Man and his environment: Class 10 Biology

Key points

The environment is everything that surrounds an organism, including living (biotic) and non living (abiotic) factors. Ecology is the study of the relationships between organisms and their environment. An ecosystem is a community of living things interacting with their non living surroundings, such as a pond, forest or farm.

Every ecosystem has producers, consumers and decomposers. Producers are green plants that make food by photosynthesis. Consumers eat other organisms: primary consumers are herbivores, secondary consumers are carnivores that eat herbivores, and tertiary consumers eat other carnivores. Decomposers, such as bacteria and fungi, break down dead matter and return minerals to the soil.

A food chain shows the transfer of food energy from one organism to the next, such as grass to grasshopper to frog to snake to eagle. Each step is a trophic level. Several linked chains form a food web. Energy flows in one direction and only about ten percent passes to the next level, since most is lost as heat in respiration and in waste. This is why food chains are short and top carnivores are few.

A pyramid of numbers or biomass is wide at the bottom with producers and narrows towards the top consumers. It shows that producers are more in number or mass than the animals that feed on them.

Two important cycles in nature:

• Carbon cycle: plants take in carbon dioxide by photosynthesis. Animals get carbon by eating plants. Respiration, decay and burning of fuels return carbon dioxide to the air.
• Nitrogen cycle: air is nearly 78 percent nitrogen, but most organisms cannot use it directly. Nitrogen fixing bacteria in root nodules of legumes and lightning change it to nitrates. Plants take up nitrates and make proteins. Animals eat the plants. Decomposers change dead matter to ammonium compounds, nitrifying bacteria change these to nitrates, and denitrifying bacteria return some nitrogen to the air.

Human population has grown quickly because of better food supply, medical care and living conditions. A larger population uses more food, water, fuel and land. It leads to cutting of forests, more waste and more pollution. So population growth has to be balanced against the resources of the Earth.

Pollution is the addition of harmful substances to the environment. The main kinds are:

• Air pollution: caused by smoke and gases from vehicles, factories and burning of wastes. Carbon monoxide, sulphur dioxide and oxides of nitrogen cause breathing problems and acid rain.
• Water pollution: caused by sewage, industrial wastes, fertilisers and oil spills. It spreads diseases such as cholera and typhoid and harms aquatic life.
• Soil pollution: caused by plastic, chemicals, pesticides and solid waste, which reduce soil fertility.
• Noise pollution: loud sound from traffic and machines that causes stress and hearing loss.

Global warming is the gradual rise in the average temperature of the Earth. It is mainly due to increase of greenhouse gases such as carbon dioxide and methane, which trap heat from the Sun. Burning fossil fuels and cutting forests add carbon dioxide. Effects include melting of glaciers, rise in sea level, floods and changes in weather. Pakistan is among countries affected by floods and heat waves.

The ozone layer in the upper atmosphere absorbs harmful ultraviolet rays from the Sun. Chlorofluorocarbons (CFCs) from old refrigerators, aerosol sprays and air conditioners break down ozone. Thinning of the layer increases skin cancer and eye cataracts and harms crops. The use of CFCs has been reduced by international agreement.

Acid rain is rain with a pH lower than normal, formed when sulphur dioxide and nitrogen oxides dissolve in rainwater. It damages buildings, soil, crops and forests, and kills life in lakes.

Deforestation is the large scale cutting of forests. It causes soil erosion, floods, loss of habitat and rise of carbon dioxide. Afforestation, which is planting trees on new land, and reforestation help to repair the damage.

Conservation is the careful use and protection of natural resources. Ways include saving water and energy, recycling paper, glass and metals, using renewable energy such as solar and wind power, protecting wildlife through national parks and sanctuaries, and growing more trees. Every person can help by reducing waste, reusing items and recycling.

Study tip: Learn the carbon and nitrogen cycles as labelled diagrams with arrows, and always give causes, effects and remedies when answering questions on pollution. In food chains, start from the producer and point arrows towards the consumer.

Definitions and flashcards

Say the meaning first, then open the card to check.

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Common mistakes

• Confusing global warming with ozone depletion. The first is caused by greenhouse gases and the second by CFCs.
• Saying that energy is recycled in an ecosystem. Matter is recycled, but energy flows one way.
• Drawing arrows in a food chain from the eater to the food. Arrows point from the food to the eater.
• Writing that plants use nitrogen gas directly from the air.
• Leaving out decomposers when listing the parts of an ecosystem.

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Practice questions and timed test Printable worksheet