What is an ecosystem?

An ecosystem is a community of living things (plants and animals) interacting with each other and with the non-living (abiotic) parts of their environment — soil, water, climate and light.

Every ecosystem has three key living roles:

• Producers — green plants that make their own food by photosynthesis (the start of every food chain).
• Consumers — animals that get energy by eating plants (herbivores) or other animals (carnivores).
• Decomposers — bacteria and fungi that break down dead material, returning nutrients to the soil.

Scale: ecosystems range from a tiny local pond to a huge global biome such as the tropical rainforest.

Food chains, webs and nutrient cycling

Energy flows through an ecosystem along a food chain, from producer to consumer. Because most animals eat several things, chains connect into a food web.

Nutrient cycling: when plants and animals die, decomposers break them down and release nutrients back into the soil. Growing plants then take these nutrients up again — so nutrients are recycled around the ecosystem.

Change ripples through the whole system

The parts of an ecosystem are interdependent — they rely on one another. Removing or changing one part affects the rest.

Take a small UK freshwater pond (or Epping Forest) as an example. If a disease wiped out the pond insects:

• The fish and birds that eat the insects lose their food, so their numbers fall.
• Plants once eaten by those insects may overgrow the pond.
• Fewer animals means fewer droppings and dead bodies, so nutrient cycling slows.

Key idea: ecosystems are balanced systems. A change to one component ripples outwards and can upset the whole web.

Who does what?

The global pattern of biomes

Large-scale ecosystems are called biomes. Their global distribution is controlled mainly by climate — especially temperature and rainfall — which is why biomes form broad bands linked to latitude.

Rainforests straddle the Equator (hot and wet all year); hot deserts sit around 30° north and south (the tropics); grasslands, temperate forests and cold tundra lie further towards the poles.

Inside a tropical rainforest

Tropical rainforests are hot (around 26–28°C all year) and very wet (over 2000 mm of rain a year), with no real seasons. This constant warm, wet climate supports huge biodiversity — over half the world's plant and animal species live here.

Everything is interdependent: the climate, water, soils, plants, animals and people all rely on one another.

• Dense vegetation grows in layers, with tall trees forming a leafy canopy.
• Fallen leaves rot quickly in the heat, so nutrients are recycled fast — but the soil itself is thin and poor because most nutrients are stored in the plants.

Key link: if the trees are cleared, heavy rain washes away the thin topsoil, and the nutrient store is lost — the whole system unravels.

Adapted for heat, rain and low light

Rainforest plants and animals are highly adapted to their environment:

• Drip-tips — pointed leaf tips let heavy rain run off quickly, stopping rot and mould.
• Buttress roots — huge above-ground roots support very tall trees in thin soil.
• Lianas — woody vines that climb the trees to reach the sunlight.
• Epiphytes — plants that grow high on branches to reach the light, taking water from the air.

Match the adaptation to its purpose

Tap an adaptation on the left, then tap its correct purpose on the right.

Why drip-tips?

Why rainforests are being cleared

Deforestation is the large-scale clearing of forest. AQA expects you to know several causes:

• Commercial & subsistence farming — clearing land for cattle ranching and crops (a major cause in the Amazon).
• Logging — felling trees for valuable hardwood timber.
• Mineral extraction — mining for iron ore, gold and other minerals.
• Road building — routes such as the Trans-Amazonian Highway open up the forest.
• Energy development — flooding valleys for hydro-electric dams.
• Settlement & population growth — more people need land, homes and resources.

The impacts of deforestation

Clearing the forest brings both benefits and serious costs:

• Economic development — farming, mining and logging create jobs, exports and income (a positive impact).
• Soil erosion — with no tree roots or leaf cover, heavy rain washes away the thin topsoil.
• Loss of biodiversity — habitats are destroyed and species are lost.
• Contribution to climate change — burning and clearing releases stored carbon dioxide, and fewer trees means less CO₂ absorbed.

Balance it: a good answer weighs the economic gains against the environmental costs — that's the classic "value of the rainforest" debate.

Cause or impact of deforestation?

Tap an item, then tap the box it belongs in.

Why rainforests are worth saving

Rainforests have huge value: they store carbon and help regulate the climate, hold over half of all species (biodiversity), provide medicines, food, timber and water, and support indigenous peoples and ecotourism.

That's why the world tries to manage them sustainably — meeting today's needs without destroying the forest for the future.

Your turn — impacts of deforestation

Managing rainforests sustainably

A range of strategies aim to protect rainforests while still allowing some use:

• Selective logging & replanting — only some trees are felled and new ones are planted.
• Conservation & education — protected areas plus teaching people the forest's value.
• Ecotourism — low-impact tourism that gives locals an income from the living forest.
• International agreements & debt reduction — wealthier nations cancel debt in return for forest protection.

Sustainable or unsustainable?

Tap whether each rainforest activity is a sustainable or an unsustainable use.

Case study: the Amazon rainforest, Brazil

The Amazon is the world's largest tropical rainforest, most of it in Brazil. It has suffered large-scale deforestation.

• Main causes: cattle ranching and soya farming, commercial logging, mining (e.g. the Carajás iron-ore project), road building (the Trans-Amazonian Highway) and hydro-electric dams.
• Impacts: loss of habitats and biodiversity, soil erosion, displacement of indigenous peoples, and the release of stored carbon that adds to climate change.
• Management: selective logging, replanting, protected areas and national parks, ecotourism, and international agreements to reduce deforestation.

Exam tip: a strong case-study answer names the place (the Amazon, Brazil), gives specific causes and impacts, and includes a real management strategy.

Understanding the Amazon

Life in a hot desert

Hot deserts are found around 30° north and south of the Equator. They are very dry (under 250 mm of rain a year), extremely hot by day yet often cold at night, with sparse vegetation and thin, sandy or stony soils.

As in every ecosystem, the climate, water, soils, plants and animals are interdependent — but here the limiting factor is the lack of water.

• Low biodiversity — far fewer species than a rainforest, because conditions are so harsh.
• The plants and animals that do survive are specially adapted to heat and drought.

Adapted to heat and drought

Desert species survive by saving water and coping with extreme temperatures:

• Cacti — thick waxy stems/leaves reduce water loss; spines cut water loss and deter animals; wide, shallow roots grab rain fast.
• Camels — store fat in their hump, can go days without water and lose very little through sweat.
• Many animals are nocturnal, sheltering underground by day to avoid the heat.

Opportunities and challenges

Hot deserts offer development opportunities but also real challenges:

• Opportunities: mineral extraction (mining), energy (huge solar potential), irrigated farming, and tourism.
• Challenges: extreme temperatures, water supply shortages, and inaccessibility (few roads, remote location).

Standard example: the Thar Desert (India/Pakistan) — mining, solar energy and irrigated farming are opportunities, while extreme heat and scarce water are challenges.

Opportunity or challenge?

Desertification and how to reduce it

Desertification is when once-productive land on the edges of deserts turns into desert. It threatens millions of people, especially in areas bordering the Sahara.

• Causes: climate change (less rainfall), population growth, removal of fuel wood, overgrazing, over-cultivation, and the soil erosion that follows.
• Strategies to reduce it: water and soil management, tree planting (roots bind the soil), and appropriate technology (e.g. simple stone lines that trap rainwater).

Exam tip: link each strategy to the cause it tackles — e.g. tree planting reduces soil erosion; water management tackles water shortage.

Your turn — reducing desertification

The key ideas to know

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