Ecology, for the exam.

Part 1Levels of organisation

Ecology has a ladder of words and examiners want them used precisely. A habitat is the place where an organism lives. A population is all the organisms of one species in a habitat. A community is all the populations of different species living together.

An ecosystem is the community of living things plus the non-living surroundings they interact with. So an ecosystem is alive and non-living together — a pond, a wood, a rock pool.

Part 2Competition & interdependence

To survive and breed, organisms need resources — and where resources are limited, they compete for them. Plants compete for light, water, mineral ions and space. Animals compete for food, mates and territory.

Species also rely on each other. This is interdependence: animals depend on plants for food and oxygen, plants depend on animals for pollination and seed dispersal. A community where every species depends on others and stays roughly steady is described as a stable community — the populations and abiotic conditions stay in balance over time.

Part 1Three types of adaptation

An adaptation is a feature that helps an organism survive and reproduce in its environment. AQA splits them into three kinds, and you should be able to give an example of each.

Structural adaptations are features of the body — shape, colour, body parts. A camel's thick foot pads spread its weight on sand; a polar bear's white fur is camouflage. Behavioural adaptations are the way an organism acts — birds migrating to warmer places, or animals being active at night to avoid the heat. Functional adaptations happen inside the body — how it works, such as a camel producing very little, concentrated urine to save water, or some animals slowing their metabolism to hibernate.

Part 2Extremophiles

Some organisms — mostly bacteria — are adapted to live in extreme conditions that would kill most life. These are called extremophiles. They survive very high temperatures (such as deep-sea volcanic vents), very high pressure, or very salty (high-salt) water.

Part 1Food chains & trophic levels

A food chain shows what eats what. It always starts with a producer — usually a green plant or algae that makes its own food by photosynthesis. Producers are eaten by primary consumers (herbivores), which are eaten by secondary consumers, then tertiary consumers. A consumer that kills and eats another animal is a predator; the one eaten is its prey.

Part 2Predator–prey cycles

In a stable community, the numbers of predators and their prey rise and fall in cycles. When there is plenty of prey, predators have lots of food, so the predator population grows. More predators eat more prey, so the prey numbers fall. With less food, predator numbers then fall — which lets the prey recover, and the cycle repeats.

The two graphs stay out of step: the predator peak comes slightly after the prey peak, because the predators take time to respond to the change in food.

Part 1Carbon goes round and round

Carbon is constantly recycled. It moves out of the air, into living things, and back to the air again. There are four processes you must know — and you should be able to say whether each one removes carbon dioxide from the air or returns it.

Photosynthesis removes carbon dioxide from the air: plants and algae take in CO₂ and lock the carbon into glucose and then into their tissues. This is the only process that removes CO₂ from the atmosphere. Respiration returns CO₂ to the air — every living thing (plants, animals and microorganisms) respires. Combustion (burning fuels and wood) returns CO₂ to the air. Decomposition returns it too: when decomposers (microorganisms) break down dead matter and waste, they respire and release CO₂.

Part 1The journey of water

The water cycle provides fresh water for plants and animals on land before it drains back to the sea, and it keeps going round and round. The energy that drives it comes from the Sun.

Energy from the Sun makes water evaporate from the seas, lakes and rivers, turning liquid water into water vapour. As the vapour rises and cools, it condenses back into tiny droplets, forming clouds. The water then falls as precipitation — rain, snow or hail. Some of this water is taken up by plants and returned to the air by transpiration; the rest runs back into rivers and the sea, and the cycle repeats.

Part 1Biodiversity & pollution

Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem. High biodiversity makes ecosystems more stable, because species depend on each other less for any single resource. Many human activities are reducing biodiversity, and that is a serious problem for the future.

A rapidly growing human population uses more resources, and we produce more waste and pollution. Pollution kills plants and animals and reduces biodiversity. It happens in three places:

Water — by sewage, fertiliser run-off and toxic chemicals. Land — by landfill and toxic chemicals such as pesticides and herbicides. Air — by smoke and acidic gases like sulfur dioxide.

Part 2Land use, deforestation & global warming

Humans reduce the land and resources available for other organisms by using more for building, quarrying, farming and dumping waste. The destruction of peat bogs to produce compost is one example — it also releases the stored carbon as CO₂, adding to the problem.

Deforestation — cutting down large areas of forest, often in tropical regions — happens to clear land for farming (cattle and rice) and to grow crops for biofuels. It causes more carbon dioxide to be released (from burning and from the activity of decomposers), reduces the rate at which CO₂ is removed from the air by photosynthesis, and reduces biodiversity by destroying habitats.

Levels of carbon dioxide and methane in the atmosphere are increasing, and these are greenhouse gases. They cause global warming: a rise in the average temperature of the Earth. The consequences include changing climates, rising sea levels (from melting ice and the expansion of warmer water), and changes to the distribution of species — meaning some species lose their habitats and biodiversity falls.

Part 1Protecting what's left

Scientists and governments run programmes to reduce the loss of biodiversity. You need to know the main methods and be able to say what each one achieves.

Breeding programmes for endangered species help keep their numbers up (for example in zoos). Protecting and regenerating rare habitats — such as mangroves, heathland and coral reefs — keeps whole communities alive. Reintroducing field margins and hedgerows in farmland, where farmers grow only one type of crop, gives more species somewhere to live. Reducing deforestation and the carbon emissions that go with it protects forests and the climate. And recycling resources rather than dumping waste in landfill reduces pollution.

Part 2The trade-off

Conservation can be expensive and is sometimes in conflict with the pressures of feeding and housing a growing human population, or with local economic interests. Decisions often involve a trade-off between protecting biodiversity and the costs to people. In the exam you may be asked to weigh up both sides using data — so look for the figures and refer to them.