Trophic levels, biomass pyramids, the carbon cycle, biodiversity loss and conservation — all of GCSE ecology covered in depth.
Ecology at GCSE covers a huge range of content — from the basic mechanics of food webs through to complex discussions of human impact on biodiversity and global warming. Exam questions range from straightforward recall to extended writing questions worth six marks. This guide covers all of it.
An ecosystem is all the living organisms in an area (the community) together with the non-living physical environment (abiotic factors such as temperature, light, water and mineral content of soil). A habitat is the specific place where an organism lives within an ecosystem. A population is all the individuals of the same species living in the same area at the same time.
Abiotic factors — temperature, light intensity, water availability, oxygen concentration, soil pH, wind speed — affect where organisms can survive and how well they can grow and reproduce. Biotic factors — predation, competition for food or territory, disease, parasitism — affect populations through interactions between organisms.
A food chain shows the transfer of energy from one organism to the next. Arrows point in the direction of energy flow — from prey to predator, not the other way.
Producers are always at the start of a food chain — these are plants and other photosynthetic organisms that make their own food from sunlight. Primary consumers eat producers. Secondary consumers eat primary consumers. Tertiary consumers eat secondary consumers.
A food web shows multiple interconnected food chains. Exam questions frequently ask what happens to a population when one organism in the web is removed or its numbers change. The key is to trace the effects through every pathway — a change in one species can have knock-on effects on species it is not directly connected to.
When answering food web questions, always consider both predator-prey relationships AND competition. If a prey species decreases, its predator may also decrease (less food). But if that predator also eats something else, the other prey may increase as predation pressure drops.
Each level of a food chain is called a trophic level. Producers are at trophic level 1, primary consumers at level 2, and so on. Only about 10% of the energy at one trophic level is transferred to the next. The other 90% is lost through: respiration (heat energy released), movement, undigested material in faeces, and parts of organisms that are not eaten.
This energy loss explains why pyramids of biomass always narrow as you move up trophic levels — there is less total mass of organisms at each successive level because there is less energy to support them. It also explains why food chains rarely extend beyond 4 or 5 links — there simply isn't enough energy left to support another level.
A pyramid of numbers shows the number of organisms at each trophic level. These can be oddly shaped — a single oak tree (producer) can support thousands of insects (primary consumers), giving an inverted base. A pyramid of biomass is almost always a conventional pyramid shape, because total mass decreases at each trophic level. If a question shows an unusual pyramid shape, check whether it's a pyramid of numbers or biomass before trying to explain it.
Carbon is constantly cycling between living organisms and the environment. The main processes:
Fossil fuels (coal, oil, natural gas) are the compressed remains of organisms that lived millions of years ago. When burned, the carbon that was locked away for millions of years is released as CO₂ in a very short time — disrupting the balance of the carbon cycle and contributing to the enhanced greenhouse effect.
Biodiversity refers to the variety of living organisms in an area — the number of different species (species richness) and the relative abundance of each. High biodiversity generally indicates a healthy, stable ecosystem. Low biodiversity indicates a stressed or degraded one.
Biodiversity matters for several reasons: ecosystems with high biodiversity are more resilient to change (if one species is lost, others can fill the ecological role); many medicines are derived from plant compounds that would be lost if species became extinct; organisms provide ecosystem services (pollination, water filtration, soil formation) that human life depends on.
Human activities have significantly reduced global biodiversity:
This is a specific process tested in detail. Fertilisers containing nitrates and phosphates wash into rivers and lakes. The nutrients cause algae to grow rapidly (algal bloom). The algae block light from reaching aquatic plants, which die. Decomposers break down the dead plants, using up oxygen in the water through respiration. Fish and other aquatic organisms die from lack of oxygen. The result is a nearly lifeless body of water.
Conservation aims to preserve biodiversity and ecosystems:
The full AQA Biology ecology specification is at the AQA GCSE Biology specification page.
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