Our Environment

An ecosystem is a functional unit of nature in which all the living organisms (plants, animals, microbes) interact among themselves and also with their physical, non-living environment. A pond, a forest, a grassland or a lake are examples of natural ecosystems, while a garden, a crop field and an aquarium are artificial (human-made) ecosystems.

Every ecosystem has two types of components. The biotic components are all the living things, and the abiotic components are the non-living, physical factors such as temperature, rainfall, wind, soil, minerals, light, water and air.

Based on their role, the biotic components are grouped as:

• Producers (Autotrophs): Green plants and blue-green algae that make their own food by photosynthesis using sunlight.
• Consumers (Heterotrophs): Organisms that depend on producers for food, directly or indirectly. They are further divided into herbivores (plant-eaters, e.g. deer), carnivores (flesh-eaters, e.g. lion), omnivores (eat both, e.g. humans) and parasites.
• Decomposers: Micro-organisms such as bacteria and fungi that break down the dead remains of plants and animals into simple substances, returning nutrients to the soil.

For example, in a pond ecosystem, the water, mud, dissolved gases and sunlight are abiotic components; the algae and aquatic plants are producers; the fish, insects and frogs are consumers; and the bacteria and fungi in the mud are decomposers. Decomposers are vital because they recycle the nutrients and keep the environment clean.

A food chain is a sequence of living organisms in a community in which one organism eats the organism below it and is in turn eaten by the organism above it. In this way, food and the energy stored in it are transferred from one organism to the next. Every food chain begins with a producer.

A simple grassland food chain is:

Grass → Deer → Lion

Here grass (producer) is eaten by the deer (herbivore / primary consumer), and the deer is eaten by the lion (carnivore / secondary consumer). Another example from a field is: Grass → Grasshopper → Frog → Snake → Hawk.

In nature, an organism rarely eats only one type of food and is rarely eaten by only one predator. So the different food chains in an ecosystem are interconnected, forming a food web. For example, grass may be eaten by grasshoppers, rabbits and deer; a frog may be eaten by a snake or a hawk. These branching, criss-crossing links make a food web.

Food webs are important because they give an ecosystem stability. If one type of food is scarce, an animal can switch to another food source, so the whole ecosystem does not collapse. Food chains and food webs also show how energy and nutrients keep moving through the living world.

The various steps in a food chain at which the transfer of food (and energy) takes place are called trophic levels. Producers form the first trophic level (T₁), herbivores (primary consumers) the second (T₂), small carnivores (secondary consumers) the third (T₃), and large carnivores (tertiary consumers) the fourth (T₄).

The energy that enters an ecosystem comes from the Sun. Green plants capture only about 1% of the sunlight falling on their leaves and convert it into chemical energy by photosynthesis. This energy then flows from one trophic level to the next.

According to Lindeman’s 10% Law, only about 10% of the energy present at one trophic level is passed on to the next trophic level. The remaining 90% is lost as heat, and is used up in the life processes such as respiration, movement, growth and reproduction. For example, if the producers store 10,000 J of energy, the herbivores get only 1000 J, the small carnivores get 100 J, and the top carnivores get only 10 J.

Because so much energy is lost at each step, food chains generally have only three or four trophic levels — there is not enough energy left to support many more levels. Two important features of energy flow are: it is unidirectional (energy flows only one way, from Sun to producers to consumers, and never back), and it is non-cyclic (energy is not reused).

A related harmful effect is biological magnification: harmful, non-biodegradable chemicals (like certain pesticides) enter a food chain and become more and more concentrated at each higher trophic level. Since humans occupy the top of food chains, we receive the maximum concentration of these poisons.

Human activities such as agriculture, industry, deforestation and the burning of fuels affect the environment. One of the most serious effects is the depletion of the ozone layer.

Ozone (O₃) is a molecule made of three atoms of oxygen. At the higher levels of the atmosphere (the stratosphere), ozone forms a layer that acts as a shield. It absorbs most of the harmful ultraviolet (UV) radiation coming from the Sun, protecting living things on the Earth’s surface.

Ozone is formed from oxygen (O₂). High-energy UV rays first split an oxygen molecule into free oxygen atoms, and these atoms then combine with oxygen molecules to form ozone:

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

The main cause of ozone depletion is a group of man-made chemicals called chlorofluorocarbons (CFCs), once widely used in refrigerators, air conditioners and aerosol sprays. In the stratosphere, UV rays break CFCs to release chlorine atoms, and each chlorine atom destroys a large number of ozone molecules:

• CFCl₃ → Cl + CFCl₂ (under UV)
• Cl + O₃ → ClO + O₂

In 1985 a sharp drop in ozone (an “ozone hole”) was detected over Antarctica. Ozone depletion is a cause for concern because increased UV radiation causes skin cancer, cataracts (damage to the eyes), weakened immunity, and harm to crops and marine life. To control the damage, the United Nations Environment Programme (UNEP) forged an agreement in 1987 called the Montreal Protocol to freeze and then phase out the production of CFCs worldwide.

The huge amount of waste (garbage) we generate must be managed to keep the environment clean. Based on how they decompose, wastes are classified into two types.

Biodegradable substances are those that can be broken down into simpler, harmless substances by the action of micro-organisms (decomposers such as bacteria and fungi). Examples include kitchen waste (vegetable peels, leftover food), paper, cotton cloth, wood, cow dung and dead plants and animals.

Non-biodegradable substances are those that cannot be broken down by micro-organisms; they persist in the environment for a very long time. Examples include plastics, polythene bags, glass, metal cans, DDT and other pesticides, and synthetic fibres.

Substances are biodegradable or non-biodegradable because decomposers have specific enzymes that can act only on natural substances; man-made materials like plastic have no matching enzymes, so they are not broken down.

Problems caused by wastes: Even biodegradable waste, if it accumulates, produces foul smells, spreads disease and gives off gases. Non-biodegradable wastes are worse — they pollute soil and water, clog drains, kill animals that swallow them, and non-biodegradable pesticides enter food chains and cause biological magnification.

Better methods of garbage management include: segregating waste at source into biodegradable and non-biodegradable bins; composting and preparing biogas or manure from biodegradable waste; recycling and reusing non-biodegradable materials such as paper, glass and metals; sewage treatment; and following the principle of the three R’s — Reduce, Reuse and Recycle.