Our Environment

AN ECOSYSTEM is all the interacting organisms in an area TOGETHER WITH the non-living constituents of the environment. It therefore has BIOTIC components (all living organisms) and ABIOTIC components (physical factors such as temperature, rainfall, wind, soil and minerals).

TYPES OF ECOSYSTEM: NATURAL — forests, ponds and lakes, which maintain themselves. ARTIFICIAL (man-made) — a garden, a crop field or an aquarium, which are created and maintained by human beings. A garden IS an ecosystem, because the plants and animals in it interact with each other and with the soil and weather.

PRODUCERS — all GREEN PLANTS and BLUE-GREEN ALGAE. They make their own food from sunlight by photosynthesis. ALL the energy that enters an ecosystem enters through the producers, which is why they are also the first trophic level.

CONSUMERS — organisms that depend on producers, directly or indirectly, for food. They are classed as HERBIVORES (eat plants), CARNIVORES (eat other animals), OMNIVORES (eat both) and PARASITES (live on a host).

DECOMPOSERS — BACTERIA and FUNGI. They break down the dead remains and waste products of organisms into simple substances, which are returned to the soil and used again by the plants. IF DECOMPOSERS DISAPPEARED: dead bodies and waste would pile up, and the nutrients locked in them would never be returned to the soil — the cycle of materials in nature would stop. (A very common 2–3 mark question.)

A FOOD CHAIN is a series of organisms feeding on one another, each forming a level. Example: GRASS → GRASSHOPPER → FROG → SNAKE → HAWK.

A TROPHIC LEVEL is each step, or level, of the food chain. Producers = 1st trophic level; herbivores (primary consumers) = 2nd; small carnivores (secondary consumers) = 3rd; larger carnivores (tertiary consumers) = 4th.

ENERGY FLOW IS UNIDIRECTIONAL — it flows from the producers onwards and NEVER back. The energy captured by the producers does not revert to the Sun, and the energy passed to herbivores does not go back to the plants.

HOW MUCH SUNLIGHT DO PLANTS ACTUALLY CAPTURE? Green plants in a terrestrial ecosystem capture about 1% of the energy of the sunlight that falls on their leaves and convert it into food energy. (NCERT states this figure explicitly — do not confuse the 1% captured from sunlight with the 10% passed between trophic levels.)

THE TEN PER CENT LAW (the key idea of the chapter): an average of only 10% of the food eaten by an organism is turned into its own body and made available to the NEXT level of consumers. The remaining 90% is used up in its own life processes and lost, largely as heat.

CONSEQUENCE OF THE 10% LAW — WHY FOOD CHAINS ARE SHORT: because so little energy is available to each successive level, food chains generally consist of only THREE or FOUR steps. NCERT's exact wording is worth learning: 'the loss of energy at each step is so great that very little usable energy remains after four trophic levels'.

NUMBERS IN AN ECOSYSTEM: there are generally a GREATER NUMBER of individuals at the LOWER trophic levels, and the greatest number of all is that of the PRODUCERS.

A FOOD WEB — in nature the relationships are not a simple straight line. Each organism is generally eaten by two or more kinds of organism, and itself eats more than one kind, so the various food chains CROSS AND INTERCONNECT to form a network called a food web. This interlinking gives an ecosystem STABILITY.

BIOLOGICAL MAGNIFICATION — pesticides and other harmful chemicals sprayed on crops are washed into the soil and water, taken up by plants along with water and minerals, and then passed along the food chain. Because these chemicals are NOT broken down and NOT excreted, their concentration INCREASES at each successive trophic level.

Since HUMAN BEINGS occupy the TOP of most food chains, the MAXIMUM concentration of these chemicals accumulates in our own bodies. This is why food grains such as wheat and rice, vegetables, fruits and even meat contain pesticide residues, which cannot always be removed by washing.

OZONE (O₃) is a molecule of THREE oxygen atoms — distinct from the O₂ we breathe. AT GROUND LEVEL ozone is a DEADLY POISON. But HIGH in the atmosphere it performs an essential function: it SHIELDS the surface of the earth from ULTRAVIOLET (UV) radiation from the Sun, which is highly damaging and is known to cause SKIN CANCER in human beings.

HOW OZONE FORMS: high-energy ultraviolet radiation splits an oxygen molecule into two free oxygen atoms (O₂ → O + O). Each free atom then combines with another oxygen molecule to form ozone (O + O₂ → O₃).

OZONE DEPLETION: the culprit is a group of man-made chemicals called CHLOROFLUOROCARBONS (CFCs), widely used in REFRIGERATORS and FIRE EXTINGUISHERS. On reaching the upper atmosphere they break ozone down, and the amount of ozone began to drop sharply from the 1980s.

THE INTERNATIONAL RESPONSE — learn this exactly as NCERT states it: in 1987 the UNITED NATIONS ENVIRONMENT PROGRAMME (UNEP) succeeded in forging an agreement to FREEZE CFC PRODUCTION AT 1986 LEVELS. It is now mandatory for all manufacturing companies to make CFC-free refrigerators throughout the world. (The question usually asks WHO forged the agreement and in WHICH year.)

BIODEGRADABLE substances are broken down by BIOLOGICAL PROCESSES — that is, by the action of bacteria and other decomposers. Examples: vegetable peels, fruit peels, paper, cotton cloth, wood, cow dung, cake, leather.

NON-BIODEGRADABLE substances are NOT broken down by biological processes; they persist in the environment for a very long time and may harm it. Examples: plastics, glass, metal cans, aluminium foil, pesticides such as DDT.

WHY are some substances biodegradable and others not? Because ENZYMES ARE SPECIFIC IN THEIR ACTION — each enzyme can act only on a particular kind of substance. The decomposers have enzymes that break down NATURAL materials, but MAN-MADE materials such as plastics have structures that no available enzyme can act upon, so nothing breaks them down. (A direct NCERT question and a standard 2-mark answer.)

BIODEGRADABLE WASTE IS NOT HARMLESS EITHER (a commonly asked 2-mark question). If it is not disposed of properly it: (a) rots and produces a FOUL SMELL and gases such as METHANE, which is a greenhouse gas; (b) attracts FLIES, RATS and other organisms that SPREAD DISEASE; and (c) if washed into ponds and rivers, pollutes the water and harms the aquatic life there.

WHY THE WASTE PROBLEM IS GROWING: our changing LIFESTYLE has greatly increased the amount and the kind of waste we generate — far more packaging, more plastics and more disposable items than before.

METHODS OF WASTE DISPOSAL: COMPOSTING (for biodegradable waste), RECYCLING (paper, glass, metal), SEWAGE TREATMENT (for waste water) and LANDFILL (for the remainder). The best long-term answer, however, is to GENERATE LESS WASTE in the first place.

Ecosystem

ecosystem = BIOTIC (living) + ABIOTIC (physical factors)

Abiotic = temperature, rainfall, wind, soil, minerals. Natural: forest, pond, lake. Artificial: garden, crop field, aquarium.

The three functional groups

PRODUCERS → CONSUMERS → DECOMPOSERS

Producers make food; consumers eat; decomposers return nutrients to the soil.

A food chain

grass → grasshopper → frog → snake → hawk

Each step is one TROPHIC LEVEL. Producers are the 1st level.

The ten per cent law

only ~10% passes to the next trophic level; ~90% is lost

1000 units → 100 → 10 → 1. This is why chains have only 3–4 steps.

Direction of energy flow

producers → consumers, ONE WAY only (unidirectional)

Energy never flows back down the chain.

Biological magnification

concentration of harmful chemicals RISES at each trophic level

Not broken down, not excreted. Humans are at the top, so we accumulate the most.

Ozone

O₂ → O + O (by UV); O + O₂ → O₃

O₃ shields the earth from UV. At ground level ozone is a poison.

The CFC agreement

UNEP · 1987 · freeze CFC production at 1986 levels

Learn who (UNEP) and when (1987) — that is what is asked.

Waste

BIODEGRADABLE (broken down by decomposers) vs NON-BIODEGRADABLE

Peels, paper, cotton, wood vs plastic, glass, metal, DDT.