Communities and competition, abiotic and biotic factors, adaptations, levels of organisation and feeding relationships, the carbon and water cycles, biodiversity and the human impact on ecosystems including pollution, land use and climate change.

What this topic is asking

AQA wants you to describe communities and competition, distinguish abiotic and biotic factors, explain adaptations and feeding relationships, describe the carbon and water cycles, and explain biodiversity and how humans affect ecosystems.

Communities, competition and adaptations

Organisms compete with each other for resources. Plants compete for light, water, space and mineral ions; animals compete for food, mates and territory. Within a species, individuals compete with each other (intraspecific competition); between species, those needing the same resources compete (interspecific competition). A stable community is one in which the populations and abiotic factors are in balance, so population sizes stay roughly constant over time.

• Abiotic factors: light intensity, temperature, moisture, soil pH and mineral content, wind intensity and direction, and carbon dioxide and oxygen levels. A change in any one (for example a drought reducing moisture) shifts the distribution of organisms.
• Biotic factors: availability of food, the arrival of a new predator, a new pathogen, and competition between species (one outcompeting another for the same resource).

Feeding relationships and cycles

Feeding relationships are shown in food chains, which always begin with a producer (usually a green plant or alga) that makes glucose by photosynthesis, transferring light energy into the chain as chemical energy in biomass. Primary consumers (herbivores) eat producers, secondary consumers eat primary consumers, and so on; an animal that hunts another is a predator and the hunted is its prey. In a stable community, predator and prey populations rise and fall in linked cycles: more prey allows the predator population to grow, which then reduces the prey, which in turn reduces the predators, and so on.

The carbon cycle recycles the element carbon between living things and the environment: photosynthesis removes carbon dioxide from the air and fixes it into glucose and then biomass, while respiration, combustion (burning) and decay by microorganisms return carbon dioxide to the air. Decomposers (bacteria and fungi) are central because they release the carbon and mineral ions locked in dead material back into the cycle, and they work fastest in warm, moist, well-oxygenated conditions. The water cycle provides fresh water on land through evaporation from oceans, condensation into clouds, and precipitation, with water returning to the sea by rivers.

Biodiversity and human impact

Biodiversity is the variety of all the different species of organisms on Earth, or within an ecosystem. High biodiversity makes ecosystems more stable because species are less dependent on one another and on particular abiotic conditions, so the system copes better with change. Human activities reduce biodiversity through pollution of water (sewage, fertiliser), air (smoke, acidic gases) and land (landfill, toxic chemicals); through land use for farming, building and quarrying; through deforestation to provide land and timber; and through peat bog destruction, which both releases stored carbon dioxide and destroys habitats. Burning fossil fuels and farming livestock increase carbon dioxide and methane, the main greenhouse gases driving global warming and climate change. Programmes to maintain biodiversity include breeding endangered species, protecting and regenerating rare habitats, reintroducing field margins and hedgerows, reducing deforestation, and recycling rather than landfilling waste.