Our Environment Notes

An ecosystem is a self-contained unit of living organisms (biotic components) and their non-living surroundings (abiotic components — temperature, rainfall, wind, soil, minerals) interacting with each other. A forest, a pond, or a lake is a natural ecosystem; a crop field or an aquarium is a human-made ecosystem.

The biotic components are grouped by how they obtain food. Producers (green plants, some bacteria) make food by photosynthesis. Consumers eat other organisms — herbivores eat plants, carnivores eat other animals, omnivores eat both, and parasites live on a host. Decomposers (bacteria and fungi) break down dead remains and return simple nutrients to the soil, water, and air.

Decomposers are essential: they replenish the store of inorganic nutrients so that producers can use them again, keeping the ecosystem running.

A food chain is a series of organisms through which energy passes, each feeding on the one before it: for example grass → grasshopper → frog → snake → hawk. Each step is a trophic level. Producers form the first trophic level, herbivores the second, small carnivores the third, and so on.

In nature, organisms usually eat, and are eaten by, more than one kind of organism, so food chains are interconnected into a food web. Food webs make the ecosystem more stable, because if one species declines, others can still be eaten.

Energy from the Sun enters through producers. As we move to higher trophic levels, the amount of usable energy falls sharply because energy is lost as heat, in movement, and in life processes at every level.

The flow of energy in an ecosystem is one-way (unidirectional). Solar energy captured by producers does not go back to the Sun, and energy passed to herbivores does not return to the producers.

According to the 10 per cent law (Lindeman), only about 10 per cent of the energy at one trophic level is passed on to the next level; the other 90 per cent is used up in respiration, movement, and heat loss, or remains in parts that are not eaten.

Because so much energy is lost at each step, food chains generally have only four or five trophic levels — there is not enough energy left to support more. It also explains why the number (or biomass) of top carnivores is small.

Some harmful chemicals, such as certain pesticides, are not broken down by the body and are not excreted. They stay in the tissues of organisms.

As we move up the food chain, these chemicals become more and more concentrated because a predator eats many prey, each already carrying the chemical. This progressive increase in concentration of a persistent substance at successive trophic levels is called biological magnification.

Human beings, who often occupy the top of food chains, can therefore accumulate the highest concentrations of such chemicals.

Biodegradable substances (vegetable peels, paper, cotton, sewage) are broken down by decomposers. Non-biodegradable substances (most plastics, glass, DDT, aluminium cans) are not broken down by biological processes; they persist for a long time and can harm ecosystems and choke drains and soil.

The ozone layer high in the atmosphere absorbs most of the Sun's harmful ultraviolet (UV) radiation. Man-made chemicals called chlorofluorocarbons (CFCs), once used in refrigerators, air conditioners, and aerosol sprays, break down ozone. Thinning of the ozone layer lets in more UV, which can cause skin cancer and cataract and damage crops. In 1987 many countries signed the Montreal Protocol to freeze and then reduce CFC production.

Managing waste well — segregating biodegradable and non-biodegradable waste, reducing single-use plastic, composting, and recycling — lowers the load on the environment.