Free Ecosystems OCR A Level Biology revision notes – covering specification points 6.3.1(a).
An ecosystem is a community of organisms together with the physical and chemical environment with which they interact.
The environment is all of the factors surrounding an organism that can affect it.
An environment consists of abiotic and biotic factors:
- Abiotic factors are the non-living components of an ecosystem (e.g. pH, temperature, humidity and light intensity).
- Biotic factors are the living components of an ecosystem (e.g. predation, disease and competition).
The environment influences the abundance, behaviour and location of organisms within an ecosystem due to its abiotic and biotic factors.
Abiotic factors
Some examples of abiotic factors from the environment which can affect organisms include:
- Humidity, the amount of water vapour in the air, affects the rate of water loss from organisms.
- Light intensity affects the rate of photosynthesis and, therefore, the food available to a community.
- Oxygen availability, which is needed for aerobic respiration, can be limited in water or waterlogged soil.
- Temperature affects the rate of enzyme-controlled reactions and, therefore, the activity and survival of organisms.
Biotic factors
Some examples of biotic factors arising from organisms interacting with each other include:
- Competition: When organisms compete for the same limited resources, such as food, light, water, mineral ions or space.
- Disease: When pathogens infect organisms and can reduce the size of a population.
- Herbivory: When an animal feeds on a plant.
- Predation: When one organism hunts, kills and eats another.
Dynamic Ecosystems
The components of an ecosystem change over time as a result of internal and external factors interacting with each other, which makes them dynamic.
The table below outlines the main dynamic changes affecting example ecosystems:
| Rock pool | Large Tree | Playing field | African grassland |
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