Our Environment
1. What This Chapter Covers
The environment is the sum of physical and biological factors, along with their chemical interactions, that affect an organism. The physical factors are abiotic — land, air, water, sunlight — and the biological factors are biotic, the flora and fauna. The place where an organism lives is its habitat.
Living organisms always try to maintain a balance among themselves and with both sets of factors. The book states the consequence sharply: an organism cannot completely defy the balance to suit its own need, because doing so affects the balance in a way that puts the survival of the organism causing the damage at stake.
The index allots this chapter 10 periods in December and runs it from page 205 to page 224.
2. Food Chains (Textbook 9.1)
A food chain shows who eats what in a particular habitat, and the arrows between two organisms always point from the food to the feeder.
The chapter's examples:
Grass -> grasshopper -> frog -> snake -> hawk
Grass -> rabbit -> fox -> wolf
Grass -> goat -> man
Two patterns appear as soon as you look at real chains. Most food chains are quite short, rarely more than four steps, and the number of organisms decreases as you move from producer to primary, secondary and tertiary consumers.
The reason for both is energy. All organisms derive energy from food and sunlight is its main source. At each transfer, 80 to 90 per cent of the energy is dissipated as heat produced during respiration and other reactions, so after three steps there is very little left for top carnivores.
Ecosystems and niches
Terrestrial ecosystems are determined largely by climatic variation between the Poles and the Equator. Climbing a mountain such as Kilimanjaro in equatorial Africa or the Himalayas compresses the same sequence into a single slope, from tropical rain forest at the base to perpetual snow and ice at the summit.
The main climatic influences are rainfall, temperature and availability of light. Forests are associated with high rainfall, but the type of forest depends on temperature and light; deserts occur where rainfall is extremely low.
A niche is not simply an animal's position in the food web. In deciduous woodland there is a niche for insects such as aphids that suck up leaf juices, another for caterpillars with strong jaws for biting off pieces of leaf, and another for relatively large animals such as deer that browse on vegetation. All three feed on leaves but differ in size and manner of feeding.
The book's formulation is the one to remember: a habitat is the place where an animal lives; a niche describes its occupation — the way it goes about its business and earns its living.
3. Food Webs (Textbook 9.2)
In nature, food chains are usually not simple and linear. At every trophic level a consumer has several alternative foods to choose from: snakes prey on frogs, mice and small birds; cranes prey on fish and frogs.
Chains therefore become branched and interlinked, producing a web-like structure — the food web. The book notes that "chain" is misleading precisely because it suggests rigidity: aphids are eaten by many insectivorous birds as well as ladybirds and other insects, and eagles prey on a considerable variety of birds and small mammals.
Food web is the better term when being precise, because it reflects that the whole community is a complex interconnected unit through which the sun's original energy flows from one trophic level to the next.
4. Ecological Pyramids (Textbook 9.3)
A graphic representation of the trophic levels of an ecosystem in the shape of a pyramid is an ecological pyramid. It was first introduced by the British ecologist Charles Elton in 1927.
The producers form the base — the first trophic level — with successive levels stacked above and top-level carnivores at the tip. There are three types: pyramid of numbers, pyramid of biomass, and pyramid of energy.
Pyramid of numbers (9.3.1)
Each bar represents the number of individuals at a trophic level. Moving up a chain there is normally an increase in size but a decrease in number.
The book's forest example: aphids are very small and occur in astronomical numbers; ladybirds that feed on them are distinctly larger and not so numerous; insectivorous birds are larger still and present in small numbers; and there may be only a single pair of hawks, much larger again. That gives an upright pyramid.
But it does not always look like a pyramid. It can invert if the producer is a large plant such as a tree, or if an organism at some level is very small. The book's two contrasting exercises make the point: banyan tree -> insects -> woodpecker against grass -> rabbit -> wolf. Whatever the shape, the producers stay at the bottom.
Pyramid of biomass (9.3.2)
Biomass is organic material of biological origin, ultimately derived from the fixation of carbon dioxide by trapping solar energy during photosynthesis — trees, shrubs, crops, grasses, algae, aquatic plants, agricultural and forest residues, and all forms of human, animal and plant waste. Any plant or animal material that can be converted into energy is biomass, and when used for energy production it is biofuel.
The pyramid of biomass represents the quantity of living matter at each trophic level.
In terrestrial ecosystems biomass decreases progressively from producers to top carnivores, so the pyramid is upright. In an aquatic ecosystem the biomass of phytoplankton is quite negligible compared with the crustaceans and small herbivorous fish that feed on them, and the biomass of the large carnivorous fish is greater still — so the pyramid is inverted.
The book gives a transfer figure: 10 to 20 per cent of the biomass is transferred from one trophic level to the next.
Why so little? When animals eat, only a small proportion of the food is converted into new tissue; part is used to provide the energy needed for staying alive, and the rest passes out of the body. Many animals convert not more than 10 per cent of their food into body tissue, and in some herbivores it is even less.
Biomass can be used as an energy source to reduce dependence on fossil fuels and help reduce air pollution. Using it still puts carbon dioxide back into the atmosphere — but it is the same carbon dioxide that was taken from the air as the biomass grew.
Pyramid of energy (9.3.3)
Food is chemical energy, and in stored form it is potential energy.
Energy enters the ecosystem from the sun as solar radiation, and no organisms except green plants and photosynthetic bacteria can absorb it and convert it into chemical energy. From producers the chemical energy passes to consumers, level by level, through food.
Solar energy -> producers, chemical energy -> consumers, chemical energy, with heat lost at every arrow.
At each level, organisms use most of the food energy they assimilate for their metabolic requirements — work, growth and reproduction. Because biological energy transformations are inefficient, a substantial proportion is lost unused as heat. Hence the pyramid of energy is always upright.
The comparison the book draws is with a car: most of the energy in petrol is lost as heat in the engine rather than becoming motion. In natural communities, energy used for work or dissipated as heat cannot be consumed by another organism and is forever lost to the ecosystem — which is different from matter, since minerals keep cycling between organisms and the environment in the biogeochemical cycles.
Not all food can be digested. Hair, feathers, insect exoskeletons, cartilage and bone in animal food, and cellulose and lignin in plant food, are indigestible for most animals and are ejected by defecation or regurgitated. Assimilated energy not lost through respiration or excretion is available for new biomass through growth and reproduction; biomass lost by death, disease or annual leaf-drop enters the detritus pathway.
The one-line test for any pyramid question: energy can only go down, so its pyramid can never invert. Numbers and biomass count things rather than energy, so either can stand on its head when the producer happens to be very large or very small.
5. Kolleru Lake (Textbook 9.4.1)
Kolleru is one of the largest fresh water lakes in India, lying between the West Godavari and Krishna districts of Andhra Pradesh. Its catchment extends to 6121 km², and it discharges excess water into the Bay of Bengal through a twisty channel called Upputeru, about 60 km long. The wetland receives huge quantities of nutrient-rich sediment from the flood plains.
In November 1999 the Government of India declared the lake a Bird Sanctuary. It hosts 193 species of birds and a variety of flora and fauna including medicinal plants, attracts migratory birds from northern Asia and Eastern Europe between October and March at an estimated 20,00,000 birds per year, and was an important habitat for an estimated 20 million residents.
| Portion of the lake | Area in 1967, km² | Area in 2004, km² |
|---|---|---|
| Water spread area | 70.70 | 0 |
| With sparse weed | 0 | 47.45 |
| With dense weed | 0 | 15.20 |
| Liable to flood in rainy season | 100.97 | 0 |
| Aquaculture ponds | 0 | 99.74 |
| Rice fields | 8.40 | 16.62 |
| Encroachment | 0.31 | 1.37 |
| Total | 180.38 | 180.38 |
Read the two columns against each other: the total is unchanged, but open water has gone to zero, and aquaculture ponds have gone from zero to nearly 100 km².
What happened
Aquaculture, being profitable, was started extensively in the eighties and spread through the Krishna-Godavari delta, attracting many investors. By 1996 almost the entire lake was under cultivation, with bunds constructed to keep water out to protect the crops. That diversion affected the natural flow system and significantly reduced the water-holding capacity.
Agriculture and industry grew in the catchment, so drains and rivulets began carrying substantial pollutants in. The major sources are agricultural runoff with agrochemical and fertilizer residues, fish tank discharges, industrial effluents, and municipal and domestic sewage.
Excessive nutrient addition, especially from human sources, led to explosive weed growth — Eichhornia and Pistia — which is eutrophication, and the balance of the lake was damaged.
The water turned more alkaline, turbid, nutrient rich, low in Dissolved Oxygen (DO) and high in Biological Oxygen Demand (BOD). Water-borne diseases like diarrhoea, typhoid and amoebiasis became common among local inhabitants unaware of the pollution, and mosquito-borne diseases increased. Prawn and fish were affected by disease and some farms were abandoned — and the abandoned land is useless for agriculture too.
The lake is now prone to siltation, encroachment and blocked canals. Fifteen local varieties of fish are endangered because of aquaculture, and as oxygen dissolution falls, the number of fish living in surface water has increased.
Which activity causes which problem
The book's Table 2 marks + where an activity influences a problem and − where it does not.
| Problem | Agricultural practices | Aquaculture practices | Industrial activities | Human activities |
|---|---|---|---|---|
| Decreased migratory birds | − | + | − | − |
| Population loss of flora and fauna | − | + | − | − |
| Pathogens | − | − | − | + |
| Eutrophication | + | + | − | + |
| Toxic contamination | + | + | + | − |
| Siltation | + | + | − | − |
| Flooding | + | + | − | + |
The pattern the table brings out is that aquaculture is the only activity marked against every problem except pathogens, while industrial activity is marked only against toxic contamination.
The Ministry of Environment and Forest constituted a committee, Operation Kolleru, to bring back the ecological balance of the lake.
6. Pesticides and Monoculture (Textbook 9.4)
When a forest is cut down and a food crop grown in its place, a natural ecosystem with a vast number of species in dynamic equilibrium is replaced by a monoculture — an unnatural concentration of a single crop.
Growing crops in large concentrations gives food in abundance, which is optimum for pests and parasites such as fungi. The larger the quantity of food, the more rapid their multiplication and the greater the damage. To prevent this we use toxic chemicals — pesticides, herbicides and fungicides — many of them very effective, but their use has created new problems.
The book states the difficulty plainly: the perfect pesticide would destroy one particular pest and be completely harmless to every other form of life, and no such pesticide exists or is likely to.
Pesticides are often indiscriminate. They may destroy predators that naturally feed on the pests, or prey that other animals depend on, causing unpredictable changes in food chains and upsetting the ecosystem's balance — and the effect is more dangerous still when they mix into the soil.
Degradable and non-degradable
Some pesticides and herbicides are degradable, broken down into harmless substances in a comparatively short time, usually a year. Others are non-degradable, including those containing mercury, arsenic or lead.
Non-degradable pesticides accumulate in the bodies of animals and pass right through the food web, being further concentrated at each step until animals at the top of the pyramid receive enough to do considerable harm.
The book gives the two terms precisely:
- Bioaccumulation is the process of entry of pollutants into a food chain.
- Biomagnification is the tendency of pollutants to concentrate as they move from one trophic level to the next.
Minamata disease was first discovered in Minamata city, Kumamoto prefecture, Japan, in 1956. It was caused by the release of methyl mercury in industrial wastewater from the Chisso corporation's chemical factory, which continued from 1932 to 1968. The chemical bioaccumulated in shellfish and fish in Minamata Bay and the Shiranui Sea, and eating them caused mercury poisoning; cat, dog, pig and human deaths continued for 36 years.
7. Heavy Metals in Fish at Edulabad (Textbook 9.4.2)
Aquatic bodies around urban areas in India pose a serious risk to aquatic organisms through excessive nutrient input, acidification, heavy metal contamination and organic pollution. Fish are now considered bioindicators of metal contamination in environmental monitoring, because fish species respond strongly to stress.
The study assessed contamination by lead (Pb), cadmium (Cd), chromium (Cr), manganese (Mn), nickel (Ni) and iron (Fe) in Edulabad Water Reservoir (EBWR) in Medchal district, Telangana, which is highly polluted with industrial effluents.
Cyprinus carpio, the common scale carp, was chosen because it is a cheap, high-protein fish eaten by people in the surrounding areas. Heavy metals in the water and their accumulation in liver, kidney and gill were analysed, along with glycogen and lipid contents.
A parallel study was run at a fresh water reservoir at Bibinagar, Yadadri-Bhuvanagiri district, 30 km away and less polluted, as a comparison. Bioaccumulation was higher and glycogen and lipid contents lower in the EBWR fish.
Samples were collected in three seasons — pre-monsoon February to May, monsoon June to September, post-monsoon October to January — three water samples at three stations, three times a season, 27 samples in total, analysed from June 2005 to May 2007.
Metal concentrations in EBWR were higher than Indian standard limits, in the sequence Fe > Pb > Cr > Ni > Cd. Bioaccumulation in the fish tissues followed a different sequence: Cd > Cr > Fe > Ni > Pb.
That difference is the finding. Higher bioaccumulation factors for cadmium in liver, gill and kidney indicate the fish are sensitive to cadmium even at low concentrations — a metal low in the water can be high in the tissue.
Bioaccumulation was lower in the monsoon than in the pre- and post-monsoon seasons. The metals reach human beings through the food chain and cause physiological disorders such as hypertension, sporadic fever and renal damage.
The study concluded that unplanned urban settlement, unorganized small-scale industry and sewage contaminated the reservoir, and that such accumulation disturbs aquatic life and increases health risk in humans through the food chain.
8. The Sparrow Campaign (Textbook 9.5)
The book uses a documented historical case to show what removing one link does.
In 1958 a radical campaign was set in motion in China to increase industrial output rapidly by mobilizing the rural peasantry, in an agrarian society, so that the country would catch up with the rest of the world.
One famous initiative was forming co-operatives of up to 5,000 families, which initially doubled the crop. That success led to ambitious goals for the next year, but the weather did not cooperate. Fewer crops were harvested, and agricultural officials overstated the amount of grain for fear of missing their quotas, creating an imbalance between demand and supply.
The sparrows were accused of pecking away at warehouse supplies at an officially estimated four pounds of grain per sparrow per year. In cities and outskirts almost half the labour force was mobilized into an anti-sparrow army.
People trapped, poisoned and killed sparrows in large numbers. Free-fire zones were set up. People beat drums to stop the birds landing, so the sparrows kept flying until they dropped dead from fatigue. Nests were torn down, eggs broken, nestlings killed, and rewards and recognition were offered to schools, work units and government agencies according to the number killed.
Then scientists cut open the digestive systems of dead sparrows. Three-quarters of the contents were insects harmful to crops; only one-quarter was grain. Sparrows were basically a beneficial bird for humans.
Crop yields after the campaign were substantially decreased, not increased. The government ended the campaign once the truth was known, but it was too late. With no sparrows to eat them, the locust populations swarmed the country.
Locusts together with bad weather led to the Great Chinese Famine, and the use of pesticides against the locusts degraded the land further. Millions of farmers left their villages to work in industry instead of the fields, very little area was left under agriculture, and food shortages became an everyday occurrence.
9. Steps Towards Prevention (Textbook 9.6)
The instant reaction to the harm pesticides cause is to ban them — but the pests still have to be kept in check, and even with pesticides a significant amount of food is lost to them.
The long-term solution is to find effective methods of control that do far less harm and rest on sound biological principles.
Rotation of crops. Growing different crops on a particular piece of land in successive years reduces the occurrence of pests and the damage they do from year to year.
Studying the life histories of pests. Understanding how a pest spreads and what its life stages are lets seeds be sown at a time that reduces damage.
Biological control. Introducing the natural predators or parasites of the pest.
Genetic strains. Developing genetically modified plants resistant to a certain pest.
Environmental ethics. This concerns the morality of human activities as they affect the environment. Besides the laws, people need to know what is right and what is wrong in view of the environment. As the book puts it: protecting nature means protecting yourselves.
Key words from the chapter
Food chain, food web, niche, ecological pyramid, biomass, pesticides, bioaccumulation, biomagnification, ecofriendly activities, environmental ethics, metal contamination.
10. Summary
A food chain shows how energy passes from one organism to another, and the arrows between trophic levels always point from the food to the feeder.
Ecological pyramids are ways of showing food relationships and the flow of energy among living things. The pyramid of numbers shows the population of organisms at each trophic level, and the pyramid of biomass represents the available food as a source of energy at each level.
Biomass can also be used as a biofuel. The pyramid of energy is always upright, because a large fraction of energy is lost as heat at every transfer and heat lost to an ecosystem cannot be recovered.
The pesticides used to control pests are toxic and threaten the environment. Bioaccumulation is the entry of pollutants into the food chain; biomagnification is their tendency to concentrate as they move from one trophic level to the next.
There are several alternatives to pesticides through which more yield can be obtained with less damage — rotation of crops, biological control, and developing resistant strains among them.
The two Indian case studies in this chapter are documented research, not illustrations: the Kolleru figures come from a 2006 research paper on the lake's status between 1967 and 2004, and the Edulabad study was published in the International Journal of Life Sciences, Biotechnology and Pharma Research.
