The concept of an ecosystem, the balance between living and non-living components, food chains, food webs, nutrient cycling, a small-scale UK ecosystem, and the global distribution and characteristics of large-scale biomes.

What this dot point is asking

This is AQA GCSE Geography (8035) Paper 1, Section B (3.1.2 The living world). AQA expects you to define an ecosystem and distinguish producers, consumers and decomposers, explain how food chains, food webs and nutrient cycling link the living and non-living parts, show how a change to one part affects the rest using a small-scale UK example, and describe and locate the world's large-scale biomes. This topic is the foundation for the rainforest and hot desert case studies that follow.

The structure of an ecosystem

Energy enters almost every ecosystem as sunlight. Producers (green plants and algae) capture this energy and convert it into stored chemical energy (food) by photosynthesis: they are the base of the system. Consumers cannot make their own food, so they eat plants (primary consumers, herbivores) or other animals (secondary and tertiary consumers, carnivores). Decomposers (bacteria and fungi) break down dead plants, animals and waste, releasing the nutrients locked inside back into the soil. At each step, most energy is lost as heat and movement, which is why food chains rarely have more than four or five links.

Food chains, food webs and nutrient cycling

A food chain shows a single line of energy flow from a producer to a top consumer (for example oak leaves to caterpillar to blue tit to sparrowhawk). A food web links many overlapping chains together; it is a more realistic model because most animals eat more than one type of food and are eaten by more than one predator. This overlapping is what makes an ecosystem resilient to small changes but vulnerable to losing a key species.

A small-scale UK example is a freshwater pond. The producers are algae and pondweed; primary consumers include water fleas and tadpoles; secondary consumers include small fish; and herons are top predators. If fertiliser run-off pollutes the pond, algae bloom and then die, decomposing bacteria use up the dissolved oxygen (eutrophication), the fish suffocate, and the herons lose their prey. This shows the interdependence of biotic and abiotic factors: a change in water quality (abiotic) collapses the living community.

Global distribution of large-scale biomes

A biome is a large-scale ecosystem covering a huge area with a distinctive climate, soil, plant and animal community. The major biomes are arranged in broad latitudinal belts because climate is controlled by latitude and global atmospheric circulation:

• Tropical rainforest in a belt along the Equator, where rising air gives heat and heavy rain all year.
• Hot desert around 30 degrees north and south, under the dry, sinking air of the Hadley cell.
• Tropical grassland (savanna) between the rainforest and the deserts, with a marked wet and dry season.
• Temperate forest and grassland in the mid-latitudes (for example the UK and the North American prairies).
• Tundra and polar near the poles, where it is cold and dry all year.

Climate, especially the combination of temperature and rainfall, is the main control on where each biome is found, with soil type and altitude as local modifiers.

Try this

Q1. Define the term ecosystem. [2 marks]

• Cue. A community of living organisms interacting with the non-living environment in a particular area.

Q2. Explain how the removal of one species can affect a whole food web. [2 marks]

• Cue. Organisms are interdependent, so losing one species removes food for those above it and reduces grazing pressure below it, disrupting the balance.