Human Influences on Ecosystems

Feeding a growing population

The human population has risen very quickly over the last two centuries. Better food supply and better medicine and sanitation mean fewer people die young, so more survive to have children of their own. A larger population needs more food, and that demand is what has driven farmers to produce more from the same land.

Agricultural machinery - tractors, combine harvesters and seed drills - lets a farmer work much larger areas of land, and work them faster and more efficiently, so a smaller workforce produces far more food. Chemical fertilisers supply the nitrate and other mineral ions that crop plants need for healthy growth, so yields improve. Insecticides kill the insect pests that eat and damage crops, which improves both the quality and the yield, while herbicides kill the weeds that compete with the crop for light, water and mineral ions.

Selective breeding improves production by choosing parents that show the best characteristics and breeding from them generation after generation - high-yielding wheat, or a cow that gives more milk. Genetic modification transfers a useful gene from one organism into a crop plant, so a single generation gains a new characteristic such as resistance to a pest or to a herbicide.

Monocultures, intensive livestock production and organic farming

A monoculture is a large area of land planted with a single crop. Its advantages are practical: the same machinery can be used across the whole field, planting, spraying and harvesting are easier and cheaper, and the whole crop ripens at the same time. Its disadvantages matter just as much. A single crop is a single food source, so a pest or disease that attacks it can sweep through the entire field, and monocultures are usually grown with heavy applications of fertilisers and pesticides. They also reduce biodiversity, because only one plant species lives there.

Intensive livestock production keeps animals at very high density, often indoors; the classic example is battery hens kept in crowded cages. Food, water, temperature and lighting are all controlled, disease is easier to manage, far less land is needed, and production is high and cheap. The cost is animal welfare: the animals have almost no space to move, which causes stress, and an infection can spread very quickly through a crowded flock.

Organic farming avoids artificial fertilisers and pesticides and uses natural methods instead - manure and compost for nutrients, crop rotation, and biological pest control. It causes less chemical pollution of soil and water, but yields are usually lower and the food is more expensive.

Habitat destruction, biodiversity and extinction

Biodiversity is the number of different species that live in an area. Humans reduce it every time they destroy a habitat, because the species living there lose their food, their shelter and their breeding sites. Land is cleared for housing, for crop plant production and for livestock production, and habitats are also destroyed to extract natural resources such as timber, minerals and oil. Freshwater and marine pollution damages habitats too, for example when untreated sewage or industrial waste is discharged into a river or the sea.

Destroying a habitat alters food webs and food chains. If one species disappears, the predators that fed on it lose a food source and the species it fed on may increase in number, so the whole community is disturbed and further species can die out. Extinction is the permanent loss of a species.

Deforestation - the clearing of forests - is the standard example of habitat destruction. You must be able to explain its undesirable effects one by one: reduced biodiversity, extinction, loss of soil, flooding and an increase of carbon dioxide in the atmosphere.

Pollution of water: sewage, fertiliser and eutrophication

Untreated sewage discharged into a river or lake carries organic matter and nitrate ions. The organic matter is broken down by bacteria, and those decomposers use up the dissolved oxygen in the water during aerobic respiration, so fish and other organisms that need dissolved oxygen suffocate. Excess fertiliser washed off farmland causes the same final result by a different route: the nitrate ions leached from the soil into the water act as nutrients, so the producers in the water - algae and other plants - grow extremely quickly. This sequence is called eutrophication.

Eutrophication runs through a fixed sequence of steps, and examiners want them in order: (1) nitrate and other ions leach into the water, so their availability increases; (2) the growth of producers increases, producing a bloom of algae; (3) the producers die and are decomposed; (4) the decomposers increase in number and respire aerobically; (5) the dissolved oxygen in the water is reduced; (6) organisms that require dissolved oxygen, such as fish, die.

Non-biodegradable plastics pollute both aquatic and terrestrial ecosystems. Because they are not broken down by microorganisms they persist for hundreds of years, they entangle and choke animals, and when fish and birds eat them the plastic fills their stomachs so they cannot eat real food and starve.

• Sewage and fertiliser run-off both end in the same place: too little dissolved oxygen for fish.
• In sewage the oxygen is used up by decomposers breaking down organic matter; in eutrophication the decomposers break down a bloom of dead producers.
• Eutrophication is a sequence, so write the steps in order and keep 'increased aerobic respiration by decomposers' as its own step.
• Non-biodegradable plastics do not decay, so they accumulate in food chains and in the environment.

Air pollution, the greenhouse effect and acid rain

The greenhouse effect is natural and essential to life. Greenhouse gases in the atmosphere, especially carbon dioxide and methane, absorb the infrared radiation given out by the Earth's surface and re-radiate some of it back, so heat is trapped and the Earth stays warm enough for living things. Carbon dioxide is released by burning fossil fuels and by deforestation; methane is released by cattle, by rice paddy fields, from landfill sites and during the extraction of fossil fuels.

The enhanced greenhouse effect is the extra warming caused by the extra greenhouse gases that humans add to the atmosphere. More heat is trapped, so average global temperatures rise - that is global warming - and the resulting change in weather patterns over long periods is climate change. The consequences you must know are rising sea levels (ice sheets and glaciers melt and warm water expands, so low-lying land is flooded), changes in species distribution (species move towards the poles or to higher, cooler ground, and those that cannot move fast enough may become extinct) and extreme weather such as more frequent droughts, storms and heatwaves.

Acid rain is caused by sulphur dioxide released when fossil fuels containing sulphur are burned. The sulphur dioxide dissolves in rainwater to form a weak acid. Acid rain lowers the pH of lakes, killing fish and other aquatic organisms; it releases toxic ions such as aluminium from the soil and washes nutrients away, so trees lose their leaves and grow poorly; and it corrodes the stonework and metal of buildings, statues and bridges.

Conservation: protecting species, habitats and resources

A sustainable resource is one that is produced as rapidly as it is removed from the environment, so that it does not run out. Some resources can be conserved and managed sustainably, and the syllabus limits this to forests and fish stocks. Forests can be conserved by education, protected areas, quotas and replanting. Fish stocks can be conserved by education, closed seasons, protected areas, controlled net types and mesh size, quotas and monitoring.

Organisms become endangered or extinct because of climate change, habitat destruction, hunting, overharvesting, pollution and introduced species. An introduced species often does the most damage on an island, where it may have no natural predator and so outcompetes or eats the native species. Endangered species can be conserved by monitoring and protecting species and habitats (for example in national parks and nature reserves), by education, by captive breeding programmes in zoos, and by seed banks that store the seeds of many plant varieties so that crops and wild plants are not lost.

There are clear reasons for running conservation programmes: to maintain or increase biodiversity, to reduce extinction, to protect vulnerable ecosystems, and to maintain ecosystem functions - nutrient cycling and the provision of resources such as food, drugs, fuel and genes. A small population is a risk in itself, because it has reduced genetic variation; with fewer alleles there is less chance that some individuals will survive a new disease or a change in the environment, so the species may become extinct.

In captive breeding programmes, artificial insemination (AI) and in vitro fertilisation (IVF) are used to produce offspring from animals that cannot or will not mate naturally, and to increase the number of offspring produced by a rare animal.

Reducing the effects of human activity

Each problem in this topic has a matching strategy, and examiners like answers that link the two together rather than listing solutions. Renewable energy - wind, solar, hydroelectric, tidal and geothermal power - replaces the burning of fossil fuels, so less carbon dioxide and less sulphur dioxide are released. Reducing deforestation and replanting cleared land protects the soil and the species that live in the forest, and the new trees remove carbon dioxide from the atmosphere by photosynthesis.

Treating sewage before it is discharged removes the organic matter and the nutrients, so decomposers do not use up the dissolved oxygen in the river and the water stays able to support fish. Conserving resources means using them sustainably: reduce, reuse and recycle materials, use sustainable timber and fish stocks, and keep the rate of removal below the rate at which the resource is replaced.