Our Environment — Class 10 Science
Science · Chapter 13
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Chapter summary
Key ideas & terms
- Ecosystem
- A community of organisms interacting with their physical environment.
- Food chain
- A sequence showing the flow of energy from one organism to the next.
- Trophic level
- A step or feeding level in a food chain.
- Biodegradable waste
- Waste that can be broken down by natural processes.
Extra questions & answers
Why are food chains usually limited to three or four trophic levels?
Only about 10% of energy passes to the next level, so too little energy remains to support many levels.
Why is the depletion of the ozone layer a cause for concern?
Ozone absorbs harmful ultraviolet radiation; its depletion lets more UV reach Earth, causing skin cancer, cataracts and ecological damage.
What is an ecosystem? Name its two components.
A community of interacting organisms together with their physical environment; its two components are the biotic (living) and abiotic (non-living) components.
What are producers, consumers and decomposers?
Producers (green plants) make food by photosynthesis; consumers eat other organisms; decomposers (bacteria and fungi) break down dead matter and recycle nutrients.
What is a food chain and a trophic level?
A food chain is a series of organisms showing who eats whom and how energy flows; each step or feeding level in it is called a trophic level.
State the ten per cent law of energy flow.
Only about 10% of the energy at one trophic level is passed on to the next; the rest is lost as heat, which limits food chains to three or four levels.
Differentiate between biodegradable and non-biodegradable substances with examples.
Biodegradable substances are broken down by natural processes (e.g. paper, vegetable peels), while non-biodegradable substances are not (e.g. plastics, DDT) and cause pollution.
Why is the ozone layer important, and what damages it?
The ozone layer absorbs the sun’s harmful ultraviolet radiation; chemicals such as CFCs (chlorofluorocarbons) deplete it, increasing the risk of skin cancer and cataracts.
Why are food chains usually not very long?
Because only about 10% of the energy passes to each higher level, too little energy is left to support many trophic levels.
How can the problems caused by non-biodegradable waste be reduced?
By reducing their use, reusing and recycling materials, segregating waste, and using eco-friendly biodegradable alternatives.
Activity 13.1
You might have seen an aquarium. Let us try to design one.
An aquarium can be designed using a sufficiently large transparent container filled with clean water. It should provide free swimming space and contain suitable aquatic plants, fish, decomposers and non-living components such as water, dissolved oxygen and minerals.
What are the things that we need to keep in mind when we create an aquarium? The fish would need a free space for swimming (it could be a large jar), water, oxygen and food.
The aquarium must have enough space for the fish to swim, clean water, dissolved oxygen, suitable food, aquatic plants and appropriate light and temperature. The number and kinds of organisms should be balanced so that they can coexist.
We can provide oxygen through an oxygen pump (aerator) and fish food which is available in the market.
An aerator can supply oxygen and fish food can provide nutrition. Aquatic plants may also release oxygen during photosynthesis and help support the aquarium ecosystem.
If we add a few aquatic plants and animals it can become a self-sustaining system. Can you think how this happens? An aquarium is an example of a human-made ecosystem.
Aquatic plants act as producers by making food and releasing oxygen. Animals consume plants or other organisms, while decomposers break down wastes and dead matter and return nutrients to the water. These interactions allow matter to be recycled and make the aquarium partly self-sustaining.
Can we leave the aquarium as such after we set it up? Why does it have to be cleaned once in a while? Do we have to clean ponds or lakes in the same manner? Why or why not?
An aquarium cannot usually be left completely unattended because uneaten food, wastes and dead organisms may accumulate faster than they can be decomposed, making the water unsuitable. It therefore needs occasional cleaning. Natural ponds and lakes generally do not require cleaning in the same way because they are larger, more complete ecosystems with many producers, consumers and decomposers that recycle materials naturally, although excessive human pollution can disturb this balance.
Activity 13.2
While creating an aquarium did you take care not to put an aquatic animal which would eat others? What would have happened otherwise?
Yes, the feeding habits of the animals should be considered. If a predator were placed with suitable prey, it could eat the other animals, reduce their population and disturb the balance of the aquarium.
Make groups and discuss how each of the above groups of organisms are dependent on each other.
Producers prepare food and supply energy to consumers. Consumers obtain food directly or indirectly from producers and may also help maintain population balance. Decomposers break down dead organisms and wastes, returning simple inorganic substances to the environment for reuse by producers. Thus, all three groups are interdependent.
Write the aquatic organisms in order of who eats whom and form a chain of at least three steps. → →
Aquatic plants → small fish → large fish.
Would you consider any one group of organisms to be of primary importance? Why or why not?
No single group can be considered independently important because producers, consumers and decomposers perform connected roles. Removing any one group would disrupt energy flow, nutrient recycling and the balance of the ecosystem.
Activity 13.3
Newspaper reports about pesticide levels in ready-made food items are often seen these days and some states have banned these products. Debate in groups the need for such bans.
A ban may be justified when pesticide residues exceed safe limits because such chemicals can enter food chains, accumulate in organisms and reach high concentrations at higher trophic levels. However, decisions should be based on reliable testing, proper safety standards and strict monitoring. Manufacturers should also be required to identify and reduce the source of contamination.
What do you think would be the source of pesticides in these food items? Could pesticides get into our bodies from this source through other food products too?
The pesticides may come from crops treated with agricultural chemicals, contaminated water, animal feed or ingredients containing pesticide residues. Yes, they can also enter our bodies through grains, fruits, vegetables, milk, eggs and meat because they pass through food chains and may undergo biological magnification.
Discuss what methods could be applied to reduce our intake of pesticides.
Pesticide intake can be reduced by limiting unnecessary pesticide use, adopting safer pest-control methods, monitoring residues in food, washing and peeling produce where appropriate, avoiding contaminated products and enforcing safe limits for agricultural chemicals. Since some residues occur inside food tissues, washing alone may not remove them completely.
QUESTIONS
1. What are trophic levels? Give an example of a food chain and state the different trophic levels in it.
Each step in a food chain at which energy is transferred is called a trophic level. For example: grass → deer → tiger. Grass is the producer at the first trophic level, deer is the primary consumer at the second trophic level, and tiger is the secondary consumer at the third trophic level.
2. What is the role of decomposers in the ecosystem?
Decomposers such as bacteria and fungi break down dead organisms and waste products into simpler inorganic substances. These substances return to the soil or water and can be reused by producers. Decomposers therefore recycle nutrients and prevent dead matter from continuously accumulating.
Activity 13.4
Find out from the library, internet or newspaper reports, which chemicals are responsible for the depletion of the ozone layer.
The chapter identifies chlorofluorocarbons, or CFCs, as synthetic chemicals responsible for ozone-layer depletion. They were used as refrigerants and in fire extinguishers.
Find out if the regulations put in place to control the emission of these chemicals have succeeded in reducing the damage to the ozone layer. Has the size of the hole in the ozone layer changed in recent years?
The chapter states that an international agreement was made in 1987 to freeze CFC production at 1986 levels and that manufacturers were required to make CFC-free refrigerators. It does not provide recent measurements of the ozone hole, so its recent change cannot be determined from this chapter alone.
Activity 13.5
Collect waste material from your homes. This could include all the waste generated during a day, like kitchen waste (spoilt food, vegetable peels, used tea leaves, milk packets and empty cartons), waste paper, empty medicine bottles/strips/bubble packs, old and torn clothes and broken footwear.
The collected waste should include a representative mixture of biodegradable materials, such as food remains, vegetable peels, tea leaves and paper, and non-biodegradable materials, such as plastic packets, medicine strips, bottles and some synthetic materials.
Bury this material in a pit in the school garden or if there is no space available, you can collect the material in an old bucket/flower pot and cover with at least 15 cm of soil.
The materials should be placed in a pit, bucket or flower pot and covered with at least 15
textcm of soil so that their changes under moist environmental conditions can be observed safely.
Keep this material moist and observe at 15-day intervals.
The material should be kept moist and examined at intervals of 15 days. Changes in colour, texture, smell, shape and degree of decomposition should be recorded without directly handling unsafe waste.
What are the materials that remain unchanged over long periods of time?
Materials such as plastic packets, medicine strips, glass bottles and other non-biodegradable substances are likely to remain largely unchanged for long periods.
What are the materials which change their form and structure over time?
Spoilt food, vegetable peels, used tea leaves, paper and other biodegradable materials change their form and structure as microorganisms decompose them.
Of these materials that are changed, which ones change the fastest?
Soft organic kitchen wastes, such as spoilt food, vegetable peels and used tea leaves, generally change fastest because decomposers can break them down readily.
Activity 13.6
Use the library or internet to find out more about biodegradable and non-biodegradable substances.
Biodegradable substances can be broken down by biological processes involving organisms such as bacteria and fungi. Examples include food waste, leaves and paper. Non-biodegradable substances are not broken down in this manner and may persist for long periods; examples include many plastics, glass and synthetic materials.
How long are various non-biodegradable substances expected to last in our environment?
The chapter does not give exact lifetimes. It states that non-biodegradable substances persist in the environment for a long time because bacteria and other saprophytes cannot break them down under ordinary environmental conditions.
These days, new types of plastics which are said to be biodegradable are available. Find out more about such materials and whether they do or do not harm the environment.
Plastics described as biodegradable are intended to break down more easily through biological processes under suitable conditions. Their environmental effect depends on whether they actually decompose completely and whether the substances formed are harmless. The chapter does not provide enough information to judge particular products, so their claims would need to be tested before they could be considered environmentally safe.
QUESTIONS
1. Why are some substances biodegradable and some non-biodegradable?
Decomposer organisms possess specific enzymes that can break down certain natural substances. Such substances are biodegradable. Many human-made substances, including many plastics, cannot be acted upon by these enzymes under ordinary environmental conditions and are therefore non-biodegradable.
2. Give any two ways in which biodegradable substances would affect the environment.
Biodegradable substances can be decomposed and return nutrients to the soil, supporting natural nutrient cycles. However, if they accumulate in large quantities, their decay can produce foul smells, attract disease-carrying organisms and pollute the surroundings.
3. Give any two ways in which non-biodegradable substances would affect the environment.
Non-biodegradable substances persist and accumulate in soil and water, creating long-term pollution. They may also enter food chains, harm organisms and become progressively concentrated at higher trophic levels.
Activity 13.7
Find out what happens to the waste generated at home. Is there a system in place to collect this waste?
This must be observed locally. In a suitable system, household waste is placed in separate containers and collected by an authorised local service. Biodegradable and non-biodegradable waste should be segregated before collection.
Find out how the local body (panchayat, municipal corporation, resident welfare association) deals with the waste. Are there mechanisms in place to treat the biodegradable and non-biodegradable wastes separately?
The exact arrangement depends on the locality and must be verified from the local body. A proper system collects segregated waste, sends biodegradable material for composting or biological treatment, directs recyclable material for recycling and disposes of the remaining waste safely.
Calculate how much waste is generated at home in a day.
The amount should be calculated by collecting and weighing all household waste produced over one complete day. The total daily waste is the sum of the masses of the biodegradable and non-biodegradable portions.
How much of this waste is biodegradable?
Separate food scraps, vegetable peels, tea leaves, paper and other decomposable materials, weigh them, and record their combined mass as the biodegradable part of the household waste.
Calculate how much waste is generated in the classroom in a day.
Collect and weigh all waste produced in the classroom during one school day. The result will vary with class size and activities and therefore must be measured rather than assumed.
How much of this waste is biodegradable?
Separate biodegradable classroom waste, such as paper, pencil shavings and food remains, and weigh it. Its combined mass gives the biodegradable portion.
Suggest ways of dealing with this waste.
Waste should be reduced at its source, separated into biodegradable and non-biodegradable categories, and reused whenever possible. Biodegradable waste can be composted, recyclable materials can be sent for recycling, and hazardous or remaining waste should be disposed of through authorised systems.
Activity 13.8
Find out how the sewage in your locality is treated. Are there mechanisms in place to ensure that local water bodies are not polluted by untreated sewage.
The exact system must be checked locally. Sewage should be carried through drains or sewers to a treatment plant, where solids and harmful contaminants are removed before the treated water is released. Direct discharge of untreated sewage into local water bodies should be prevented.
Find out how the local industries in your locality treat their wastes. Are there mechanisms in place to ensure that the soil and water are not polluted by this waste?
This must be verified for the industries in the locality. Industrial waste should be treated in suitable effluent-treatment facilities, with harmful chemicals removed or neutralised before discharge. Safe storage, recycling and monitoring are needed to prevent soil and water pollution.
Think it over
Disposable cups in trains
If you ask your parents, they will probably remember a time when tea in trains was served in plastic glasses which had to be returned to the vendor. The introduction of disposable cups was hailed as a step forward for reasons of hygiene. No one at that time perhaps thought about the impact caused by the disposal of millions of these cups on a daily basis. Some time back, kulhads, that is, disposable cups made of clay, were suggested as an alternative. But a little thought showed that making these kulhads on a large scale would result in the loss of the fertile top-soil. Now disposable paper-cups are being used. What do you think are the advantages of disposable paper-cups over disposable plastic cups?
Paper cups are generally more biodegradable than plastic cups and therefore do not persist in the environment for as long. Paper may also be recycled, while discarded plastic can accumulate and harm organisms. However, paper cups still require raw materials and energy, so reusable cups would usually create less waste than either disposable option.
Activity 13.9
Search the internet or library to find out what hazardous materials have to be dealt with while disposing of electronic items. How would these materials affect the environment?
The chapter does not list particular electronic substances. In general, electronic waste may contain hazardous materials that can enter soil and water if discarded carelessly. Such persistent or toxic materials may harm organisms and may pass through food chains. Electronic items should therefore be handled through authorised collection and recycling systems.
Find out how plastics are recycled. Does the recycling process have any impact on the environment?
Plastic waste is generally collected, sorted, cleaned and processed into material that can be used again. Recycling reduces the amount of waste requiring disposal and lowers demand for new raw material, but collection and processing still use energy and may produce pollution if they are poorly managed.
QUESTIONS
1. What is ozone and how does it affect any ecosystem?
Ozone, O
3
, is a molecule made of three oxygen atoms. At higher levels of the atmosphere, it protects ecosystems by absorbing harmful ultraviolet radiation from the Sun. If the ozone layer becomes depleted, more ultraviolet radiation reaches Earth and can damage humans and other organisms.
2. How can you help in reducing the problem of waste disposal? Give any two methods.
I can reduce waste by avoiding unnecessary disposable products and reusing items such as bags and containers. I can also separate biodegradable waste for composting and send recyclable materials for proper recycling.
EXERCISES
1. Which of the following groups contain only biodegradable items?
(a) Grass, flowers and leather
(b) Grass, wood and plastic
(c) Fruit-peels, cake and lime-juice
(d) Cake, wood and grass
The groups containing only biodegradable items are (a), (c) and (d). Plastic in option (b) is non-biodegradable.
2. Which of the following constitute a food-chain?
(a) Grass, wheat and mango
(b) Grass, goat and human
(c) Goat, cow and elephant
(d) Grass, fish and goat
Option (b), Grass, goat and human, constitutes a food chain because the goat feeds on grass and a human may obtain food from the goat.
3. Which of the following are environment-friendly practices?
(a) Carrying cloth-bags to put purchases in while shopping
(b) Switching off unnecessary lights and fans
(c) Walking to school instead of getting your mother to drop you on her scooter
(d) All of the above
Option (d), All of the above. Each practice reduces waste, saves energy or lowers fuel consumption and pollution.
4. What will happen if we kill all the organisms in one trophic level?
The food chain and the balance of the ecosystem will be disturbed. Organisms at the next higher trophic level may lose their food source and decline, while organisms at the preceding level may increase excessively. These changes can spread through the entire food web.
5. Will the impact of removing all the organisms in a trophic level be different for different trophic levels? Can the organisms of any trophic level be removed without causing any damage to the ecosystem?
Yes, the immediate effects will differ according to the trophic level removed. Removing producers would cut off the main source of food energy, while removing consumers could cause the populations of their prey to increase and their predators to decline. No trophic level can be removed completely without damaging the ecosystem because all levels are interdependent.
6. What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?
Biological magnification is the progressive increase in the concentration of persistent harmful chemicals at successive trophic levels of a food chain. Yes, the concentration is different at different levels: it is generally lowest in producers and becomes progressively greater in consumers, reaching its maximum in organisms at the highest trophic level.
7. What are the problems caused by the non-biodegradable wastes that we generate?
Non-biodegradable wastes remain in the environment for long periods and accumulate in land and water. They may harm organisms, obstruct natural processes, reduce environmental quality and allow toxic substances to enter food chains and become biologically magnified.
8. If all the waste we generate is biodegradable, will this have no impact on the environment?
No. Even biodegradable waste can cause environmental problems if generated faster than decomposers can break it down. Large accumulations may produce foul smells, attract pests, spread disease and pollute soil and water. Such waste must still be reduced and managed properly.
9. Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?
The ozone layer shields Earth from harmful ultraviolet radiation. Its depletion allows more ultraviolet radiation to reach living organisms and can cause serious damage, including skin cancer in humans. To limit the damage, international regulations were introduced to control CFC production and manufacturers were required to adopt CFC-free technologies.