. Dust storms with strong wind are common. Deserts occur on every continent except Europe. There are four types.

Tundra Ecosystem

The tundra is the coldest biome, with frost-shaped land, very low temperature and little rain. Few organisms live here and plants are scarce: shrubs, mosses and grasses. It has two types.

Arctic Tundra

1. Circles the north pole and has cold desert-like conditions.
2. Soil below the surface is permanently frozen (permafrost), containing gravel and little fine material, and holds a lot of methane.
3. Plants are short and grow close together to resist cold and low light.
4. Arctic hares, arctic foxes, wolves, polar bears, bumble bees, blackflies and moths live here (Alaska, northern Canada, Greenland).

Alpine Tundra

1. A biome without trees because of high altitude; it is too cold, harsh and windy for trees.
2. Found on mountains all over the world; snow and ice remain even in summer.
3. Plants are dwarf shrubs; no single animal is typical, but wild mountain goats, Himalayan tahr and yak are seen.

Mountain Ecosystem

This ecosystem has the living things of mountain regions. Climate is harsh at great heights and trees are absent in the alpine zone. The Himalayas are the highest range and Mount Everest the highest peak. Plant and animal life changes with climate, rainfall, altitude and soil: the climate goes from tropical at the foot to permanent ice and snow at the top, and rainfall rises from west to east along the southern front. This variety supports many distinct plant and animal communities.

Aquatic Ecosystem

Water covers about 71 percent of the earth, and organisms in water depend on each other. Corals look like rocks but are animals with hard skeletons of calcium carbonate. The two types are freshwater and marine.

Freshwater Ecosystem

1. Covers 0.78 percent of the earth's surface but holds 41 percent of the world's fish.
2. Lentic (lacustrine): still or slow water such as pools and ponds.
3. Lotic (riverine): flowing water such as streams and rivers.
4. Wetlands: marshy or swampy land, for example Loktak Lake in Manipur.

Marine Ecosystem

1. The largest aquatic ecosystem, with salty water, covering about two-thirds of the earth.
2. Holds a huge number of living things.
3. Includes oceans, seas and river estuaries.

Functions of an Ecosystem

An ecosystem performs three main functions: productivity, energy flow and the nutrient (geo-chemical) cycle.

Productivity and Energy Flow

Energy enters an ecosystem through photosynthesis. Green plants (producers) trap sunlight and change it into chemical energy stored in glucose, which then passes to the organisms that feed on plants. One organism feeding on another in a straight line is a food chain; many food chains linked together form a food web.

The position of an organism in a food chain is its trophic level: producers are in the first level, herbivores (primary consumers) in the second, carnivores (secondary consumers) in the third, and carnivores that eat carnivores (tertiary consumers) in the fourth. Energy decreases from lower to higher levels because each organism spends part of the energy it receives on its own activities, and only the rest passes on.

In this food web, grass is eaten by rabbit, fieldmouse and sparrow; rosehip berries by fieldmouse and blackbird; blackberries by fieldmouse, blackbird and slug. The fox eats the rabbit and fieldmouse, the sparrowhawk eats the fieldmouse and sparrow, and the blackbird eats the slug.

The pyramids show that the producers hold the most energy, and each higher level holds less because energy is lost as heat into the atmosphere at every level.

Nutrient Cycle

The passing and exchange of nutrients among the parts of an ecosystem is the nutrient cycle. Important examples are the carbon, nitrogen and phosphorus cycles.

Carbon Cycle

All living things contain carbon, mainly in carbohydrates. In the air it is present as carbon dioxide. Plants take in CO₂ by photosynthesis, and animals get carbon by eating plants. To keep the CO₂ level steady, carbon is also returned to the air: mainly by respiration of plants and animals, and also by burning fossil fuels (combustion). Dead plants and animals are decayed by decomposers.

Nitrogen Cycle

Nitrogen is essential for life, especially for plants, but the free gas in air cannot be used by living things. It must first be changed into a usable form, which is called nitrogen fixation. Special bacteria (such as Azotobacter) fix nitrogen so plants can use it, while other bacteria do the reverse and return nitrogen to the air as gas.

In the figure: nitrogen gas in the air is converted to nitrates in the soil by nitrogen-fixing bacteria and lightning; roots absorb nitrates and the plants are eaten by animals (feeding); animals add organic remains by death and excretion; decomposition turns these into ammonium compounds; nitrifying bacteria oxidise ammonium to nitrates; and denitrifying bacteria break nitrates down, releasing nitrogen gas back to the air.

Forest

Forests are an essential part of the ecosystem. In ancient times about two-fifths of the earth was forest, but now more than one-third of that has become barren land. Loss of forests has led to a rise in temperature, less rain, heavy soil erosion, floods, extinction of species, loss of forest products and imbalance in the ecosystem.

Significance of Forest

1. Shelter for wildlife.
2. Natural home of many plants, animals and tribal groups.
3. Commercial goods such as timber, honey, animal products and medicines.
4. Control of climate, rainfall and soil erosion.
5. Firewood.

Exploitation of Forest Land

Large-scale commercial use destroys forests. Trees are cleared for buildings, industries and factories as cities grow, and to get land for farming. Wild forest fires also destroy forests, and people cut trees for timber and firewood. Reckless cutting is deforestation. Planting trees again where they were cut is reforestation, and making a large new plantation or an artificial forest is afforestation.

Steps to Conserve Forests

1. Slow down deforestation.
2. Control forest fires.
3. Check forest clearing for farming and housing.
4. Control diseases spreading through forest plants.
5. Use forest resources wisely.

Pollution

Pollution is the entry of harmful substances into the environment, which damages the environment and living things. The harmful substances are called pollutants. There are four types.

1. Air pollution: harmful gases from factories, industries, vehicles and burning fuels released into the air. Breathing it causes respiratory diseases and allergies. Emissions from burning oil and coal (with contaminants such as mercury) can react with oxygen in sunlight or heat to form smog, which can make allergic people seriously ill.
2. Water pollution: industrial and domestic waste entering ponds, rivers and lakes without treatment; washing and bathing in water bodies also pollute them. Polluted water is dangerous to drink or use at home.
3. Soil pollution: dumping of biodegradable wastes (which rot gradually) and non-biodegradable wastes (which do not) on land. Piles of waste breed flies and mosquitoes that spread disease, and decaying waste smells and makes the place unfit for plants, animals and people.
4. Noise pollution: unpleasant, loud and harsh sound that disturbs us, such as a loudspeaker at night, which can cause sleeplessness and headaches. Very high noise can lead to permanent hearing loss. The decibel (dB) is the unit for the intensity of sound.

Prevention of Pollution

Air pollution:

1. Locate smoky factories far from homes.
2. Fit smoke filters or use less polluting raw materials.
3. Encourage public transport to cut vehicle fumes.
4. Prefer CNG (compressed natural gas) to petrol and diesel, as it gives fewer emissions.

Water pollution:

1. Treat sewage at treatment plants before releasing it into water.
2. Use as little fertilizer as possible.
3. Provide good sanitation to people.
4. Discourage throwing waste into water bodies.

Soil pollution:

1. Encourage organic fertilizers on fields.
2. Burn (incinerate) toxic industrial waste before disposal.
3. Separate waste into biodegradable and non-biodegradable before disposal.
4. Do not dump mercury and lead batteries on dumping sites; use alternative lead-free batteries.

Noise pollution:

1. Avoid unnecessary honking on roads.
2. Do not use loudspeakers in open areas at night; the rules allow loudspeakers at night (10 p.m. to 6 a.m.) only inside closed places such as auditoriums, conference rooms, community halls and banquet halls.