Levels of organisation in an ecosystem, abiotic and biotic factors, interdependence and competition, the carbon, water and nitrogen cycles, and the effect of human activity on biodiversity.

What this dot point is asking

Edexcel wants you to describe the levels of organisation in an ecosystem, distinguish abiotic and biotic factors, explain interdependence and competition, describe the carbon, water and nitrogen cycles, and explain how human activity affects biodiversity.

Levels of organisation

A food chain shows the feeding relationships, starting with a producer (usually a plant or alga that photosynthesises), then primary, secondary and tertiary consumers. Energy is lost at each stage, so food chains are usually short.

Abiotic and biotic factors

• Abiotic (non-living) factors: light intensity, temperature, water and mineral ion availability, carbon dioxide and oxygen levels.
• Biotic (living) factors: availability of food, new predators or pathogens, and competition.

Organisms show interdependence: a change in one species (for example a predator) affects others. They also compete for resources: plants compete for light, water and minerals; animals compete for food, water, territory and mates.

The carbon cycle

The water and nitrogen cycles

In the water cycle, water evaporates from oceans and transpires from plants, condenses to form clouds, and falls as precipitation, returning to rivers and the sea. It provides fresh water for organisms.

In the nitrogen cycle, plants need nitrogen to make proteins but cannot use nitrogen gas directly. Nitrogen-fixing bacteria turn nitrogen gas into compounds plants can use; decomposers release nitrogen from dead matter; and denitrifying bacteria return nitrogen gas to the air.

Human impact on biodiversity

Biodiversity is the variety of different species in an ecosystem. A high biodiversity makes an ecosystem more stable, because species depend less on any single other species and the whole system copes better with change. Human activities reduce it: deforestation destroys habitats and reduces the number of species, pollution (of air, water and land) harms or kills organisms, and global warming changes where species can live so some cannot survive in their old range. Conservation measures such as protecting and restoring habitats, replanting forests, recycling and reducing pollution all help maintain biodiversity.

Try this

Q1. Name the process that removes carbon dioxide from the air in the carbon cycle. [1 mark]

• Cue. Photosynthesis.

Q2. Give one example of an abiotic factor. [1 mark]

• Cue. Light intensity, temperature, water or mineral ion availability (any one).