Ecosystems (AQA GCSE Biology Combined Science): Revision Notes
Ecosystems
Levels of organisation
Living things are arranged in different levels from smallest to largest. Understanding these levels helps us see how ecosystems work.
Organism - This is a single living thing. It could be one fish, one tree, or one rabbit.
Population - This is a group of organisms that are all the same species living in the same place. For example, all the oak trees in a forest or all the goldfish in a pond.
Community - This includes all the different populations living together in one habitat. A woodland community might have populations of oak trees, squirrels, birds, and insects all living together.
Ecosystem - This is the biggest level. It includes the community of living things plus all the non-living parts of their environment that affect them.
The levels of organisation form a hierarchy: organism → population → community → ecosystem. Each level builds upon the previous one, with ecosystems being the most complex level that includes both living and non-living components.
Habitats and environments
A habitat is the place where an organism lives. This could be a pond, a forest, or even someone's garden.
The environment includes all the conditions around an organism in its habitat. These conditions come from two types of factors:
- Abiotic factors (non-living things)
- Biotic factors (living things)
It's important to understand the difference: a habitat is the place where something lives, while the environment includes all the conditions in that place that affect the organism.
Abiotic factors
Abiotic factors are non-living things that can affect organisms in an ecosystem. These factors are essential for determining which organisms can survive in a particular habitat.
Why Abiotic Factors Matter: Abiotic factors determine the basic conditions for life. If these factors are outside an organism's tolerance range, it cannot survive in that habitat, regardless of other conditions.
The main abiotic factors include:
- Light intensity - How bright the light is affects plant growth
- Temperature - All organisms need certain temperatures to survive
- Moisture levels - How much water is available in the environment
- pH levels - How acidic or alkaline the soil or water is
- Mineral content of soil - What nutrients are available for plants
- Wind intensity and direction - Affects plant growth and animal movement
Special abiotic factors for different habitats:
- Oxygen levels are very important for fish and other water animals
- Carbon dioxide levels affect how well plants can do photosynthesis
Biotic factors
Biotic factors are living things that affect other organisms. These factors create complex interactions between different species in an ecosystem.
Remember: Biotic factors involve interactions between living organisms. Changes in one species can have cascading effects throughout the entire ecosystem.
These include:
- Food availability - Whether there's enough food for animals to eat
- New predators arriving - Animals that hunt other animals in the ecosystem
- New pathogens arriving - Diseases that can harm organisms
- Competition - When organisms compete with each other for resources
Competition in ecosystems
Competition happens when organisms need the same resources but there isn't enough for everyone. This is one of the most important biotic factors affecting population sizes.
Animals compete for:
- Food
- Mates (partners for reproduction)
- Territory (space to live)
Plants compete for:
- Light (needed for photosynthesis)
- Water
- Mineral ions from the soil
- Space (both above ground for leaves and below ground for roots)
Example of Competition in Action:
In a forest ecosystem, two bird species might compete for the same nesting sites in tree holes. If there are only 20 suitable holes but 50 birds need nests, competition occurs.
Result: The stronger competitor gets more nesting sites and breeds successfully, while the weaker competitor may have fewer offspring or be forced to find less suitable nesting areas.
When one species is better at competing, it may outcompete another species. This means the weaker competitor has fewer individuals and might struggle to breed.
Key Points to Remember:
- Ecosystems are organised in levels: organism → population → community → ecosystem
- Habitats are where organisms live, environments are all the conditions around them
- Abiotic factors are non-living (light, temperature, moisture, pH, wind)
- Biotic factors are living (food, predators, diseases, competition)
- All organisms compete for resources they need to survive and reproduce