Ecosystem concepts and biodiversity; nutrient cycling and succession; biomes and their functioning; ecological responses to environmental change; and the management of fragile ecosystems under threat.

What this dot point is asking

AQA section 3.1.6 wants you to understand ecosystem concepts and biodiversity, explain energy flow, nutrient cycling and succession, describe the functioning of biomes, analyse ecological responses to environmental and human change, and evaluate the management of fragile and threatened ecosystems. The synoptic idea is that ecosystems are dynamic systems with energy flows and nutrient cycles, and that human pressure disrupts these flows faster than the system can re-equilibrate.

Ecosystem concepts and biodiversity

An ecosystem is a community of organisms (the biotic component) interacting with each other and with their abiotic environment (climate, soil, water). Energy flows in one direction through trophic levels (producers, primary, secondary and tertiary consumers, and decomposers) shown in food chains and webs. Energy is lost as heat at each transfer through respiration, typically around 90 percent per level, so the number of links is limited and pyramids of biomass narrow upward. Energy is therefore not recycled, in contrast to nutrients.

Nutrient cycling and succession

Succession is the directional change in communities over time. A prisere starts on a previously unvegetated surface; named examples are a lithosere (bare rock), psammosere (sand dune), hydrosere (fresh water) and halosere (salt marsh). Each seral stage modifies conditions until a climatic climax is reached, unless human activity (grazing, burning, mowing) arrests it at a plagioclimax such as managed heathland or chalk grassland.

Biomes and ecological responses

Biomes reflect the global distribution of temperature and precipitation, which together set the limits on plant productivity. Within them, ecosystems respond to environmental change (long-term climate shifts, fire, disease) and increasingly to human activity: deforestation, agricultural expansion, pollution, over-exploitation, the spread of invasive species and urbanisation all reduce biodiversity, simplify food webs and disrupt nutrient and energy flows. These responses can be gradual or, once a threshold is crossed, abrupt and difficult to reverse.

Managing fragile ecosystems

Fragile ecosystems (coral reefs, wetlands, tropical rainforests, tundra) recover slowly and are highly vulnerable to climate change and direct exploitation. Management includes protected areas and national parks, sustainable resource use (selective logging, ecotourism), rewilding and restoration, international agreements (CITES controlling trade in endangered species, the Ramsar Convention protecting wetlands) and local community involvement that balances conservation with livelihoods. The hardest cases, such as coral reefs, are limited by the fact that the main threat is global climate change rather than a local, manageable pressure.

Try this

Q1. Define a biome. [2 marks]

• Cue. A large-scale ecosystem defined by dominant vegetation and controlled mainly by climate.

Q2. Name the three stores in the Gersmehl nutrient cycle model. [2 marks]

• Cue. Biomass, litter and soil.

Q3. Explain why tropical rainforest soils are poor despite the lush vegetation. [4 marks]

• Cue. Nutrients are held in the biomass store, not the soil; rapid decomposition and uptake recycle nutrients quickly, while heavy rainfall leaches the soil.