A-Level Geography: Ecosystems Under Stress

'Ecosystems under stress' is an optional physical geography section of the AQA A level specification (3.1.6), and many of its ideas, such as succession, nutrient cycling and threats to coral reefs, also appear in other boards' content on the biosphere and carbon cycle. This material trains the reasoning behind those ideas through short scenarios.

It starts with how ecosystems work. You follow energy through trophic levels, applying the approximate 10% transfer rule to energy and to biomass, and link net primary productivity to what the rest of the food web can support.

The succession questions trace change over time. A disused quarry in lowland Britain becomes a lithosere, from lichens and mosses through grasses, shrubs and birch to oak and ash woodland, the climatic climax. A hydrosere fills open water and turns it into land. Heather moorland and lowland heathland show the plagioclimax, held in place by burning, grazing and scrub cutting, and why conservation sometimes means continuing a disturbance rather than stopping it.

Biome questions compare the tropical rainforest, where most nutrients are held in living biomass and the soil store is small, with the savanna, with its wet and dry seasons and fire-adapted vegetation. The marine section sets out the conditions coral reefs need: warm, clear, shallow, sunlit water of normal salinity. It then explains bleaching, when heat-stressed corals expel their symbiotic algae, and ocean acidification, which reduces the carbonate ions corals use to build their skeletons.

Finally you consider biodiversity under pressure from human activity, including an invasive riverbank plant that spreads without its natural controls, and weigh up conservation strategies for local ecosystems.

The material offers a quiz with explanations, flashcards for the key terms, a printable written sheet of explanations and short calculations, and an oral exam in which an examiner asks one question at a time, including about the local ecosystem and region you have studied, and gives brief feedback at the end.

• Apply trophic-level transfer to energy and biomass and interpret net primary productivity
• Describe lithosere and hydrosere succession to the climatic climax
• Explain plagioclimax and sub-climax communities and their management
• Compare nutrient cycling and characteristics of tropical rainforest and savanna
• Explain the conditions for coral reefs and the threats of bleaching and acidification
• Explain causes of biodiversity decline and evaluate conservation strategies

Practice material written by Zestly, based on board specifications for the ecosystems option, such as AQA A-level Geography 7037, section 3.1.6 (Ecosystems under stress: ecosystems and processes, biomes, ecosystems in the British Isles over time, marine ecosystems, local ecosystems).