CBSE Class 10 Science Our Environment Notes Set 02

Weightage Extract

• Maximum total weightage is of Topic 13.1 Ecosystem - What are its Components?
• Maximum MCQ, VSA and SA II type questions were asked from Topic 13.1 Ecosystem - What are its Components?
• Maximum SA I type questions were asked from Topic 13.2 How do Our Activities Affect the Environment?

QUICK RECAP

Environment : It is the sum total of external factors and conditions that affect organisms without becoming an integral part of them. It includes everything surrounding the organism, i.e., both non-living (abiotic) and living (biotic) components. The abiotic components are the physical factors such as climatic factors (e.g., temperature, light, wind, humidity, precipitation, water, etc.) and edaphic factors (e.g., soil texture, substratum, topography, background, minerals, pH, etc.). The biotic components, on the other hand, include all kinds of living organisms.

Ecosystem and its components: An ecosystem is self-supporting unit of living organisms (plants, animals and decomposers), and their non-living environment (soil, air and water).

• Natural ecosystem : They operate in nature by themselves without any human interference, e.g., desert, grassland, pond, lakes, etc.
• Artificial ecosystem : These are man made and also maintained by them, e.g., cropfields, aquariums, etc.

Abiotic components of ecosystem

These include inorganic substances, organic compounds and climatic factors.

• Inorganic substances: These substances, e.g., carbon, nitrogen, oxygen, calcium, phosphorus, etc., and their compounds (water, carbon dioxide, etc.) constitute the main abiotic components. These occur either in the form of compounds dissolved in water, in the soil or in free state in the air.
• Organic compounds: These include carbohydrates, proteins, lipids, nucleic acids, etc. These are present in living organisms and dead organic matter. The dead organic matter is broken down by the action of decomposers (e.g., bacteria, fungi on decayed matter) into inorganic substances for their recycling.
• Climatic factors: These include light, temperature, humidity, wind, rainfall, water, etc., and also edaphic factors (e.g., soil and substrate, topography, mineral, pH, etc.)

Biotic components of ecosystem

The biotic community (or living community) of an ecosystem includes three types of organisms:

• Producers (or autotrophs) are organisms which synthesise their own food. All the green plants and some blue-green algae are producers. Biomass is the total amount of living organic matter in an ecosystem. Mostly producers have maximum level of biomass.
• Consumers (or heterotrophs) are organisms which are dependent on others for food directly or indirectly. All the animals are consumers. Consumers can be further divided into three groups:
  • Those animals which eat only plants are called herbivores. Herbivores are primary consumers. E.g., cow, buffalo, goat, sheep, horse, deer, camel, etc.
  • Those animals which eat only the flesh of other animals are called carnivores. The small carnivores which feed on herbivores are called secondary consumers. The large carnivores which feed upon the small carnivores are called tertiary consumers. E.g., lion, tiger, vulture, etc.
  • Those animals which eat both, plants and animals, are called omnivores. E.g., dog, crow, sparrow, bear, mynah and ant.
• Decomposers (or saprotrophs) are those which consume the dead remains of other organisms. Certain bacteria and fungi are decomposers and they replenish the soil and increase its fertility.

Food chains

The sequential interlinking of organisms involving transfer of food energy from the producers, through a series of organisms with repeated eating and being eaten is called the food chain. In ecosystem, different food chains may have two, three, four or maximum five trophic levels. A food chain may end at the:

• herbivore (primary consumer) level
• primary carnivore (secondary consumer) level
• secondary carnivore (tertiary consumer) level or
• tertiary carnivore (quaternary consumer) level.

Example: \( \text{Grass (Producer)} \rightarrow \text{Grasshopper (Herbivore)} \rightarrow \text{Frog (Primary carnivore)} \rightarrow \text{Snake (Secondary carnivore)} \rightarrow \text{Eagle (Top or tertiary carnivore)} \)

• The distinct sequential steps in the food chain where transfer of energy occurs are referred to as different trophic levels. For example, green plants (producers) form the first trophic level. The plant eaters (herbivores), belong to the second trophic level and the flesh eaters (carnivores), represent the third trophic level.
• A food chain is always straight and proceeds in a progressive straight line. In a food chain, there is unidirectional flow of energy from sun to producers and subsequently to series of consumers.
• The quantum of available energy in a food chain successively decreases at each trophic level as a result of energy which is wasted as heat in metabolic reactions such as respiration, excretion, etc.

Food web

In nature, food chains do not operate in isolation. It is so because in natural environment, each organism is generally eaten by two or more kinds of organisms which, in turn, are eaten by several other organisms. Thus, instead of straight line food chain, the relationship between organisms forms a sort of interlinking pattern due to alternate pathways, called a food web.

• Food web is a network of food chains which become interlinked at various trophic levels forming a number of feeding connections amongst various organisms in a biotic community.
• Food webs provide stability to ecosystem, keep a check on the population of species of animals and plants having high reproductive rates and help in development of ecosystem.
• Some of pesticides (e.g., D.D.T.), industrial chemicals, heavy metals and radioactive substances are very harmful. This is so because these enter the food chains and their concentration goes on increasing from one trophic level to the next. This phenomenon is known as biomagnification which results in many harmful effects in human beings and other animals.

Ecological pyramids

An ecological pyramid is a graphic representation of an ecological parameter (numbers or biomass or amount of accumulated energy) at different trophic levels in a food chain in an ecosystem. Ecological pyramids can be of following types on the basis of ecological parameters.

• Pyramid of numbers : It is the graphic representation depicting the arrangement of number of individuals of different trophic levels in a food chain of an ecosystem. The shape of pyramid of numbers may be upright (e.g., in pond ecosystem and in grassland ecosystem) or inverted (e.g., in parasitic food chain) depending upon whether the number of organisms gradually decreases or increases at successive trophic levels from producers onwards to top consumers.
• Pyramid of energy: A pyramid of energy is a graphic representation of amount of energy per unit area at different trophic levels of a food chain in an ecosystem. The pyramid of energy is always upright because the available energy is the highest at producer level. According to second law of thermodynamics, there is gradual decrease in available energy at successive trophic levels.

Flow of energy in an ecosystem

In an ecosystem, energy comes from the sunlight which is further converted from one form to another. Energy gets continuously transferred from one trophic level to the other by repeated food consumption in a food chain. Loss of energy occurs as heat at each step of energy transfer (trophic level) in the environment. At each trophic level, organisms utilise energy in respiration, reproduction, growth, etc.)

• The green plants in a terrestrial ecosystem capture about one per cent of the energy of sunlight that falls on their leaves and convert it into food energy.
• Ten percent law given by Lindeman in 1942 states that during transfer of energy from one trophic level to the next level, only about 10% energy is available to the higher trophic level and the remaining 90% is lost in respiration and heat. This limits the trophic levels to 3 or maximum of 4 or 5 in food chain.

Ozone depletion

Ozone is a triatomic molecule made up of three atoms of oxygen (\( \text{O}_3 \)). Very little quantity of ozone is present in the lower part of atmosphere, called troposphere. However, good amount of ozone is present in the upper part of atmosphere, called stratosphere.

• Though ozone is present throughout the stratosphere, its concentration is maximum 23-25 km above equator and 11-18 km above poles. This rich zone of ozone in the stratosphere is called ozone layer or ozone shield because this region intercepts high energy ultraviolet (UV) radiations to reach the Earth’s surface.
• In the stratosphere, ozone is being photo-dissociated and generated simultaneously by the absorption of harmful ultraviolet (UV) radiations coming from the Sun.
  \( \text{O}_3 \xrightarrow{\text{UV radiations from Sun}} \text{O}_2 + [\text{O}] \)
• The two reactions (photodissociation of \( \text{O}_3 \) and its generation) are in equilibrium thereby maintaining steady concentration of ozone in the stratosphere (18-50 Km above sea level).
• The thinning of ozone layer is known as ozone depletion. The amount of ozone in the atmosphere began to drop sharply in 1980s. Ozone shield is important for the survival and existence of life on Earth but it is being depleted by air pollutants like chlorofluorocarbons (CFCs), methane (\( \text{CH}_4 \)) and oxides of nitrogen (\( \text{NO}_x \)).
• Chlorofluorocarbons (CFCs) are synthetic, harmful chemicals which are widely used in refrigerators and air conditioners as coolants, in fire extinguishers, in aerosol sprayers and as propellants. Once released in the air, these harmful chemicals produce active chlorine (Cl and ClO radicals) in the presence of UV radiations.
• These radicals, through chain reactions, convert ozone into oxygen. Due to this, the ozone layer in the upper atmosphere (i.e., stratosphere) becomes thinner.
• The thinning of ozone layer allows more UV radiations to pass through it which then strike the Earth. UV radiations reaching the Earth have following harmful effects on man, animals and plants:
  • In humans, UV radiations may cause sun burn, skin cancer, eye cataract, etc.
  • Damage to immune system in human beings and animals and hence lowering the body’s resistance to diseases.
  • Increased mortality of developing embryos in the mother’s uterus.
  • In plants, UV radiations may interfere with the process of photosynthesis. Decreased photosynthesis would increase the concentration of \( \text{CO}_2 \) in the atmosphere which will result in global warming.
• In 1987, the United Nations Environment Programme (UNEP) forged an agreement between nations to limit CFCs production to half the level of 1986. It is now mandatory for all the manufacturing companies to make CFC - free refrigerators throughout the world.

Types of wastes

In our daily lives, we generate a lot of useless materials and discard them. The useless left over or discarded materials are termed as wastes. The waste materials can be gaseous (e.g., automobile exhausts, smoke from chimneys of industries and house), liquid (e.g., effluents from industries, sewage water) or solid (e.g., kitchen waste, cow dung, human excreta, trash and rubbish, etc.)

• Solid wastes : These accumulate in the environment due to human activities and are the main sources of soil pollution. The affluent life style has forced people to start using more of disposable articles, e.g., plastic cups, bags, etc., which keep on accumulating in the environment and lie undecomposed, there by negatively affecting the environment.
• Solid wastes can be categorised into two types: biodegradable and non-biodegradable wastes.
  • Biodegradable wastes can be easily degraded by natural means, i.e., by the action of microorganisms such as bacteria and fungi, complex organic matters can be converted into simpler, harmless substances in due course of time. However, biodegradable wastes can also act as pollutants when their quantity becomes large, i.e., they start accumulating as they are not broken down into simpler substances by the action of microorganisms at the pace at which they are generated. These affect the human life in various ways:
    • (i) Flies breed on heaps of solid wastes containing biodegradable substances. These carry the germs and spread diseases such as diarrhoea, typhoid, tuberculosis, cholera, conjunctivitis, etc.
    • (ii) These biodegradable wastes may also block the drains, creating pools of water which become the breeding sites of mosquitoes. The latter are the carriers of diseases like malaria, chikungunya, etc.
  • Non-biodegradable wastes cannot be degraded by natural means, i.e., by the action of microorganisms, into simpler, harmless substances in due course of time.
• Excessive use of fertilisers and pesticides, and dumping of industrial chemical wastes affect the soil fertility and subsequently reduces the crop yield. The soil, thus, may become acidic or alkaline.
• Modes of solid waste disposal :
  • (i) In urban areas, majority of the solid wastes are buried in low lying areas to level the uneven surface of land. This method of waste disposal is commonly called landfilling.
  • (ii) Number of solid wastes (e.g., paper, plastics, metals, etc.) can be recycled by sending them to respective recycling units. This is known as recycling of wastes.
  • (iii) Preparation of compost : Household waste should be segregated and the biodegradable wastes such as fruit peel and vegetables, left-over food, fallen dead leaves of plants, etc., should be converted to compost and used as manure.