The human population has a dramatic impact on the environment. Only a few hundred years ago the human population was much smaller and lived a very different type of lifestyle. Many people were nomadic, they were wanderers, and travelled around to find food. Because they didn’t stay in one place very long, they did not cause much damage to the environment. To provide enough food, housing, and roads for today’s population, more land has to be cleared of its natural vegetation. The amount of waste we produce causes many problems; sewage and refuse can cause health risks; industrial wastes are often toxic and accumulate in living organisms. There is also a problem with how to dispose of these wastes. We use up vast amounts of the Earth’s resources to provide energy and raw materials to make numerous consumer goods. Quarrying for raw materials causes pollution as well as damaging the habitats of plants and animals.

Many human activities cause air pollution. The burning of fossil fuels; coal, oil and gas; in power stations, factories, vehicle engines and in our homes, causes a great deal of pollution. The information in this table is incomplete. Can you match the effects with their corresponding pollutants? Using the mouse, drag the letters beside the pollutants to match their effects.

Some pollutant gases are called ‘greenhouse gases’. This is because they form a layer in the atmosphere, which acts like the glass in a greenhouse. Radiation from the Sun reaches the Earth during the day and warms it up. Much of this energy is reflected off the Earth towards space. However, the greenhouse gases absorb a lot of heat so the Earth isn’t cooling down as it should at night and so is gradually getting warmer. This is called global warming. The main greenhouse gases are carbon dioxide and methane.

Photosynthesis is the process by which plants make their food using carbon dioxide and water. Plants are therefore very important in controlling the amount of carbon dioxide in the air, so we should not destroy vast areas of plant life, such as the tropical rainforests. We should also try to cut down on the amount of carbon dioxide produced from burning fossil fuels by using less electricity.

Methane is produced when bacteria digest plant material. The increasing number of rice fields has increased the amount of methane released into the atmosphere. Thousands of tonnes of methane are released each year from the guts of herbivores like cattle. There is great concern about the effects of global warming. A small increase in temperature may melt the polar ice caps, causing a rise in sea level. Low lying areas of the world may be flooded. The change in world climate would affect crop growth in important food producing areas of the world. We have already experienced unusual weather conditions around the world such as violent storms, hurricanes and flooding.

The burning of fossil fuels also causes acid rain. Sulphur dioxide, carbon dioxide and oxides of nitrogen are all acidic gases. These gases contribute to the formation of acid rain, but it’s the sulphur dioxide that’s the main cause of acid rain. The sulphur dioxide gas produced by power stations, industry, and vehicles rises into the atmosphere and may be carried long distances by the wind. The gas dissolves in the water in clouds, forming sulphuric acid that is released as acid rain. Acid rain may damage trees directly, and if the water in rivers and lakes becomes too acidic, plants and animals cannot survive there. Acid rain damages buildings made of limestone and marble, by chemically reacting with the stone. It also corrodes metals. The obvious solution is to burn less fossil fuel. Many countries are investing in cleaner sources of energy, such as wind and water, for the production of electricity. Another solution is to remove the sulphur dioxide from chimney smoke with scrubbers.

In the U.K. there are strict laws which prevent anyone polluting water in rivers, ponds or the sea, but sometimes these laws are broken or accidents happen. Dying fish or birds are often the first signs of water pollution. Water pollution mainly consists of sewage and chemicals from industry and agriculture. Sometimes untreated sewage is passed into the sea through a long pipe and the tide carries the sewage back towards the beach. This looks unpleasant and is a health hazard for bathers. When human wastes are treated, the breakdown products contain nutrients for bacteria. These micro-organisms increase in numbers and reduce the amount of dissolved oxygen available for other animals. Farmers often add fertilisers to the soil to promote crop growth. Fertilisers contain nitrates and phosphates. These can be washed out of the soil into streams and rivers where they provide nutrients for algae, resulting in their rapid growth. This is called an algal bloom. These poisons can contaminate drinking water supplies. High levels of nitrates in drinking water are a health concern. Nitrates damage red blood cells, causing blue-baby syndrome. There could also be links with cancer in animals and humans.

Pesticides are sprayed onto crops to control weeds, insects and fungi. Pesticides are especially dangerous when they enter living organisms, because they accumulate in fat and other tissues. They are then passed along food chains, becoming more and more concentrated, because organisms further along the chain will eat a larger number of the previous organism. The animals that get the biggest dose of the toxin are those at the top of the food chain.

In recent years there have been several accidents at sea involving oil tankers. The oil from damaged tankers escapes, resulting in large oil slicks. Crude oil is a very toxic substance that sticks to the feathers of birds, and the fur of seals and otters. These animals are poisoned by the oil when they try to clean it off.

Pollutants such as chemical fertilisers and sewage can kill a lake by a process called eutrophication. Chemical fertilisers contain high levels of the plant nutrients nitrates and phosphates. Micro-organisms feed on sewage and release nitrates and phosphates from it. When the nutrients enter lakes, they cause a massive growth of algae. The algae form a thick green blanket on the surface of the water, preventing light from reaching submerged water plants. The plants compete for the light available and many die. Microbes then decompose the dead plants. Because there is plenty of food for the bacteria, they rapidly increase in numbers and reduce the amount of oxygen in the water as they respire. Fish and other aquatic animals die because of the shortage of oxygen.

Modern farming methods have had a big impact on the environment. Habitats have been lost as more and more land is used for agriculture. Toxins have entered food chains due to the widespread use of pesticides, and fertilisers have contaminated water supplies. Can anything be done to maintain sufficient food supplies but also protect our environment? As an alternative to pesticides, biological control methods can sometimes be used. This involves introducing a natural predator into the habitat to kill the pests. For example, greenfly are pests because they feed on crops. Ladybirds are their natural predator, so they can control the greenfly population with little effect on other organisms. However, introducing a predator can sometimes have unexpected results. Cane toads were introduced into Australia to kill beetles, but the toad population has increased so much that they are now considered to be a pest! Some farmers avoid the use of chemical fertilisers by adding natural manure to the soil instead. This breaks down slowly so nutrients are returned gradually to the soil, reducing the chance of nutrients being washed out of the soil and contaminating water supplies. Manure also improves soil structure and drainage.

Marine ecosystems have been disturbed by over-fishing, resulting in a reduction in the number of younger fish. As a result, the breeding stock is reduced and populations of fish decrease. Fishing in some areas is no longer worthwhile, but there are measures that could be taken to increase fish populations. Avoid fishing in areas where fish are breeding. Only remove fish above a certain size. Limit the number of fish that can be caught by setting fishing quotas. Reduce the amount of pollution being released into the sea. Ban fishing for certain species. Some methods of fishing also affect other species. The large driftnets used to catch tuna fish also trap and drown dolphins. The fall in fish populations doesn’t just affect humans. Birds, seals, otters, and other fish-eating animals also decline in numbers as their food becomes scarce.

Deforestation is the destruction of vast areas of forests. Forests are usually cleared to provide land for agriculture and roads, and sometimes for timber. Deforestation has far reaching effects. All living organisms need oxygen for respiration and this gas is produced when plants photosynthesise. The plants also remove carbon dioxide from the atmosphere. A reduction in the number of trees might lead to an upset in the balance of these gases in the atmosphere. The increase in carbon dioxide contributes to the Greenhouse Effect and global warming. This effect is made worse when forests are burnt to clear them. This releases even more carbon dioxide into the atmosphere. When forests are cleared, the soil is left exposed to wind and rain. Without the tree roots to bind the soil together, and the canopy of leaves to protect the soil from direct winds and rain, the fertile topsoil can be blown or washed away leaving barren land. This is called soil erosion.

Deforestation also affects the water cycle in the area. Rain forests are humid places. The trees absorb water from the soil, which passes up the tree to be evaporated from the leaves. This water vapour rises and eventually condenses to form clouds, which release the water as rain to be reabsorbed by the plants. If the trees are removed, rainwater flows away. The atmosphere becomes less humid resulting in less rainfall. The forest can become more like a desert - dry and barren. Destroying rain forests damages the habitats of many different species of plants and animals, which may become extinct. Many of our modern day drugs originated from organisms living in the rainforests. If species become extinct, we could lose possible sources of new medicines.

If all the many species of organisms which exist on the Earth today are to be given the chance to survive, ecosystems must be allowed to remain in balance. The human population threatens to disrupt this delicate balance by pollution, intensive farming methods, and destruction of habitats. What can be done to limit man’s destructive influences? Conservation means taking measures to improve the survival of plants and animals and the places where they live.

Pollution can be reduced by developing alternatives to fossil fuels for producing electricity. This will also conserve resources. Animals that we kill for food, like fish and whales, must not be over-hunted. Local habitats can be saved by controlling building in green-belt areas. Some habitats can be legally protected as nature reserves and national parks. Endangered species such as bats, whales, white rhinos, and orchids should be protected. Natural forests should not be destroyed. Timber should be cut from plantations where felled trees are replaced by new seedlings. This is an example of sustainable development. We should encourage organic farming to reduce the use of harmful chemicals. Resources such as glass, metals, paper, and plastics should be recycled wherever possible to save energy and raw materials, as well as protecting habitats. You could try to find out about local conservation projects in your area.

When organisms die, they begin to decay or decompose. This is an important process, because it releases valuable nutrients back into the environment to be reused by other organisms. It also ensures that dead organic material does not ‘pile up’. Micro-organisms like bacteria and fungi are important decomposers which grow and feed on dead plants and animals. The rate of decomposition depends on a number of factors. Decomposition is faster if it is warmer because the reactions involved are controlled by enzymes. Enzymes are biological catalysts that speed up chemical reactions. They work faster if it is warmer, up to about 40°C. Above this temperature, the enzymes become denatured. The rate is also increased in moist conditions, and most decomposers are more active when there is plenty of oxygen.

Some people have a compost heap in their garden. They pile dead leaves and plants together and leave them to decay. They can later put the partially decomposed material onto their soil to fertilise it without the need for chemicals. Micro-organisms are used in sewage works to decompose the wastes from humans.

Carbon is found in all living organisms. It is a vital chemical element needed to make carbohydrates, fats, proteins, and other compounds that make up living things. Carbon dioxide in the air is the source of carbon for living organisms. The amount of carbon dioxide in the air remains constant at 0.03% because it is returned to the air at the same rate as it is removed. Plants absorb carbon dioxide from the air and use it to make glucose in the process of photosynthesis. Inside the plant, the glucose is converted to other carbon compounds such as carbohydrates, fats, and proteins.

Animals obtain carbon compounds by eating plants or other animals which have eaten plants. They use the carbon to produce their own proteins, fats, and carbohydrates. When plants and animals die, they provide food for other animals, or are decomposed by micro-organisms. All living organisms plants, animals, bacteria, and fungi return carbon dioxide to the air as they respire. Some carbon is converted to carbon compounds in coal, oil, and gas in the process of fossilisation. When fuels are burned, carbon dioxide is produced.

Nitrogen is an essential element needed by all living organisms to make proteins. The air contains 78% nitrogen, but plants and animals cannot absorb nitrogen gas. Nitrogen gas must first be changed into nitrates or ammonium compounds that plants can absorb from the soil through their roots. Animals obtain nitrogen by eating plants, or other animals that have eaten plants.

Lightning makes the air so hot that the nitrogen and oxygen combine to make nitrogen oxides. These dissolve in rainwater and get washed into the soil where they react to form nitrates, which can be absorbed by plants. The only organisms that can use the nitrogen in the air directly are nitrogen-fixing bacteria, which live in the soil and in root nodules on the roots of some plants such as peas, beans and clover. They make nitrates, which are released into the soil or plants. Farmers sometimes grow a crop of these plants and then plough them back into the soil to replace lost nitrates. When plants and animals die they are decomposed by microbes in the soil. The protein in their bodies is broken down into ammonium compounds. Ammonium compounds are also produced when microbes break down animal waste products such as urine and faeces.

Nitrifying bacteria in the soil convert ammonium compounds into nitrates, which are then absorbed by the plants. The nitrogen is incorporated into amino acids. These are the building blocks of proteins. The valuable soil nitrates can be changed back into nitrogen by denitrifying bacteria that live in waterlogged soils. In addition to these processes in living organisms, nitrogen can be added to the cycle when farmers add fertilisers to the soil to replace nitrogen. The nitrogen is removed when crops are harvested.