Ecosystems (AQA GCSE Biology): Revision Notes
Ecosystems
What are ecosystems?
Ecosystems are organised at different levels. Think of it like a pyramid, starting with individual organisms at the bottom and building up to whole ecosystems at the top.
The pyramid structure helps us understand how life is organised - from the smallest unit (a single organism) all the way up to complex ecosystems with thousands of different species working together.
Levels of organisation
Organism
- A single living individual
- Example: one fish swimming in a pond
Population
- All the organisms of the same species living in one habitat
- Example: all the fish of one type living in the same pond
Community
- All the populations of different species living together in a habitat
- Example: all the fish, plants, and other creatures living in the pond
Ecosystem
- A community of living organisms interacting with the non-living parts of their environment
- Example: the pond community plus the water, rocks, mud, and air around it
Habitat and environment
A habitat is simply where an organism lives. The environment includes all the conditions that surround an organism in its habitat. These conditions come from two types of factors:
- Abiotic factors - non-living things
- Biotic factors - living things
Abiotic factors
Abiotic factors are non-living conditions that can affect a community. The main ones you need to know are:
- Light intensity - affects how well plants can photosynthesise
- Temperature - affects how active organisms are
- Moisture levels - determines what can survive in different areas
- pH and mineral content of soil - affects which plants can grow
- Wind intensity and direction - affects plant growth and seed dispersal
Special cases:
- Oxygen levels are vital for aquatic animals like fish
- Carbon dioxide levels are important for plants because they use it for photosynthesis
Biotic factors
Biotic factors are living things that can affect a community:
- Food availability - determines how many organisms can survive
- New predators arriving - can reduce prey populations
- New pathogens arriving - can cause disease and reduce populations
- Competition between individuals of the same or different species
Competition explained
Animals often compete with each other for:
- Food - not enough to go around
- Mates - to reproduce successfully
- Territory - safe places to live and breed
When one species outcompetes another, it can leave too few individuals of the losing species to breed successfully. This can lead to local extinction of the weaker competitor.
Plants also compete, but for different things:
- Space below ground - room for their roots to spread
- Space above ground - room for their shoots and leaves
- Light and water - essential for photosynthesis
- Nutrients - needed to make proteins and grow
Examples of competition
Competition Example: Forest Light Competition
Plants often compete with each other for light. This is why forest floors can be quite bare - the tall trees block most of the sunlight, making it hard for smaller plants to survive underneath.
Competition Example: Root System Competition
Plants also compete for water and mineral ions in the soil. Those with the best root systems usually win this competition.
Key Points to Remember:
- Ecosystems are organised in levels: organism → population → community → ecosystem
- Abiotic factors are non-living conditions like temperature, light, and pH
- Biotic factors are living things like food availability and predators
- Competition happens when organisms need the same resources
- Plants compete mainly for light, water, space and nutrients
- Animals compete mainly for food, mates and territory