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All the interacting organisms in an area, together with the non-living parts of the environment, form an ecosystem. Plants, animals, microorganisms and human beings interact with each other and with their surroundings and keep a balance in nature.
A well-made aquarium needs free space for swimming, water, oxygen (from an aerator) and food. With a few aquatic plants and animals it can become a self-sustaining system. It still needs cleaning now and then, unlike a pond or lake, which look after themselves.
Green plants and certain bacteria make organic compounds like sugar and starch from inorganic substances using sunlight, in the presence of chlorophyll. They are the producers.
Organisms that depend on producers, directly or indirectly, for food are consumers. They are grouped as:
Bacteria and fungi break down the dead remains and wastes of organisms into simple inorganic substances that go into the soil and are used again by plants. They are the decomposers. Without them, garbage and dead bodies would pile up and the soil would not be replenished.
A series of organisms, each feeding on the one before it, is a food chain. Example: grass → grasshopper → frog → snake → hawk.
Each step of a food chain is a trophic level.
Most organisms are eaten by more than one kind of animal, which in turn are eaten by others. So instead of a single straight chain, the relationships look like a network of branching lines called a food web.
Green plants in a land ecosystem capture only about 1% of the sunlight that falls on their leaves and convert it into food energy. From the producers the energy passes to consumers and decomposers.
When a herbivore eats a plant, a great deal of energy is lost as heat to the environment, some goes into digestion and doing work, and the rest goes to growth and reproduction. On average only about 10% of the food energy is turned into body matter and is available to the next level.
Pesticides and other chemicals used to protect crops are washed into the soil and water bodies. Plants absorb them from the soil along with water and minerals, and aquatic plants and animals take them from water. In this way they enter the food chain.
These chemicals are not degradable, so they build up at each trophic level. Human beings are at the top of food chains and get the highest concentration. This increase in concentration of harmful chemicals along the food chain is biological magnification.
Wheat, rice, vegetables, fruits and even meat contain some pesticide residues, and washing does not always remove them. Using fewer pesticides and safer methods of pest control reduces our intake.
Ozone (O₃) is a molecule of three oxygen atoms. Ordinary oxygen (O₂) is essential for aerobic life, but ozone is a poisonous gas at ground level. High up in the atmosphere, however, it is essential, because it shields the Earth's surface from ultraviolet (UV) radiation of the Sun. UV radiation can cause skin cancer and damage other organisms.
At high levels, UV radiation splits some oxygen molecules into free oxygen atoms. These atoms combine with molecular oxygen to form ozone.
O₂ → O + O (UV)O + O₂ → O₃The amount of ozone began to fall sharply in the 1980s. This was linked to synthetic chemicals called chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers.
In 1987 the United Nations Environment Programme (UNEP) got an agreement to freeze CFC production at 1986 levels. It is now mandatory for manufacturers all over the world to make CFC-free refrigerators.
Substances that are broken down by biological processes are biodegradable. Substances that are not broken down in this way are non-biodegradable.
| Biodegradable | Non-biodegradable | |
|---|---|---|
| Examples | Kitchen waste, vegetable peels, paper, cotton cloth, dead plants and animals | Plastics, polythene, glass, metals, some chemicals |
| Broken down by | Bacteria and other saprophytes | Not by living organisms; only slowly by physical processes |
Enzymes are specific in their action: a particular enzyme breaks down only a particular substance. That is why we cannot get energy by eating coal. In the same way, many human-made materials like plastics are not broken down by bacteria or other saprophytes, and last for a long time in the environment.
Better living standards, more disposable items and changes in packaging have increased the amount of waste, and much of it is now non-biodegradable.
| Level | Organisms | Name |
|---|---|---|
| 1 | Green plants | Producers |
| 2 | Herbivores | Primary consumers |
| 3 | Small carnivores | Secondary consumers |
| 4 | Large carnivores | Tertiary consumers |
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Each step or level of a food chain is a trophic level.
Example: Grass → Grasshopper → Frog → Snake → Hawk.
Trophic levels: grass is the first (producer); grasshopper the second (primary consumer); frog the third (secondary consumer); snake the fourth (tertiary consumer); hawk the fifth (top carnivore).
Decomposers (bacteria and fungi) break down the dead remains and waste products of organisms into simple inorganic substances. These go into the soil and are used again by plants. They clean the environment and replenish the soil with nutrients.
Enzymes are specific in their action. Biodegradable substances can be broken down by the enzymes of bacteria and other decomposers. Non-biodegradable substances (like plastics) have no suitable enzymes to act on them, so they persist.
Ozone (O₃) is a molecule of three oxygen atoms. In the upper atmosphere it forms a layer that absorbs the Sun's harmful ultraviolet radiation and so protects living things from skin cancer and other damage. Near the ground ozone is a poison. Depletion of the layer lets more UV reach the Earth and harms ecosystems.
Plastic is not biodegradable, which rules out (b). Leather is not easily broken down, so (a) is out. Both (c) and (d) contain only items that bacteria and fungi break down.
Answer: (c) and (d)
Only (b) has organisms that feed on one another in a line: grass → goat → human.
Answer: (b)
All three reduce waste or save energy.
Answer: (d)
The food chain is broken. The organisms of the level above lose their food and decline, while the level below increases without control. The balance of the ecosystem is disturbed.
Yes, the impact differs. Removing the producers is the most serious, because all the energy of the ecosystem enters through them and all other organisms would be affected. Removing herbivores makes producers increase and carnivores starve; removing carnivores lets herbivores increase and overgraze. No trophic level can be removed without damaging the ecosystem.
Biological magnification is the progressive increase in the concentration of harmful non-degradable chemicals, such as pesticides, along the food chain. Yes, the levels differ: the concentration is lowest in producers and increases at each higher trophic level, being maximum in the top carnivores, including human beings.
No. Even biodegradable waste has an impact if it is produced in very large amounts. It piles up faster than it decomposes, gives out foul smell, attracts flies and rats, spreads disease and can pollute water. Its decomposition may also give out gases such as methane.
The ozone layer protects living things from the Sun's harmful ultraviolet rays. If it is damaged, more UV reaches the Earth, which can cause skin cancer and harm organisms. In 1987 the United Nations Environment Programme (UNEP) got an agreement to freeze the production of CFCs at 1986 levels, and it is now mandatory to make CFC-free refrigerators.