Levels of organisation in an ecosystem, biotic and abiotic factors, interdependence and competition, the carbon cycle and the nitrogen cycle, decomposition, biodiversity, and human impacts on the environment.

What this topic is asking

OCR wants you to describe the levels of organisation in an ecosystem, explain biotic and abiotic factors and how organisms compete and depend on each other, describe the carbon and nitrogen cycles and decomposition, and explain biodiversity and human impacts.

Levels of organisation and interactions

Organisms in a community show interdependence: they rely on each other for food, shelter, pollination and seed dispersal, so a change in one population affects others. They also compete for resources that are in limited supply: plants compete for light, water, space and mineral ions, while animals compete for food, water, territory and mates. Abiotic factors (such as temperature, light intensity, water and soil pH) and biotic factors (such as the availability of food, new predators, or disease) determine how well a species survives in a given place. Organisms are adapted to their environment, and a stable community is one where the populations stay roughly constant over time.

The carbon cycle

The carbon cycle keeps the amount of carbon dioxide in the air roughly balanced.

• Photosynthesis removes carbon dioxide from the air, fixing carbon into glucose and then into other plant compounds. This carbon passes along food chains as animals eat plants.
• Respiration by plants, animals and microorganisms returns carbon dioxide to the air.
• Combustion (burning wood and fossil fuels) returns carbon dioxide to the air.
• Decomposition by microorganisms breaks down dead organisms and waste, and their respiration returns carbon dioxide.

The nitrogen cycle, decomposition and biodiversity

Plants need nitrogen to make proteins, but cannot use nitrogen gas directly, so it must be converted into nitrates. The nitrogen cycle involves bacteria: nitrogen-fixing bacteria (in the soil and in the root nodules of legumes) turn nitrogen gas into nitrogen compounds; decomposers break down proteins in dead material and release ammonia; nitrifying bacteria convert ammonia into nitrates that plants absorb; and denitrifying bacteria convert nitrates back into nitrogen gas. Decomposition by microorganisms is faster when it is warm, moist and well aerated, because these conditions help the microbes respire. Biodiversity is the variety of different species in an ecosystem; high biodiversity makes ecosystems more stable. Human activities such as pollution, deforestation and the destruction of habitats reduce biodiversity, while conservation, recycling and protecting habitats help to maintain it.