CBSE Class 10 Science Our Environment Notes Set 04

These CBSE Class 10 Science notes for Chapter 13, Our Environment, cover the ecosystem and its biotic and abiotic components, food chains, trophic levels and the food web, the flow of energy and the 10% law, biological magnification, biodegradable and non-biodegradable waste and its disposal, and the formation and depletion of the ozone layer.

Important Notes and Formulas

Environment and Ecosystem

The environment is our basic life support system. It includes all the biotic (living) and abiotic (non-living) things around us and provides the air, water, food and land that life needs. All living organisms, whether plants, animals, microorganisms or human beings, interact with each other and with their physical surroundings, and this interdependence maintains the balance of nature.

An ecosystem is a self-dependent unit of living organisms (plants, animals and microorganisms) and their non-living environment (soil, air and water). All the interacting organisms in an area together with the non-living constituents of the environment form an ecosystem. The term ecosystem was given by A. G. Tansley in 1935.

Types of Ecosystem

1. Natural ecosystem: It operates in nature by itself without any human interference, for example a forest, pond, lake, grassland, desert or ocean.
2. Artificial ecosystem: It is created and maintained by human beings, for example a crop field, orchard, garden or aquarium. It does not have a self-regulating mechanism.

Components of an Ecosystem

1. Abiotic components are the non-living physical and chemical factors of the environment, such as temperature, rainfall, wind, soil, minerals, light, water and air.

2. Biotic components are the living organisms, which are of three kinds:

1. Producers: They prepare their own food from simple inorganic substances such as carbon dioxide and water, using sunlight and chlorophyll, by photosynthesis. Examples: green plants, blue-green algae and certain bacteria. They are also called autotrophs.
2. Consumers: They depend on the producers for food, directly or indirectly, and are also called heterotrophs. Herbivores (buffalo, goat, deer, grasshopper) eat only plants and are primary consumers. Carnivores (lion, tiger, frog, snake) eat other animals; those that feed on herbivores are secondary consumers and those that feed on other carnivores are tertiary consumers. Omnivores (man, crow, bear) eat both plants and animals.
3. Decomposers: They are microorganisms, mainly bacteria and fungi, that break down the complex organic compounds of dead plants and animals and their wastes into simple inorganic substances.

Importance of decomposers:

1. They act as cleaning agents of the environment by decomposing dead bodies and wastes.
2. They recycle the materials of the biosphere, returning nutrients to the soil, where they are used again by plants.
3. They help to maintain the fertility of the soil.
4. Without decomposers the Earth would be a heap of dead organisms.

Parasites live on or inside another organism, called the host, and obtain their food from it, for example tapeworm.

Food Chain and Trophic Levels

A food chain is the sequence of living organisms in a community in which one organism eats another to transfer food energy. It is a straight line because the flow of energy is unidirectional, from the Sun to the producers and then to a series of consumers.

Each step or level of a food chain at which the transfer of food energy takes place is called a trophic level. The producers are at the first trophic level; they fix solar energy and make it available to the consumers. The herbivores (primary consumers) are at the second, the small carnivores (secondary consumers) at the third and the larger carnivores (tertiary consumers) at the fourth trophic level. The number of individuals is generally greatest at the lowest trophic level.

Other examples: Grass → Insects → Frog → Birds (terrestrial) and Phytoplankton → Zooplankton → Small fish → Large fish (aquatic).

Significance of food chains: The study of food chains helps us to understand the feeding relationships and interactions among organisms in an ecosystem, the transfer of food, energy and nutrients, and the movement of toxic substances and the problem of their biological magnification.

Food Web

The length and complexity of food chains vary greatly. Each organism is generally eaten by two or more other kinds of organisms, which in turn are eaten by several others. A network of food chains interconnected at various trophic levels, forming a number of feeding connections among the different organisms of a community, is called a food web.

A food web made of interlinked food chains

Characteristics of a food web:

1. A food web is made of many interconnected food chains and is never a straight line.
2. It provides alternative pathways of food, so if one source of food is lost the consumer can feed on another.
3. The more complex the web, the more stable the ecosystem; it also allows endangered populations to grow.
4. It keeps a check on the overpopulation of species.

Flow of Energy and the 10% Law

The Sun is the ultimate source of energy for all living organisms. Green plants in a terrestrial ecosystem capture about 1% of the energy of the sunlight that falls on their leaves and convert it into chemical energy of food. When the plants are eaten by primary consumers, a great deal of energy is lost as heat to the environment, and some is used in digestion, work, growth and reproduction. On an average only 10% of the energy of the food eaten is passed on to the next trophic level. This is the 10% law (Lindeman, 1942).

Flow of energy in an ecosystem

The 10% law: the energy decreases at each trophic level

1. The flow of energy is unidirectional. The energy captured by the producers does not return to the Sun, and the energy passed to herbivores does not come back to the producers.
2. Since only about 10% of the energy is available to the next level, the usable energy left after three or four trophic levels is too small to support another level. Hence food chains generally have only three or four steps.
3. Worked example: if the producers contain 1000 J, the herbivores get 100 J, the secondary consumers 10 J and the tertiary consumers 1 J.
4. Going backwards, multiply by 10 at each step: if the lion has 100 J, the deer had 1000 J and the plants had 10,000 J.
5. A shorter food chain gives more energy to the last consumer, which is why vegetarian food habits give more energy.

Biological Magnification

Pesticides and other chemicals used to protect crops are washed down into the soil or into water bodies. They are absorbed by plants along with water and minerals, and aquatic plants and animals take them up from the water, so they enter the food chain. These chemicals are not degraded, so they get accumulated progressively at each trophic level. This increase in the concentration of harmful chemicals at each successive trophic level of a food chain is called biological magnification (biomagnification).

1. Human beings occupy the top level in most food chains, so the maximum concentration of these chemicals accumulates in our bodies.
2. This is why our food grains, vegetables, fruits and even meat contain varying amounts of pesticide residues, which cannot always be removed by washing.
3. Example: DDT sprayed on water bodies gets accumulated in plankton, then in small fish, large fish and finally fish-eating birds in increasing concentrations.

Our Activities and the Environment: Garbage and Waste

Improvements in our lifestyle have resulted in greater amounts of waste, and changes in packaging have made much of it non-biodegradable. Solid waste includes kitchen waste, paper, plastics, glass, metal and wastes of homes, schools, industries and agriculture. When it collects in large amounts, it makes the surroundings dirty and pollutes the soil and water.

Microorganisms (saprophytes) are specific in their action. They break down natural materials such as paper but cannot break down man-made materials such as plastics, so some substances are biodegradable and some are non-biodegradable.

Effects of waste on the environment:

1. Large amounts of biodegradable waste give a foul smell and become a breeding place for flies and mosquitoes, which spread diseases such as cholera, typhoid and malaria.
2. Non-biodegradable waste accumulates in the soil and reduces its fertility, blocks drains, harms animals that swallow it, and gives toxic gases when burnt.
3. Pesticides and heavy metals enter the food chain and undergo biological magnification.

Methods of waste disposal:

1. Landfills: solid waste is dumped in low lying areas and covered with soil. It is not a complete solution, as chemicals may seep into the underground water.
2. Recycling: paper, plastics, metal cans and glass are sent to recycling units and reprocessed into new products.
3. Composting: household wastes such as vegetable and fruit peels, left-over food and dry leaves are buried in a pit and converted into compost, which is used as manure.
4. Incineration: burning of waste at a high temperature to ash, used for hospital and chemical wastes; the ash is disposed of in a landfill.
5. Biogas production: biodegradable waste can be used in biogas plants to produce biogas and manure.

What we can do: separate biodegradable and non-biodegradable waste (green bin for wet waste, blue bin for dry waste), reduce, reuse and recycle, carry cloth or jute bags instead of plastic bags, and use paper cups instead of plastic cups or kulhads, because making kulhads on a large scale would cause the loss of fertile top soil.

The Ozone Layer and its Depletion

Ozone (O₃) is a molecule formed by three atoms of oxygen. Ordinary oxygen (O₂) is essential for aerobic life, but ozone is a deadly poison near the ground. However, at the higher levels of the atmosphere, in the stratosphere, ozone forms a layer that shields the surface of the Earth from the harmful ultraviolet (UV) radiation of the Sun.

Formation of ozone: The high-energy UV radiation from the Sun splits some oxygen molecules into free oxygen atoms. These atoms are very reactive and combine with other oxygen molecules to form ozone.

O₂ → O + O (UV radiation)     O + O₂ → O₃

Depletion of ozone: The amount of ozone in the atmosphere began to drop sharply in the 1980s. A decline in the thickness of the ozone layer over a limited area is called an ozone hole; the largest one appears over Antarctica. This was linked to synthetic chemicals called chlorofluorocarbons (CFCs), which are used as coolants in refrigerators and air conditioners, in fire extinguishers and in aerosol sprays. UV radiation splits the CFCs in the stratosphere and releases chlorine atoms, which break ozone down into oxygen.

Effects of ozone depletion:

1. More UV radiation reaches the Earth, causing skin cancer, sunburn and early ageing.
2. UV rays damage the eyes and cause cataract.
3. They weaken the immune system, reducing the body’s resistance to diseases.
4. They reduce the rate of photosynthesis in plants and harm aquatic life and crops.

Steps to limit the damage: In 1987 the United Nations Environment Programme (UNEP) succeeded in forging an agreement among its member nations to freeze CFC production at the 1986 level (the Montreal Protocol). CFC-free refrigerators are now compulsory throughout the world, and CFCs have been replaced by hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), which are less harmful to the ozone layer.

Quick Revision Points

1. Ecosystem = biotic components (producers, consumers, decomposers) + abiotic components.
2. Producers are always at the first trophic level; herbivores at the second.
3. Only 10% of the energy passes to the next trophic level; plants capture only about 1% of sunlight.
4. Energy flow is unidirectional; matter is recycled by decomposers.
5. Biological magnification is the build-up of non-degradable chemicals at higher trophic levels, with the highest level in human beings.
6. Ozone (O₃) absorbs UV radiation; CFCs deplete it; UNEP froze CFC production at 1986 levels in 1987.
7. Separate biodegradable and non-biodegradable waste; compost, recycle and reuse.